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From:   Yusuf Khan <yusisamerican@gmail.com>
To:     linux-kernel@vger.kernel.org
Cc:     jasowang@redhat.com, mikelley@microsoft.com, mst@redhat.com,
        gregkh@linuxfoundation.org, javier@javigon.com, arnd@arndb.de,
        will@kernel.org, axboe@kernel.dk,
        Yusuf Khan <yusisamerican@gmail.com>
Subject: [PATCH v5] drivers: ddcci: upstream DDCCI driver
Date:   Thu, 10 Mar 2022 15:40:17 -0800
Message-Id: <20220310234017.31664-1-yusisamerican@gmail.com>
MIME-Version: 1.0
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Precedence: bulk

This patch upstreams the DDCCI driver by Christoph Grenz into
the kernel. The original gitlab page is loacted at https://gitlab
.com/ddcci-driver-linux/ddcci-driver-linux/-/tree/master.

Signed-off-by: Yusuf Khan <yusisamerican@gmail.com>
---
v2: Fix typos.

v3: Add documentation, move files around, replace semaphores with
mutexes, and replaced <asm-generic/fcntl.h> with <linux/fcntl.h>.
"imirkin"(which due to his involvement in the dri-devel irc channel
I cant help but assume to be a DRM developer) said that the DDC/CI
bus does not intefere with the buses that DRM is involved with.

v4: Move some documentation, fix grammer mistakes, remove usages of
likely(), and clarify some documentation.

v5: Fix grammer mistakes, remove usages of likely(), and clarify
some documentation.
---
 Documentation/ABI/testing/sysfs-driver-ddcci |   37 +
 Documentation/driver-api/ddcci.rst           |  107 +
 drivers/char/Kconfig                         |   11 +
 drivers/char/Makefile                        |    1 +
 drivers/char/ddcci.c                         | 1887 ++++++++++++++++++
 drivers/video/backlight/Kconfig              |   11 +
 drivers/video/backlight/Makefile             |    1 +
 drivers/video/backlight/ddcci-backlight.c    |  411 ++++
 include/linux/ddcci.h                        |  164 ++
 9 files changed, 2630 insertions(+)
 create mode 100644 Documentation/ABI/testing/sysfs-driver-ddcci
 create mode 100644 Documentation/driver-api/ddcci.rst
 create mode 100644 drivers/char/ddcci.c
 create mode 100644 drivers/video/backlight/ddcci-backlight.c
 create mode 100644 include/linux/ddcci.h

diff --git a/Documentation/ABI/testing/sysfs-driver-ddcci b/Documentation/ABI/testing/sysfs-driver-ddcci
new file mode 100644
index 000000000000..2c160ab0797e
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-driver-ddcci
@@ -0,0 +1,37 @@
+What:		/sys/bus/ddcci/ddcci<I²C bus number>i<hex address>
+Date:		February 2022
+KernelVersion:	5.17
+Contact:	Yusuf Khan <yusisamerican@gmail.com>
+Description:	This file is a internal dependent device on the I2C bus,
+		it exports the same data that the main device exports.
+
+What:		/sys/bus/ddcci/ddcci<I²C bus number>e<hex address>
+Date:		February 2022
+KernelVersion:	5.17
+Contact:	Yusuf Khan <yusisamerican@gmail.com>
+Description:	This file is a external dependent device on the I2C bus,
+		it exports the same data that the main device exports.
+
+What:		/sys/bus/ddcci/ddcci<I²C bus number>
+Date:		February 2022
+KernelVersion:	5.17
+Contact:	Yusuf Khan <yusisamerican@gmail.com>
+Description:	This file is the main device on the I2C bus, it exports
+		the following peices of information about the device, capabilities
+		which is a full ACCESS.bus capabilities string(see the ACCESS.bus
+		specification for more info), idProt, which is a full ACCESS.bus
+		protocol supported by the device(Usually "monitor"), idType,
+		a ACCESS.buss device subtype(Usually "LCD" or "CRT"), idModel, a
+		ACCESS.bus device model identifier(Usually a shortened form of the
+		device model name), idVendor, a ACCESS.bus device vendor identifier
+		(Empty if the Identification command is not supported), idModule,
+		a ACCESS.bus device module identifier(Empty if the Identification
+		command is not supported), idSerial, a 32 bit device number
+		a fixed serial number if it's positive, a temporary serial number
+		if negative and zero if the Identification command is not supported,
+		and modalias, a combined identifier for driver selection,
+		it has the form ddcci:<idProt>-<idType>-<idModel>-<idVendor>-<idModule>,
+		All non-alphanumeric characters (including whitespace) in the model,
+		vendor or module parts are replaced by underscores to prevent issues
+		with software like systemd-udevd. See Documentation/misc-devices/ddcci.rst
+		for more information on DDC/CI.
diff --git a/Documentation/driver-api/ddcci.rst b/Documentation/driver-api/ddcci.rst
new file mode 100644
index 000000000000..417bf3e216e5
--- /dev/null
+++ b/Documentation/driver-api/ddcci.rst
@@ -0,0 +1,107 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============
+DDC/CI
+==============
+
+1. Introduction
+===============
+DDC/CI is a control protocol for monitor settings supported by most
+monitors since about 2005. It is based on ACCESS.bus (an early USB predecessor).
+
+2. sysfs interface
+==================
+Each detected DDC/CI device gets a directory in /sys/bus/ddcci/devices.
+The main device on a bus is named ddcci[I²C bus number].
+Internal dependent devices are named ddcci[I²C bus number]i[hex address]
+External dependent devices are named ddcci[I²C bus number]e[hex address]
+There the following files export information about the device:
+
+capabilities
+The full ACCESS.bus capabilities string. It contains the protocol,
+type and model of the device, a list of all supported command
+codes, etc. See the ACCESS.bus spec for more information.
+
+- idProt
+ACCESS.bus protocol supported by the device. Usually "monitor".
+
+- idType
+ACCESS.bus device subtype. Usually "LCD" or "CRT".
+
+- idModel
+ACCESS.bus device model identifier. Usually a shortened form of the
+device model name.
+
+- idVendor
+ACCESS.bus device vendor identifier. Empty if the Identification command
+is not supported.
+
+- idModule
+ACCESS.bus device module identifier. Empty if the Identification command
+is not supported.
+
+- idSerial
+32 bit device number. A fixed serial number if it's positive, a temporary
+serial number if negative and zero if the
+Identification command is not supported.
+
+- modalias
+A combined identifier for driver selection. It has the form:
+ddcci:<idProt>-<idType>-<idModel>-<idVendor>-<idModule>.
+All non-alphanumeric characters (including whitespace) in the model,
+vendor or module parts are replaced by underscores to prevent issues
+with software like systemd-udevd.
+
+3. Character device interface
+=============================
+For each DDC/CI device a character device in
+/dev/bus/ddcci/[I²C bus number]/ is created.
+
+The main device on the bus is named display.
+
+Internal dependent devices are named i[hex address]
+
+External dependent devices are named e[hex address]
+
+These character devices can be used to issue commands to a DDC/CI device
+more easily than over i2c-dev devices. They should be opened unbuffered and
+may be opened with O_EXCL if you want exclusive access. To send a command just
+write the command byte and the arguments with a single write() operation.
+The length byte and checksum are automatically calculated.
+
+To read a response use read() with a buffer big enough for the expected answer.
+
+NOTE: The maximum length of a DDC/CI message is 127 bytes.
+
+4. ddcci-backlight (monitor backlight driver)
+=============================================
+For each monitor that supports accessing the Backlight Level White
+or the Luminance property, a backlight device of type "raw" named like the
+corresponding ddcci device is created. You can find them in /sys/class/backlight/.
+For convenience a symlink "ddcci_backlight" on the device associated with the
+display connector in /sys/class/drm/ to the backlight device is created, as
+long as the graphics driver allows to make this association.
+
+5. Limitations
+==============
+
+-Dependent devices (sub devices using DDC/CI directly wired to the monitor,
+like  Calibration devices, IR remotes, etc.) aren't automatically detected.
+You can force detection of external dependent devices by writing
+"ddcci-dependent [address]" into /sys/bus/i2c/i2c-?/new_device.
+
+There is no direct synchronization if you manually change the luminance
+with the buttons on your monitor, as this can only be realized through polling
+and some monitors close their OSD every time a DDC/CI command is received.
+
+Monitor hotplugging is not detected. You need to detach/reattach the I²C driver or reload the module.
+
+6. Debugging
+============
+Both drivers use the dynamic debugging feature of the Linux kernel.
+To get detailed debugging messages, set the dyndbg module parameter.
+If you want to enable debugging permanently across reboots, create a file
+/etc/modprobe.d/ddcci.conf containing lines like the following before loading the modules:
+
+options ddcci dyndbg
+options ddcci-backlight dyndbg
diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 740811893c57..2449c17e1080 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -451,4 +451,15 @@ config RANDOM_TRUST_BOOTLOADER
 	pool. Otherwise, say N here so it will be regarded as device input that
 	only mixes the entropy pool.
 
+config DDCCI
+	tristate "DDCCI display protocol support"
+	depends on I2C
+	help
+	  Display Data Channel Command Interface is an
+	  interface that allows the kernel to "talk"
+	  to most displays made after 2005. Check your
+	  display's specification to see if it has
+	  support for this. This depends on I2C to
+	  compile.
+
 endmenu
diff --git a/drivers/char/Makefile b/drivers/char/Makefile
index 264eb398fdd4..eaa2d58d50df 100644
--- a/drivers/char/Makefile
+++ b/drivers/char/Makefile
@@ -3,6 +3,7 @@
 # Makefile for the kernel character device drivers.
 #
 
+obj-$(CONFIG_DDCCI)				+= ddcci.o
 obj-y				+= mem.o random.o
 obj-$(CONFIG_TTY_PRINTK)	+= ttyprintk.o
 obj-y				+= misc.o
diff --git a/drivers/char/ddcci.c b/drivers/char/ddcci.c
new file mode 100644
index 000000000000..dd7662f78b46
--- /dev/null
+++ b/drivers/char/ddcci.c
@@ -0,0 +1,1887 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  DDC/CI sub-bus driver
+ *
+ *  Copyright (c) 2015 Christoph Grenz
+ */
+
+/*
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+#include <linux/fcntl.h>
+#include <linux/cdev.h>
+#include <linux/delay.h>
+#include <linux/device.h>
+#include <linux/fs.h>
+#include <linux/i2c.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/rwsem.h>
+#include <linux/sem.h>
+#include <linux/slab.h>
+#include <linux/uaccess.h>
+#include <linux/version.h>
+
+#include <linux/ddcci.h>
+
+#define DDCCI_RECV_BUFFER_SIZE 130
+#define DEVICE_NAME "ddcci"
+#define DDCCI_MAX_CAP_CHUNKS 200
+
+static unsigned int delay = 60;
+static unsigned short autoprobe_addrs[127] = {0xF0, 0xF2, 0xF4, 0xF6, 0xF8};
+static int autoprobe_addr_count = 5;
+
+static dev_t ddcci_cdev_first;
+static dev_t ddcci_cdev_next;
+static dev_t ddcci_cdev_end;
+static DEFINE_MUTEX(core_lock);
+
+struct bus_type ddcci_bus_type;
+EXPORT_SYMBOL_GPL(ddcci_bus_type);
+
+/* Assert neccessary string array sizes  */
+#ifndef sizeof_field
+# define sizeof_field(t, m) FIELD_SIZEOF(t, m)
+#endif
+static_assert(sizeof_field(struct ddcci_device, prot) > 8);
+static_assert(sizeof_field(struct ddcci_device, type) > 8);
+static_assert(sizeof_field(struct ddcci_device, model) > 8);
+static_assert(sizeof_field(struct ddcci_device, vendor) > 8);
+static_assert(sizeof_field(struct ddcci_device, module) > 8);
+
+/* Internal per-i2c-client driver data */
+struct ddcci_bus_drv_data {
+	unsigned long quirks;
+	struct i2c_client *i2c_dev;
+	struct mutex mut;
+	unsigned char recv_buffer[DDCCI_RECV_BUFFER_SIZE];
+};
+
+/* Replace non-alphanumeric characters in a string (used for modalias) */
+static void ddcci_modalias_clean(char *string, size_t n, char replacement)
+{
+	int i;
+
+	for (i = 0; i < n; ++i) {
+		char c = string[i];
+
+		if (c == 0)
+			return;
+		else if (c < '0' || (c > '9' && c < 'A') || (c > 'Z' && c < 'a') || c > 'z')
+			string[i] = replacement;
+	}
+}
+
+/* Write a message to the DDC/CI bus using i2c_smbus_write_byte() */
+static int __ddcci_write_bytewise(struct i2c_client *client, unsigned char addr,
+				  bool p_flag, const unsigned char *__restrict buf,
+				  unsigned char len)
+{
+	int ret = 0;
+	unsigned char outer_addr = (unsigned char) (client->addr << 1);
+	unsigned int xor = outer_addr; /* initial xor value */
+
+	/* Consistency checks */
+	if (len > 127)
+		return -EINVAL;
+
+	/* Special case: reply to 0x6E is always 0x51 */
+	if (addr == DDCCI_DEFAULT_DEVICE_ADDR) {
+		addr = DDCCI_HOST_ADDR_ODD;
+	} else {
+		/* When sending the odd address is used */
+		addr = addr | 1;
+	}
+
+	/* first byte: sender address */
+	xor ^= addr;
+	ret = i2c_smbus_write_byte(client, addr);
+	if (ret < 0)
+		return ret;
+
+	/* second byte: protocol flag and message size */
+	xor ^= ((p_flag << 7) | len);
+	ret = i2c_smbus_write_byte(client, (p_flag << 7) | len);
+	if (ret < 0)
+		return ret;
+
+	/* send payload */
+	while (len--) {
+		xor ^= (*buf);
+		ret = i2c_smbus_write_byte(client, (*buf));
+		if (ret < 0)
+			return ret;
+		buf++;
+	}
+
+	/* send checksum */
+	ret = i2c_smbus_write_byte(client, xor);
+	return ret;
+}
+
+/* Write a message to the DDC/CI bus using i2c_master_send() */
+static int __ddcci_write_block(struct i2c_client *client, unsigned char addr,
+			       unsigned char *sendbuf, bool p_flag,
+			       const unsigned char *data, unsigned char len)
+{
+	unsigned char outer_addr = (unsigned char) (client->addr << 1);
+	unsigned int xor = outer_addr;	/* initial xor value */
+	unsigned char *ptr = sendbuf;
+
+	/* Consistency checks */
+	if (len > 127)
+		return -EINVAL;
+
+	/* Special case: sender to 0x6E is always 0x51 */
+	if (addr == DDCCI_DEFAULT_DEVICE_ADDR) {
+		addr = DDCCI_HOST_ADDR_ODD;
+	} else {
+		/* When sending the odd address is used */
+		addr = addr | 1;
+	}
+
+	/* first byte: sender address */
+	xor ^= addr;
+	*(ptr++) = addr;
+	/* second byte: protocol flag and message size */
+	xor ^= ((p_flag << 7) | len);
+	*(ptr++) = (p_flag << 7) | len;
+	/* payload */
+	while (len--) {
+		xor ^= (*data);
+		*(ptr++) = (*data);
+		data++;
+	}
+	/* checksum */
+	(*ptr) = xor;
+
+	/* Send it */
+	return i2c_master_send(client, sendbuf, ptr - sendbuf + 1);
+}
+
+/*
+ * Write a message to the DDC/CI bus.
+ *
+ * You must hold the bus mutex when calling this function.
+ */
+static int ddcci_write(struct i2c_client *client, unsigned char addr,
+		       bool p_flag, const unsigned char *data,
+		       unsigned char len)
+{
+	struct ddcci_bus_drv_data *drv_data;
+	unsigned char *sendbuf;
+	int ret;
+
+	drv_data = i2c_get_clientdata(client);
+
+
+	pr_debug("sending to %d:%02x:%02x: %*ph\n", client->adapter->nr,
+		 client->addr << 1, addr, len, data);
+	if (drv_data->quirks & DDCCI_QUIRK_WRITE_BYTEWISE) {
+		ret = __ddcci_write_bytewise(client, addr, p_flag, data, len);
+	} else {
+		sendbuf = drv_data->recv_buffer;
+		ret = __ddcci_write_block(client, addr, sendbuf, p_flag, data, len);
+	}
+
+	return ret;
+}
+
+/*
+ * Read a response from the DDC/CI bus with headers directly into a buffer.
+ * Always check for DDCCI_QUIRK_SKIP_FIRST_BYTE when using this function.
+ * The returned length contains the whole unmodified response.
+ * If -EMSGSIZE is returned, the buffer contains the response up to `len`.
+ * If any other negative error code is returned, the buffer content is
+ * unspecified.
+ */
+static int __ddcci_read(struct i2c_client *client, unsigned char addr,
+			bool p_flag, unsigned long quirks, unsigned char *buf,
+			unsigned char len)
+{
+	int i, payload_len, packet_length, ret;
+	unsigned char xor = DDCCI_HOST_ADDR_EVEN;
+
+	/* Consistency checks */
+	if (len < 3)
+		return -EINVAL;
+
+	/* Read frame */
+	ret = i2c_master_recv(client, buf, len);
+	if (ret < 0)
+		goto out_err;
+	packet_length = ret;
+
+	/* Skip first byte if quirk active */
+	if ((quirks & DDCCI_QUIRK_SKIP_FIRST_BYTE) && ret > 0 && len > 0) {
+		ret--;
+		len--;
+		buf++;
+	}
+
+	/* If answer too short (= incomplete) break out */
+	if (ret < 3) {
+		ret = -EIO;
+		goto out_err;
+	}
+
+	/* validate first byte */
+	if (unlikely(buf[0] != addr)) {
+		ret = (buf[0] == '\0') ? -EAGAIN : -EIO;
+		goto out_err;
+	}
+
+	/* validate second byte (protocol flag) */
+	if (unlikely((buf[1] & 0x80) != (p_flag << 7))) {
+		if (!p_flag || !(quirks & DDCCI_QUIRK_NO_PFLAG)) {
+			ret = -EIO;
+			goto out_err;
+		}
+	}
+
+	/* get and check payload length */
+	payload_len = buf[1] & 0x7F;
+	if (3 + payload_len > packet_length)
+		return -EBADMSG;
+	if (3 + payload_len > len)
+		return -EMSGSIZE;
+
+	/* calculate checksum */
+	for (i = 0; i < 3 + payload_len; i++)
+		xor ^= buf[i];
+
+	/* verify checksum */
+	if (xor != 0) {
+		dev_err(&client->dev, "invalid DDC/CI response, corrupted data - xor is 0x%02x, length 0x%02x\n",
+			xor, payload_len);
+		ret = -EBADMSG;
+		goto out_err;
+	}
+
+	/* return result */
+	ret = payload_len + 3 + ((quirks & DDCCI_QUIRK_SKIP_FIRST_BYTE) ? 1:0);
+
+out_err:
+	return ret;
+}
+
+/*
+ * Read a response from the DDC/CI bus
+ *
+ * You must hold the bus mutex when calling this function.
+ */
+static int ddcci_read(struct i2c_client *client, unsigned char addr,
+		      bool p_flag, unsigned char *buf, unsigned char len)
+{
+	struct ddcci_bus_drv_data *drv_data;
+	unsigned char *recvbuf;
+	int ret;
+
+	drv_data = i2c_get_clientdata(client);
+	recvbuf = drv_data->recv_buffer;
+
+	/* Read frame */
+	ret = __ddcci_read(client, addr, p_flag,
+		drv_data->quirks, recvbuf, DDCCI_RECV_BUFFER_SIZE);
+	if (ret < 0)
+		return ret;
+
+	if (drv_data->quirks & DDCCI_QUIRK_SKIP_FIRST_BYTE)
+		recvbuf++;
+
+	/* return result */
+	if (buf) {
+		if (ret > 3) {
+			ret = ret-3;
+			/* copy to caller buffer */
+			memcpy(buf, &recvbuf[2], (ret < len) ? ret : len);
+
+			if (ret > len) {
+				/* if message was truncated, return -EMSGSIZE */
+				pr_debug("received from %d:%02x:%02x: [%u/%u] %*ph ...\n",
+					 client->adapter->nr, client->addr << 1,
+					 addr, ret, len, len, buf);
+				ret = -EMSGSIZE;
+			} else {
+				pr_debug("received from %d:%02x:%02x: [%u/%u] %*ph\n",
+					 client->adapter->nr, client->addr << 1,
+					 addr, ret, len, ret, buf);
+			}
+		}
+	}
+	if (!(drv_data->quirks & DDCCI_QUIRK_WRITE_BYTEWISE)) {
+		/* second read to clear buffers, needed on some devices */
+		__ddcci_read(client, addr, true, drv_data->quirks, recvbuf, 1);
+	}
+	return ret;
+}
+
+/* Request the capability string for a device and put it into buf */
+static int ddcci_get_caps(struct i2c_client *client, unsigned char addr,
+			  unsigned char *buf, unsigned int len)
+{
+	int result = 0, counter = 0, offset = 0;
+	unsigned char cmd[3] = { DDCCI_COMMAND_CAPS, 0x00, 0x00 };
+	unsigned char *chunkbuf = kzalloc(35, GFP_KERNEL);
+
+	if (!chunkbuf)
+		return -ENOMEM;
+
+	do {
+		/* Send command */
+		result = ddcci_write(client, addr, true, cmd, sizeof(cmd));
+		if (result < 0)
+			goto err_free;
+		msleep(delay);
+		/* read result chunk */
+		result = ddcci_read(client, addr, true, chunkbuf,
+				    (len > 32) ? 35 : len + 3);
+		if (result < 0)
+			goto err_free;
+
+		if (result > 0) {
+			/* check chunk header */
+			if (chunkbuf[0] != DDCCI_REPLY_CAPS) {
+				result = -EIO;
+				goto err_free;
+			}
+			if (chunkbuf[1] != cmd[1] || chunkbuf[2] != cmd[2]) {
+				result = -EIO;
+				goto err_free;
+			}
+			if (result < 3) {
+				result = -EIO;
+				goto err_free;
+			}
+			memcpy(buf, chunkbuf + 3, min((unsigned int)result - 3, len));
+
+			counter++;
+			/* adjust offset, etc. */
+			offset += result-3;
+			len -= result-3;
+			buf += result-3;
+			cmd[1] = offset >> 8;
+			cmd[2] = offset & 0xFF;
+			/* Another superfluous read to make some devices happy... */
+			ddcci_read(client, addr, true, NULL, 2);
+		}
+	} while (result > 3 && counter < DDCCI_MAX_CAP_CHUNKS);
+
+	kfree(chunkbuf);
+	return offset + result-3;
+err_free:
+	kfree(chunkbuf);
+	return result;
+}
+
+/*
+ * Request the device identification and put it into buf.
+ *
+ * Also detects all communication quirks and sets the corresponding flags
+ * in the ddcci_bus_drv_data structure associated with client.
+ *
+ * The identification command will fail on most DDC devices, as it is optional
+ * to support, but even the "failed" response suffices to detect quirks.
+ */
+static int ddcci_identify_device(struct i2c_client *client, unsigned char addr,
+				 unsigned char *buf, unsigned char len)
+{
+	int i, payload_len, ret = -ENODEV;
+	unsigned long quirks;
+	unsigned char cmd[1] = { DDCCI_COMMAND_ID };
+	unsigned char *buffer;
+	unsigned char xor = DDCCI_HOST_ADDR_EVEN;
+	struct ddcci_bus_drv_data *bus_drv_data;
+
+	bus_drv_data = i2c_get_clientdata(client);
+	quirks = bus_drv_data->quirks;
+	buffer = bus_drv_data->recv_buffer;
+
+	/* Send Identification command */
+	if (!(quirks & DDCCI_QUIRK_WRITE_BYTEWISE)) {
+		ret = __ddcci_write_block(client, addr, buffer, true, cmd, sizeof(cmd));
+		dev_dbg(&client->dev,
+			"[%02x:%02x] writing identification command in block mode: %d\n",
+			client->addr << 1, addr, ret);
+		if ((ret == -ENXIO)
+		    && i2c_check_functionality(client->adapter,
+					       I2C_FUNC_SMBUS_WRITE_BYTE)) {
+			quirks |= DDCCI_QUIRK_WRITE_BYTEWISE;
+			dev_info(&client->dev,
+				"DDC/CI bus quirk detected: writes must be done bytewise\n");
+			/* Some devices need writing twice after a failed blockwise write */
+			__ddcci_write_bytewise(client, addr, true, cmd, sizeof(cmd));
+			msleep(delay);
+		}
+	}
+	if (ret < 0 && (quirks & DDCCI_QUIRK_WRITE_BYTEWISE)) {
+		ret = __ddcci_write_bytewise(client, addr, true, cmd, sizeof(cmd));
+		dev_dbg(&client->dev,
+			"[%02x:%02x] writing identification command in bytewise mode: %d\n",
+			client->addr << 1, addr, ret);
+	}
+	if (ret < 0)
+		return -ENODEV;
+
+	/* Wait */
+	msleep(delay);
+
+	/* Receive response */
+	ret = i2c_master_recv(client, buffer, DDCCI_RECV_BUFFER_SIZE);
+	if (ret < 0) {
+		dev_dbg(&client->dev,
+			"[%02x:%02x] receiving identification response resulted in errno %d\n",
+			client->addr << 1, addr, ret);
+		return ret;
+	}
+
+	if (ret == 0) {
+		dev_dbg(&client->dev,
+			"[%02x:%02x] no identification response received\n",
+			client->addr << 1, addr);
+		return ret;
+	}
+
+	/* Skip first byte if quirk already active */
+	if (quirks & DDCCI_QUIRK_SKIP_FIRST_BYTE && ret > 1) {
+		dev_dbg(&client->dev,
+			"[%02x:%02x] doubled first byte quirk in effect\n",
+			client->addr << 1, addr);
+		ret--;
+		buffer++;
+	}
+
+	/* If answer too short (= incomplete) break out */
+	if (ret < 3) {
+		dev_dbg(&client->dev,
+			"[%02x:%02x] identification response is too short (%d bytes)\n",
+			client->addr << 1, addr, ret);
+		return -EIO;
+	}
+
+	/* validate first byte */
+	if (buffer[0] != addr) {
+		dev_dbg(&client->dev,
+			"[%02x:%02x] identification response: %*ph\n",
+			client->addr << 1, addr, (ret > 32 ? 32 : ret), buffer);
+
+		dev_dbg(&client->dev,
+			"[%02x:%02x] identification response invalid (expected first byte %02x, got %02x)\n",
+			client->addr << 1, addr, addr, buffer[0]);
+		return -ENODEV;
+	}
+
+	/* Check if first byte is doubled (QUIRK_SKIP_FIRST_BYTE) */
+	if (!(quirks & DDCCI_QUIRK_SKIP_FIRST_BYTE)) {
+		if (buffer[0] == buffer[1]) {
+			quirks |= DDCCI_QUIRK_SKIP_FIRST_BYTE;
+			dev_info(&client->dev,
+				"DDC/CI bus quirk detected: doubled first byte on read\n");
+			ret--;
+			buffer++;
+			if (ret < 3)
+				return -EIO;
+		}
+	}
+
+	/* validate second byte (protocol flag) */
+	if ((buffer[1] & 0x80) != 0x80 && !(quirks & DDCCI_QUIRK_NO_PFLAG)) {
+		dev_info(&client->dev,
+			"DDC/CI bus quirk detected: device omits protocol flag on responses\n");
+		quirks |= DDCCI_QUIRK_NO_PFLAG;
+	}
+
+	/* get and check payload length */
+	payload_len = buffer[1] & 0x7F;
+	if (3 + payload_len > ret) {
+		dev_dbg(&client->dev,
+			"[%02x:%02x] identification response: %*ph ...\n",
+			client->addr << 1, addr, ret, buffer);
+		dev_dbg(&client->dev,
+			"[%02x:%02x] identification response was truncated (expected %d bytes, got %d)\n",
+			client->addr << 1, addr, 3+payload_len, ret);
+		return -EBADMSG;
+	}
+
+	dev_dbg(&client->dev,
+		"[%02x:%02x] identification response: %*ph\n",
+		client->addr << 1, addr, 3+payload_len, buffer);
+
+	/* calculate checksum */
+	for (i = 0; i < 3 + payload_len; i++)
+		xor ^= buffer[i];
+
+	/* verify checksum */
+	if (xor != 0) {
+		dev_err(&client->dev,
+			"[%02x:%02x] invalid DDC/CI response, corrupted data - xor is 0x%02x, length 0x%02x\n",
+			client->addr << 1, addr, xor, payload_len);
+		return -EBADMSG;
+	}
+
+	/* save quirks */
+	bus_drv_data->quirks = quirks;
+
+	/* return result */
+	if (payload_len <= len) {
+		ret = payload_len;
+		memcpy(buf, &buffer[2], payload_len);
+	} else {
+		ret = -EMSGSIZE;
+		memcpy(buf, &buffer[2], len);
+	}
+	return ret;
+}
+
+/* Character device */
+
+/* Data structure for an open file handle */
+struct ddcci_fp_data {
+	struct ddcci_device *dev;
+	bool exclusive;
+	unsigned char buffer[129];
+};
+
+/* Called when the character device is opened */
+static int ddcci_cdev_open(struct inode *inode, struct file *filp)
+{
+	struct ddcci_device *dev = container_of(inode->i_cdev,
+						struct ddcci_device, cdev);
+	struct ddcci_fp_data *fp_data = NULL;
+
+	fp_data = kzalloc(sizeof(struct ddcci_fp_data), GFP_KERNEL);
+
+	if (!fp_data)
+		return -ENOMEM;
+
+	fp_data->exclusive = filp->f_flags & O_EXCL;
+
+	if (fp_data->exclusive) {
+		if (down_write_trylock(&dev->cdev_sem) == 0) {
+			kfree(fp_data);
+			return -EBUSY;
+		}
+	} else {
+		if (down_read_trylock(&dev->cdev_sem) == 0) {
+			kfree(fp_data);
+			return -EBUSY;
+		}
+	}
+
+	fp_data->dev = dev;
+	filp->private_data = fp_data;
+
+	return 0;
+}
+
+/* Called when the character device is closed */
+static int ddcci_cdev_close(struct inode *inode, struct file *filp)
+{
+	struct ddcci_fp_data *fp_data = filp->private_data;
+	struct ddcci_device *dev = fp_data->dev;
+
+	if (fp_data->exclusive)
+		up_write(&dev->cdev_sem);
+	else
+		up_read(&dev->cdev_sem);
+
+	filp->private_data = NULL;
+	kfree(fp_data);
+	return 0;
+}
+
+/* Called when reading from the character device */
+static ssize_t ddcci_cdev_read(struct file *filp, char __user *buffer,
+			       size_t length, loff_t *offset)
+{
+	struct ddcci_fp_data *fp_data = filp->private_data;
+	struct ddcci_device *dev = fp_data->dev;
+	unsigned char *buf = fp_data->buffer;
+	const bool nonblocking = (filp->f_flags & O_NONBLOCK) != 0;
+	int ret = 0;
+
+	if ((filp->f_mode & FMODE_READ) == 0)
+		return -EBADF;
+
+	/* Lock mutex */
+	if (nonblocking) {
+		if (mutex_trylock(&dev->bus_drv_data->mut))
+			return -EAGAIN;
+	} else {
+		if (mutex_lock_interruptible(&dev->bus_drv_data->mut))
+			return -ERESTARTSYS;
+	}
+
+	/* Execute read */
+	ret = ddcci_read(dev->bus_drv_data->i2c_dev, dev->inner_addr, true, buf,
+			 length);
+
+	if (ret > 0) {
+		/* Copy data from user space */
+		if (copy_to_user(buffer, buf, ret)) {
+			ret = -EFAULT;
+			goto out;
+		}
+	}
+
+out:
+	mutex_unlock(&dev->bus_drv_data->mut);
+	return ret;
+}
+
+/* Called when writing to the character device */
+static ssize_t ddcci_cdev_write(struct file *filp, const char __user *buffer,
+				size_t count, loff_t *offset)
+{
+	struct ddcci_fp_data *fp_data = filp->private_data;
+	struct ddcci_device *dev = fp_data->dev;
+	unsigned char *buf = fp_data->buffer;
+	const bool nonblocking = (filp->f_flags & O_NONBLOCK) != 0;
+	int ret = 0;
+
+	if ((filp->f_mode & FMODE_WRITE) == 0)
+		return -EBADF;
+
+	if (count > 127)
+		return -EINVAL;
+
+	/* Lock mutex */
+	if (nonblocking) {
+		if (mutex_trylock(&dev->bus_drv_data->mut))
+			return -EAGAIN;
+	} else {
+		if (mutex_lock_interruptible(&dev->bus_drv_data->mut))
+			return -ERESTARTSYS;
+	}
+
+	if (count > 0) {
+		/* Copy data from user space */
+		if (copy_from_user(buf, buffer, count)) {
+			ret = -EFAULT;
+			goto err_out;
+		}
+
+		/* Execute write */
+		ret = ddcci_write(dev->bus_drv_data->i2c_dev, dev->inner_addr,
+				  true, buf, count);
+	}
+
+	if (ret >= 0) {
+		msleep(delay);
+		mutex_unlock(&dev->bus_drv_data->mut);
+		return count;
+	}
+
+err_out:
+	mutex_unlock(&dev->bus_drv_data->mut);
+	return ret;
+}
+
+/* Called when seeking the character device */
+static loff_t ddcci_cdev_seek(struct file *filp, loff_t offset, int anchor)
+{
+	return -EINVAL;
+}
+
+static const struct file_operations ddcci_fops = {
+	.owner = THIS_MODULE,
+	.read = ddcci_cdev_read,
+	.write = ddcci_cdev_write,
+	.open = ddcci_cdev_open,
+	.release = ddcci_cdev_close,
+	.llseek = ddcci_cdev_seek
+};
+
+/* Set up the character device for a DDC/CI device */
+static int ddcci_setup_char_device(struct ddcci_device *device)
+{
+	int ret = -EINVAL;
+
+	/* Check if free minor exists */
+	if (ddcci_cdev_next == ddcci_cdev_end) {
+		dev_err(&device->dev, "no free major/minor\n");
+		ret = -ENFILE;
+		goto out;
+	}
+
+	/* Initialize rwsem */
+	init_rwsem(&device->cdev_sem);
+
+	/* Initialize character device node */
+	cdev_init(&device->cdev, &ddcci_fops);
+	device->cdev.owner = THIS_MODULE;
+
+	/* Publish char device */
+	device->dev.devt = ddcci_cdev_next;
+	ret = cdev_add(&device->cdev, ddcci_cdev_next, 1);
+	if (ret) {
+		device->dev.devt = 0;
+		goto out;
+	}
+
+	ddcci_cdev_next++;
+out:
+	return ret;
+}
+
+/* sysfs attributes */
+
+static ssize_t ddcci_attr_capabilities_show(struct device *dev,
+					    struct device_attribute *attr,
+					    char *buf)
+{
+	struct ddcci_device *device = ddcci_verify_device(dev);
+	ssize_t ret = -ENOENT;
+	size_t len;
+
+	if (device != NULL) {
+		len = device->capabilities_len;
+		if (unlikely(len > PAGE_SIZE))
+			len = PAGE_SIZE;
+		if (len == 0) {
+			ret = len;
+		} else {
+			memcpy(buf, device->capabilities, len);
+			if (len < PAGE_SIZE) {
+				buf[len] = '\n';
+				ret = len + 1;
+			}
+		}
+	}
+
+	return ret;
+}
+
+static ssize_t ddcci_attr_prot_show(struct device *dev,
+				    struct device_attribute *attr, char *buf)
+{
+	struct ddcci_device *device = ddcci_verify_device(dev);
+	ssize_t ret = -ENOENT;
+	size_t len;
+
+	if (device != NULL) {
+		len = strnlen(device->prot, sizeof(device->prot));
+		strncpy(buf, device->prot, PAGE_SIZE);
+		if (len == 0) {
+			ret = len;
+		} else if (len < PAGE_SIZE) {
+			buf[len] = '\n';
+			ret = len + 1;
+		} else {
+			ret = PAGE_SIZE;
+		}
+	}
+	return ret;
+}
+
+static ssize_t ddcci_attr_type_show(struct device *dev,
+				    struct device_attribute *attr, char *buf)
+{
+	struct ddcci_device *device = ddcci_verify_device(dev);
+	ssize_t ret = -ENOENT;
+	size_t len;
+
+	if (device != NULL) {
+		len = strnlen(device->type, sizeof(device->type));
+		strncpy(buf, device->type, PAGE_SIZE);
+		if (len == 0) {
+			ret = len;
+		} else if (len < PAGE_SIZE) {
+			buf[len] = '\n';
+			ret = len + 1;
+		} else {
+			ret = PAGE_SIZE;
+		}
+	}
+	return ret;
+}
+
+static ssize_t ddcci_attr_model_show(struct device *dev,
+				     struct device_attribute *attr, char *buf)
+{
+	struct ddcci_device *device = ddcci_verify_device(dev);
+	ssize_t ret = -ENOENT;
+	size_t len;
+
+	if (device != NULL) {
+		len = strnlen(device->model, sizeof(device->model));
+		strncpy(buf, device->model, PAGE_SIZE);
+		if (len == 0) {
+			ret = len;
+		} else if (len < PAGE_SIZE) {
+			buf[len] = '\n';
+			ret = len + 1;
+		} else {
+			ret = PAGE_SIZE;
+		}
+	}
+	return ret;
+}
+
+static ssize_t ddcci_attr_vendor_show(struct device *dev,
+				      struct device_attribute *attr, char *buf)
+{
+	struct ddcci_device *device = ddcci_verify_device(dev);
+	ssize_t ret = -ENOENT;
+	size_t len;
+
+	if (device != NULL) {
+		len = strnlen(device->vendor, sizeof(device->vendor));
+		strncpy(buf, device->vendor, PAGE_SIZE);
+		if (len == 0) {
+			ret = len;
+		} else if (len < PAGE_SIZE) {
+			buf[len] = '\n';
+			ret = len+1;
+		} else {
+			ret = PAGE_SIZE;
+		}
+	}
+	return ret;
+}
+
+static ssize_t ddcci_attr_module_show(struct device *dev,
+				      struct device_attribute *attr, char *buf)
+{
+	struct ddcci_device *device = ddcci_verify_device(dev);
+	ssize_t ret = -ENOENT;
+	size_t len;
+
+	if (device != NULL) {
+		len = strnlen(device->module, sizeof(device->module));
+		strncpy(buf, device->module, PAGE_SIZE);
+		if (len == 0) {
+			ret = len;
+		} else if (len < PAGE_SIZE) {
+			buf[len] = '\n';
+			ret = len + 1;
+		} else {
+			ret = PAGE_SIZE;
+		}
+	}
+	return ret;
+}
+
+static ssize_t ddcci_attr_serial_show(struct device *dev,
+				      struct device_attribute *attr, char *buf)
+{
+	struct ddcci_device *device = ddcci_verify_device(dev);
+	ssize_t ret = -ENOENT;
+
+	if (device != NULL)
+		ret = scnprintf(buf, PAGE_SIZE, "%d\n", device->device_number);
+
+	return ret;
+}
+
+static ssize_t ddcci_attr_modalias_show(struct device *dev,
+				      struct device_attribute *attr, char *buf)
+{
+	struct ddcci_device *device = ddcci_verify_device(dev);
+	ssize_t ret = -ENOENT;
+	char model[ARRAY_SIZE(device->model)];
+	char vendor[ARRAY_SIZE(device->model)];
+	char module[ARRAY_SIZE(device->model)];
+
+	if (device != NULL) {
+		memcpy(model, device->model, sizeof(model));
+		memcpy(vendor, device->vendor, sizeof(vendor));
+		memcpy(module, device->module, sizeof(module));
+		ddcci_modalias_clean(model, sizeof(model), '_');
+		ddcci_modalias_clean(vendor, sizeof(vendor), '_');
+		ddcci_modalias_clean(module, sizeof(module), '_');
+
+		ret = scnprintf(buf, PAGE_SIZE, "%s%s-%s-%s-%s-%s\n",
+			DDCCI_MODULE_PREFIX,
+			device->prot,
+			device->type,
+			model,
+			vendor,
+			module
+		);
+	}
+	return ret;
+}
+
+static DEVICE_ATTR(capabilities, 0444, ddcci_attr_capabilities_show, NULL);
+static DEVICE_ATTR(idProt, 0444, ddcci_attr_prot_show, NULL);
+static DEVICE_ATTR(idType, 0444, ddcci_attr_type_show, NULL);
+static DEVICE_ATTR(idModel, 0444, ddcci_attr_model_show, NULL);
+static DEVICE_ATTR(idVendor, 0444, ddcci_attr_vendor_show, NULL);
+static DEVICE_ATTR(idModule, 0444, ddcci_attr_module_show, NULL);
+static DEVICE_ATTR(idSerial, 0444, ddcci_attr_serial_show, NULL);
+static DEVICE_ATTR(modalias, 0444, ddcci_attr_modalias_show, NULL);
+
+static struct attribute *ddcci_char_device_attrs[] = {
+	&dev_attr_capabilities.attr,
+	&dev_attr_idProt.attr,
+	&dev_attr_idType.attr,
+	&dev_attr_idModel.attr,
+	&dev_attr_idVendor.attr,
+	&dev_attr_idModule.attr,
+	&dev_attr_idSerial.attr,
+	&dev_attr_modalias.attr,
+	NULL,
+};
+ATTRIBUTE_GROUPS(ddcci_char_device);
+
+/* DDC/CI bus */
+
+static int ddcci_device_uevent(struct device *dev, struct kobj_uevent_env *env)
+{
+	struct ddcci_device	*device = to_ddcci_device(dev);
+	char model[ARRAY_SIZE(device->model)];
+	char vendor[ARRAY_SIZE(device->vendor)];
+	char module[ARRAY_SIZE(device->module)];
+
+	memcpy(model, device->model, sizeof(model));
+	memcpy(vendor, device->vendor, sizeof(vendor));
+	memcpy(module, device->module, sizeof(module));
+	ddcci_modalias_clean(model, sizeof(model), '_');
+	ddcci_modalias_clean(vendor, sizeof(vendor), '_');
+	ddcci_modalias_clean(module, sizeof(module), '_');
+
+	if (add_uevent_var(env, "MODALIAS=%s%s-%s-%s-%s-%s",
+			   DDCCI_MODULE_PREFIX,
+			   device->prot,
+			   device->type,
+			   model,
+			   vendor,
+			   module
+		))
+		return -ENOMEM;
+
+	if (device->prot[0])
+		if (add_uevent_var(env, "DDCCI_PROT=%s", device->prot))
+			return -ENOMEM;
+
+	if (device->type[0])
+		if (add_uevent_var(env, "DDCCI_TYPE=%s", device->type))
+			return -ENOMEM;
+
+	if (device->model[0])
+		if (add_uevent_var(env, "DDCCI_MODEL=%s", device->model))
+			return -ENOMEM;
+
+	if (device->vendor[0]) {
+		if (add_uevent_var(env, "DDCCI_VENDOR=%s", device->vendor))
+			return -ENOMEM;
+
+		if (add_uevent_var(env, "DDCCI_MODULE=%s", device->module))
+			return -ENOMEM;
+
+		if (add_uevent_var(env, "DDCCI_UNIQ=%d", device->device_number))
+			return -ENOMEM;
+	}
+
+	return 0;
+}
+
+static void ddcci_device_release(struct device *dev)
+{
+	struct ddcci_device *device = to_ddcci_device(dev);
+	struct ddcci_driver *driver;
+
+	/* Notify driver */
+	if (dev->driver) {
+		driver = to_ddcci_driver(dev->driver);
+		if (driver->remove)
+			driver->remove(device);
+	}
+
+	/* Teardown chardev */
+	if (dev->devt) {
+		mutex_lock(&core_lock);
+		if (device->cdev.dev == ddcci_cdev_next-1)
+			ddcci_cdev_next--;
+		cdev_del(&device->cdev);
+		mutex_unlock(&core_lock);
+	}
+
+	/* Free capability string */
+	if (device->capabilities) {
+		device->capabilities_len = 0;
+		kfree(device->capabilities);
+	}
+	/* Free device */
+	kfree(device);
+}
+
+static char *ddcci_devnode(struct device *dev,
+			 umode_t *mode, kuid_t *uid, kgid_t *gid)
+{
+	struct ddcci_device *device;
+
+	device = to_ddcci_device(dev);
+	return kasprintf(GFP_KERNEL, "bus/ddcci/%d/display",
+			 device->i2c_client->adapter->nr);
+}
+
+static char *ddcci_dependent_devnode(struct device *dev,
+			 umode_t *mode, kuid_t *uid, kgid_t *gid)
+{
+	struct ddcci_device *device;
+
+	device = to_ddcci_device(dev);
+	if (device->flags & DDCCI_FLAG_EXTERNAL) {
+		if (device->outer_addr == device->inner_addr)
+			return kasprintf(GFP_KERNEL, "bus/ddcci/%d/e%02x",
+					 device->i2c_client->adapter->nr,
+					 device->outer_addr);
+		else
+			return kasprintf(GFP_KERNEL, "bus/ddcci/%d/e%02x%02x",
+					 device->i2c_client->adapter->nr,
+					 device->outer_addr, device->inner_addr);
+	} else {
+		return kasprintf(GFP_KERNEL, "bus/ddcci/%d/i%02x",
+				 device->i2c_client->adapter->nr,
+				 device->inner_addr);
+	}
+}
+
+/* Device type for main DDC/CI devices*/
+static struct device_type ddcci_device_type = {
+	.name	= "ddcci-device",
+	.uevent		= ddcci_device_uevent,
+	.groups		= ddcci_char_device_groups,
+	.release	= ddcci_device_release,
+	.devnode	= ddcci_devnode
+};
+
+/* Device type for dependent DDC/CI devices*/
+static struct device_type ddcci_dependent_type = {
+	.name	= "ddcci-dependent-device",
+	.uevent		= ddcci_device_uevent,
+	.groups		= ddcci_char_device_groups,
+	.release	= ddcci_device_release,
+	.devnode	= ddcci_dependent_devnode
+};
+
+/**
+ * ddcci_verify_device - return parameter as ddcci_device, or NULL
+ * @dev: device, probably from some driver model iterator
+ */
+struct ddcci_device *ddcci_verify_device(struct device *dev)
+{
+	if (unlikely(!dev))
+		return NULL;
+	return (dev->type == &ddcci_device_type
+		|| dev->type == &ddcci_dependent_type)
+			? to_ddcci_device(dev)
+			: NULL;
+}
+EXPORT_SYMBOL(ddcci_verify_device);
+
+/**
+ * ddcci_quirks - Get quirks for DDC/CI device
+ * @dev: Target DDC/CI device
+ */
+unsigned long ddcci_quirks(struct ddcci_device *dev)
+{
+	if (unlikely(WARN_ON(!dev)))
+		return ~0L;
+	if (unlikely(WARN_ON(!dev->bus_drv_data)))
+		return ~0L;
+	return dev->bus_drv_data->quirks;
+}
+EXPORT_SYMBOL(ddcci_quirks);
+
+/**
+ * ddcci_register_driver - register DDC/CI driver
+ * @owner: the owning module
+ * @driver: the driver to register
+ */
+int ddcci_register_driver(struct module *owner, struct ddcci_driver *driver)
+{
+	int ret;
+
+	/* Can't register until after driver model init */
+	if (unlikely(WARN_ON(!ddcci_bus_type.p)))
+		return -EAGAIN;
+
+	pr_debug("registering driver [%s]\n", driver->driver.name);
+
+	/* add the driver to the list of ddcci drivers in the driver core */
+	driver->driver.owner = owner;
+	driver->driver.bus = &ddcci_bus_type;
+
+	/* When registration returns, the driver core
+	 * will have called probe() for all matching-but-unbound devices.
+	 */
+	ret = driver_register(&driver->driver);
+	if (ret)
+		return ret;
+
+	pr_debug("driver [%s] registered\n", driver->driver.name);
+
+	return 0;
+}
+EXPORT_SYMBOL(ddcci_register_driver);
+
+/**
+ * ddcci_del_driver - unregister DDC/CI driver
+ * @driver: the driver being unregistered
+ */
+void ddcci_del_driver(struct ddcci_driver *driver)
+{
+	driver_unregister(&driver->driver);
+	pr_debug("driver [%s] unregistered\n", driver->driver.name);
+}
+EXPORT_SYMBOL(ddcci_del_driver);
+
+/**
+ * ddcci_device_write - Write a message to a DDC/CI device
+ * @dev: Target DDC/CI device
+ * @p_flag: Protocol flag, true for standard control messages
+ * @data: Data that will be written to the device
+ * @length: How many bytes to write
+ *
+ * Writes the message to the device and sleeps (see module parameter 'delay')
+ */
+int ddcci_device_write(struct ddcci_device *dev, bool p_flag,
+		       unsigned char *data, unsigned char length)
+{
+	int ret;
+
+	if (mutex_lock_interruptible(&dev->bus_drv_data->mut))
+		return -EAGAIN;
+
+	ret = ddcci_write(dev->bus_drv_data->i2c_dev, dev->inner_addr, p_flag, data, length);
+	msleep(delay);
+	mutex_unlock(&dev->bus_drv_data->mut);
+	return ret;
+}
+EXPORT_SYMBOL(ddcci_device_write);
+
+/**
+ * ddcci_device_read - Read a response from a DDC/CI device
+ * @dev: Target DDC/CI device
+ * @p_flag: Protocol flag, must match the corresponding write
+ * @buffer: Where to store data read from the device
+ * @length: Buffer size
+ */
+int ddcci_device_read(struct ddcci_device *dev, bool p_flag,
+		      unsigned char *buffer, unsigned char length)
+{
+	int ret;
+
+	if (mutex_lock_interruptible(&dev->bus_drv_data->mut))
+		return -EAGAIN;
+
+	ret = ddcci_read(dev->bus_drv_data->i2c_dev, dev->inner_addr, p_flag, buffer, length);
+	mutex_unlock(&dev->bus_drv_data->mut);
+	return ret;
+}
+EXPORT_SYMBOL(ddcci_device_read);
+
+/**
+ * ddcci_device_writeread - Write a message to a device and read the response
+ * @dev: Target DDC/CI device
+ * @p_flag: Protocol flag, true for standard control messages
+ * @buffer: Buffer used for write and read
+ * @length: How many bytes to write
+ * @maxlength: Buffer size on read
+ *
+ * Writing, sleeping and reading are done without releasing the DDC/CI bus.
+ * This provides atomicity in respect to other DDC/CI accesses on the same bus.
+ */
+int ddcci_device_writeread(struct ddcci_device *dev, bool p_flag,
+			   unsigned char *buffer, unsigned char length,
+			   unsigned char maxlength)
+{
+	int ret;
+
+	if (mutex_lock_interruptible(&dev->bus_drv_data->mut))
+		return -EAGAIN;
+
+	ret = ddcci_write(dev->bus_drv_data->i2c_dev, dev->inner_addr, p_flag, buffer, length);
+	if (ret < 0)
+		goto err;
+	msleep(delay);
+	ret = ddcci_read(dev->bus_drv_data->i2c_dev, dev->inner_addr, p_flag, buffer, maxlength);
+err:
+	mutex_unlock(&dev->bus_drv_data->mut);
+	return ret;
+}
+EXPORT_SYMBOL(ddcci_device_writeread);
+
+#define IS_ANY_ID(x) (((x)[0] == -1) && ((x)[7] == -1))
+
+/* Check if any device id in the array matches the device and return the matching id */
+static const struct ddcci_device_id *ddcci_match_id(const struct ddcci_device_id *id,
+						    const struct ddcci_device *device)
+{
+	while (id->prot[0] || id->type[0] || id->model[0] || id->vendor[0] || id->module[0]) {
+		if ((IS_ANY_ID(id->prot) || (strcmp(device->prot, id->prot) == 0))
+		 && (IS_ANY_ID(id->type) || (strcmp(device->type, id->type) == 0))
+		 && (IS_ANY_ID(id->model) || (strcmp(device->model, id->model) == 0))
+		 && (IS_ANY_ID(id->vendor) || (strcmp(device->vendor, id->vendor) == 0))
+		 && (IS_ANY_ID(id->module) || (strcmp(device->module, id->module) == 0))) {
+			return id;
+		}
+		id++;
+	}
+	return NULL;
+}
+
+static int ddcci_device_match(struct device *dev, struct device_driver *drv)
+{
+	struct ddcci_device	*device = ddcci_verify_device(dev);
+	struct ddcci_driver	*driver;
+
+	if (!device)
+		return 0;
+
+	driver = to_ddcci_driver(drv);
+	/* match on an id table if there is one */
+	if (driver->id_table)
+		return ddcci_match_id(driver->id_table, device) != NULL;
+
+	return 0;
+}
+
+static int ddcci_device_probe(struct device *dev)
+{
+	struct ddcci_device	*device = ddcci_verify_device(dev);
+	struct ddcci_driver	*driver;
+	const struct ddcci_device_id *id;
+	int ret = 0;
+
+	if (!device)
+		return -EINVAL;
+	driver = to_ddcci_driver(dev->driver);
+
+	id = ddcci_match_id(driver->id_table, device);
+	if (!id)
+		return -ENODEV;
+
+	if (driver->probe)
+		ret = driver->probe(device, id);
+
+	return ret;
+}
+
+static int ddcci_device_remove(struct device *dev)
+{
+	struct ddcci_device	*device = ddcci_verify_device(dev);
+	struct ddcci_driver	*driver;
+	int ret = 0;
+
+	if (!device)
+		return -EINVAL;
+	driver = to_ddcci_driver(dev->driver);
+
+	if (driver->remove)
+		ret = driver->remove(device);
+
+	return ret;
+}
+
+static void ddcci_device_remove_void(struct device *dev)
+{
+	ddcci_device_remove(dev);
+}
+
+/**
+ * DDCCI bus type structure
+ */
+struct bus_type ddcci_bus_type = {
+	.name		= "ddcci",
+	.match		= ddcci_device_match,
+	.probe		= ddcci_device_probe,
+	.remove		= ddcci_device_remove_void
+};
+
+/* Main I2C driver */
+
+/* Get a pointer to the closing parenthesis */
+static char *ddcci_capstr_tok(const char *s, int depth)
+{
+	const char *ptr = s;
+	char *end;
+
+	if (s == NULL || s[0] == '\0')
+		return NULL;
+
+	while ((end = strpbrk(ptr, "()"))) {
+		if (!end || depth == INT_MAX)
+			return NULL;
+		if (*end == '(')
+			depth++;
+		else if (depth > 0)
+			depth--;
+		else
+			break;
+		ptr = end + 1;
+	}
+	return end;
+}
+
+/**
+ * ddcci_find_capstr_item - Search capability string for a tag
+ * @capabilities: Capability string to search
+ * @tag: Tag to find
+ * @length: Buffer for the length of the found tag value (optional)
+ *
+ * Return a pointer to the start of the tag value (directly after the '(') on
+ * success and write the length of the value (excluding the ')') into `length`.
+ *
+ * If the tag is not found or another error occurs, an ERR_PTR is returned
+ * and `length` stays untouched.
+ */
+const char *ddcci_find_capstr_item(const char *capabilities,
+				   const char *__restrict tag,
+				   size_t *length)
+{
+	const char *src = capabilities, *ptr;
+	ptrdiff_t len;
+	int taglen = strnlen(tag, 1024);
+
+	/* Check length of requested tag */
+	if (unlikely(taglen <= 0 || taglen > 1023))
+		return ERR_PTR(-EINVAL);
+
+	/* Find tag */
+	while (src && (strncmp(src + 1, tag, taglen) != 0 || src[1 + taglen] != '('))
+		src = ddcci_capstr_tok(src + 1, -1);
+	if (!src || src[0] == '\0')
+		return ERR_PTR(-ENOENT);
+
+	/* Locate end of value */
+	src += taglen + 2;
+	ptr = ddcci_capstr_tok(src, 0);
+	if (unlikely(!ptr))
+		return ERR_PTR(-EOVERFLOW);
+
+	/* Check length of tag data */
+	len = ptr-src;
+	if (unlikely(len < 0 || len > 65535))
+		return ERR_PTR(-EMSGSIZE);
+
+	/* Return pointer and length */
+	if (length != NULL)
+		*length = (size_t)len;
+	return src;
+}
+EXPORT_SYMBOL(ddcci_find_capstr_item);
+
+/* Search the capability string for a tag and copy the value to dest */
+static int ddcci_cpy_capstr_item(char *dest, const char *src,
+				  const char *__restrict tag, size_t maxlen)
+{
+	const char *ptr;
+	size_t len;
+
+	/* Find tag */
+	ptr = ddcci_find_capstr_item(src, tag, &len);
+	if (IS_ERR(ptr))
+		return PTR_ERR(ptr);
+
+	/* Copy value */
+	memcpy(dest, ptr, min(len, maxlen));
+	return 0;
+}
+
+/* Fill fields in device by parsing the capability string */
+static int ddcci_parse_capstring(struct ddcci_device *device)
+{
+	const char *capstr = device->capabilities;
+	int ret = 0;
+
+	if (!capstr)
+		return -EINVAL;
+
+	/* capability string start with a paren */
+	if (capstr[0] != '(')
+		return -EINVAL;
+
+	/* get prot(...) */
+	ret = ddcci_cpy_capstr_item(device->prot, capstr, "prot", sizeof(device->prot)-1);
+	if (ret) {
+		if (ret == -ENOENT) {
+			dev_warn(&device->dev, "malformed capability string: no protocol tag");
+			memset(device->prot, 0, sizeof(device->prot)-1);
+		} else {
+			return ret;
+		}
+	}
+
+	/* get type(...) */
+	ret = ddcci_cpy_capstr_item(device->type, capstr, "type", sizeof(device->type)-1);
+	if (ret) {
+		if (ret == -ENOENT) {
+			dev_warn(&device->dev, "malformed capability string: no type tag");
+			memset(device->type, 0, sizeof(device->type)-1);
+		} else {
+			return ret;
+		}
+	}
+
+	/* and then model(...) */
+	ret = ddcci_cpy_capstr_item(device->model, capstr, "model", sizeof(device->model)-1);
+	if (ret) {
+		if (ret == -ENOENT) {
+			dev_warn(&device->dev, "malformed capability string: no model tag");
+			memset(device->model, 0, sizeof(device->model)-1);
+		} else {
+			return ret;
+		}
+	}
+
+	/* if there is no protocol tag */
+	if (!device->prot[0]) {
+		/* and no type tag: give up. */
+		if (!device->type[0])
+			return -ENOENT;
+
+		/* Assume protocol "monitor" if type is "LCD" or "CRT" */
+		if (strncasecmp(device->type, "LCD", sizeof(device->type)-1) == 0
+		 || strncasecmp(device->type, "CRT", sizeof(device->type)-1) == 0) {
+			memcpy(device->prot, "monitor", 7);
+		}
+	}
+
+	/* skip the rest for now */
+
+	return 0;
+}
+
+/* Probe for a device on an inner address and create a ddcci_device for it */
+static int ddcci_detect_device(struct i2c_client *client, unsigned char addr,
+			       int dependent)
+{
+	int ret;
+	unsigned char outer_addr = client->addr << 1;
+	unsigned char *buffer = NULL;
+	struct ddcci_bus_drv_data *drv_data = i2c_get_clientdata(client);
+	struct ddcci_device *device = NULL;
+
+	mutex_lock(&drv_data->mut);
+
+	/* Allocate buffer big enough for any capability string */
+	buffer = kmalloc(16384, GFP_KERNEL);
+	if (!buffer) {
+		ret = -ENOMEM;
+		goto err_end;
+	}
+
+	/* Allocate device struct */
+	device = kzalloc(sizeof(struct ddcci_device), GFP_KERNEL);
+	if (!device) {
+		ret = -ENOMEM;
+		goto err_end;
+	}
+
+	/* Initialize device */
+	device_initialize(&device->dev);
+	device->dev.parent = &client->dev;
+	device->dev.bus = &ddcci_bus_type;
+	device->outer_addr = outer_addr;
+	device->inner_addr = addr;
+	device->bus_drv_data = drv_data;
+	device->i2c_client = client;
+
+	if (!dependent) {
+		device->dev.type = &ddcci_device_type;
+		ret = dev_set_name(&device->dev, "ddcci%d", client->adapter->nr);
+	} else if (outer_addr == dependent) {
+		/* Internal dependent device */
+		device->dev.type = &ddcci_dependent_type;
+		device->flags = DDCCI_FLAG_DEPENDENT;
+		ret = dev_set_name(&device->dev, "ddcci%di%02x", client->adapter->nr, addr);
+	} else if (outer_addr == addr) {
+		/* External dependent device */
+		device->dev.type = &ddcci_dependent_type;
+		device->flags = DDCCI_FLAG_DEPENDENT | DDCCI_FLAG_EXTERNAL;
+		ret = dev_set_name(&device->dev, "ddcci%de%02x", client->adapter->nr, addr);
+	} else {
+		/* Dependent device of external dependent device Just in case something like this exists */
+		device->dev.type = &ddcci_dependent_type;
+		device->flags = DDCCI_FLAG_DEPENDENT | DDCCI_FLAG_EXTERNAL;
+		ret = dev_set_name(&device->dev, "ddcci%de%02x%02x", client->adapter->nr, outer_addr, addr);
+	}
+
+	if (ret)
+		goto err_free;
+
+	/* Read identification and check for quirks */
+	ret = ddcci_identify_device(client, addr, buffer, 29);
+	if (ret < 0) {
+		if (!dependent && (ret == -EBADMSG || ret == -EMSGSIZE))
+			dev_warn(&device->dev, "DDC/CI main device sent broken response on identification. Trying to detect solely based on capability information.\n");
+		else
+			goto err_free;
+	}
+
+	if (ret == 29 && buffer[0] == DDCCI_REPLY_ID) {
+		memcpy(device->vendor, &buffer[7], 8);
+		memcpy(device->module, &buffer[17], 8);
+		device->device_number = be32_to_cpu(*(__force __be32 *)&buffer[18]);
+	}
+
+	/* Read capabilities */
+	ret = ddcci_get_caps(client, addr, buffer, 16384);
+	if (ret > 0) {
+		/*
+		 *	Fixup unparenthesized capability strings, but only if the first
+		 *	character is an ascii lower case letter.
+		 *	This should still allow an early exit for completely garbled
+		 *	data but helps detecting devices where only the parentheses are
+		 *	missing, as the second char must be the first character of a
+		 *	keyword.
+		 */
+		if (ret > 2 && buffer[0] >= 'a' && buffer[0] <= 'z') {
+			dev_err(&device->dev, "DDC/CI device quirk detected: unparenthesized capability string\n");
+			device->capabilities = kzalloc(ret + 3, GFP_KERNEL);
+			if (!device->capabilities) {
+				ret = -ENOMEM;
+				goto err_free;
+			}
+			device->capabilities_len = ret + 2;
+			memcpy(&(device->capabilities[1]), buffer, ret);
+			device->capabilities[0] = '(';
+			device->capabilities[ret + 1] = ')';
+		} else {
+			/* Standard case: simply copy the received string */
+			device->capabilities = kzalloc(ret + 1, GFP_KERNEL);
+			if (!device->capabilities) {
+				ret = -ENOMEM;
+				goto err_free;
+			}
+			device->capabilities_len = ret;
+			memcpy(device->capabilities, buffer, ret);
+		}
+
+		ret = ddcci_parse_capstring(device);
+		if (ret) {
+			dev_err(&device->dev, "malformed capability string: \"%s\" errno %d\n", device->capabilities, ret);
+			ret = -EINVAL;
+			goto err_free;
+		}
+	}
+
+	/* Found a device if either identification or capabilities succeeded */
+	if (!device->capabilities && device->vendor[0] == '\0') {
+		dev_dbg(&client->dev,
+			"[%02x:%02x] got neither valid identification nor capability data\n",
+			client->addr << 1, addr);
+		ret = -ENODEV;
+		goto err_free;
+	}
+
+	/* Setup chardev */
+	mutex_lock(&core_lock);
+	ret = ddcci_setup_char_device(device);
+	mutex_unlock(&core_lock);
+	if (ret)
+		goto err_free;
+
+	/* Release mutex and add device to the tree */
+	mutex_unlock(&drv_data->mut);
+	pr_debug("found device at %d:%02x:%02x\n", client->adapter->nr, outer_addr, addr);
+	ret = device_add(&device->dev);
+	if (ret)
+		goto err_free;
+
+	goto end;
+err_free:
+	put_device(&device->dev);
+err_end:
+	mutex_unlock(&drv_data->mut);
+end:
+	kfree(buffer);
+	return ret;
+}
+
+/* I2C detect function: check if a main or external dependent device exists */
+static int ddcci_detect(struct i2c_client *client, struct i2c_board_info *info)
+{
+	int ret;
+	unsigned char outer_addr;
+	unsigned char inner_addr;
+	unsigned char buf[32];
+	unsigned char cmd_id[1] = { DDCCI_COMMAND_ID };
+	unsigned char cmd_caps[3] = { DDCCI_COMMAND_CAPS, 0x00, 0x00};
+	unsigned char *cmd;
+	unsigned int cmd_len;
+
+	/* Check for i2c_master_* functionality */
+	if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C)) {
+		pr_debug("i2c adapter %d unsuitable: no i2c_master functionality\n", client->adapter->nr);
+		return -ENODEV;
+	}
+
+	/* send Capabilities Request (for main) or Identification Request command (for dependent devices) */
+	outer_addr = client->addr << 1;
+	inner_addr = (outer_addr == DDCCI_DEFAULT_DEVICE_ADDR) ? DDCCI_HOST_ADDR_ODD : outer_addr | 1;
+	cmd = (outer_addr == DDCCI_DEFAULT_DEVICE_ADDR) ? cmd_caps : cmd_id;
+	cmd_len = (outer_addr == DDCCI_DEFAULT_DEVICE_ADDR) ? sizeof(cmd_caps) : sizeof(cmd_id);
+	pr_debug("detecting %d:%02x\n", client->adapter->nr, outer_addr);
+
+	ret = __ddcci_write_block(client, inner_addr, buf, true, cmd, cmd_len);
+
+	if (ret == -ENXIO || ret == -EIO) {
+		if (!i2c_check_functionality(client->adapter, I2C_FUNC_SMBUS_WRITE_BYTE)) {
+			pr_debug("i2c write failed with ENXIO or EIO but bytewise writing is not supported\n");
+			return -ENODEV;
+		}
+		pr_debug("i2c write failed with ENXIO or EIO, trying bytewise writing\n");
+		ret = __ddcci_write_bytewise(client, inner_addr, true, cmd, cmd_len);
+		if (ret == 0) {
+			msleep(delay);
+			ret = __ddcci_write_bytewise(client, inner_addr, true, cmd, cmd_len);
+		}
+	}
+
+	if (ret < 0)
+		return -ENODEV;
+
+	/* wait for device */
+	msleep(delay);
+	/* receive answer */
+	ret = i2c_master_recv(client, buf, 32);
+	if (ret < 3) {
+		pr_debug("detection failed: no answer\n");
+		return -ENODEV;
+	}
+
+	/* check response starts with outer addr */
+	if (buf[0] != outer_addr) {
+		pr_debug("detection failed: invalid %s response (%02x != %02x)\n", (cmd == cmd_id) ? "identification" : "capabilities", buf[0], outer_addr);
+		pr_debug("received message was %*ph \n", ret, buf);
+		return -ENODEV;
+	}
+
+	pr_debug("detected %d:%02x\n", client->adapter->nr, outer_addr);
+
+	/* set device type */
+	strlcpy(info->type, (outer_addr == DDCCI_DEFAULT_DEVICE_ADDR) ? "ddcci" : "ddcci-dependent", I2C_NAME_SIZE);
+
+	return 0;
+}
+
+/* I2C probe function */
+static int ddcci_probe(struct i2c_client *client, const struct i2c_device_id *id)
+{
+	int i, ret = -ENODEV, tmp;
+	unsigned char main_addr, addr;
+	struct ddcci_bus_drv_data *drv_data;
+
+	/* Initialize driver data structure */
+	drv_data = devm_kzalloc(&client->dev, sizeof(struct ddcci_bus_drv_data), GFP_KERNEL);
+	if (!drv_data)
+		return -ENOMEM;
+	drv_data->i2c_dev = client;
+	mutex_init(&drv_data->mut);
+
+	/* Set i2c client data */
+	i2c_set_clientdata(client, drv_data);
+
+	if (id->driver_data == 0) {
+		/* Core device, probe at 0x6E */
+		main_addr = DDCCI_DEFAULT_DEVICE_ADDR;
+		dev_dbg(&client->dev, "probing core device [%02x]\n",
+			client->addr << 1);
+		ret = ddcci_detect_device(client, main_addr, 0);
+		if (ret) {
+			dev_info(&client->dev, "core device [%02x] probe failed: %d\n",
+				 client->addr << 1, ret);
+			if (ret == -EIO)
+				ret = -ENODEV;
+			goto err_free;
+		}
+
+		/* Detect internal dependent devices */
+		dev_dbg(&client->dev, "probing internal dependent devices\n");
+		for (i = 0; i < autoprobe_addr_count; ++i) {
+			addr = (unsigned short)autoprobe_addrs[i];
+			if ((addr & 1) == 0 && addr != main_addr) {
+				tmp = ddcci_detect_device(client, addr, main_addr);
+				if (tmp < 0 && tmp != -ENODEV) {
+					dev_info(&client->dev, "internal dependent device [%02x:%02x] probe failed: %d\n",
+						 client->addr << 1, addr, ret);
+				}
+			}
+		}
+	} else if (id->driver_data == 1) {
+		/* External dependent device */
+		main_addr = client->addr << 1;
+		dev_dbg(&client->dev, "probing external dependent device [%02x]\n", main_addr);
+		ret = ddcci_detect_device(client, main_addr, -1);
+		if (ret) {
+			dev_info(&client->dev, "external dependent device [%02x] probe failed: %d\n",
+				 main_addr, ret);
+			if (ret == -EIO)
+				ret = -ENODEV;
+			goto err_free;
+		}
+	} else {
+		dev_warn(&client->dev,
+			 "probe() called with invalid i2c device id\n");
+		ret = -EINVAL;
+	}
+
+	goto end;
+err_free:
+	devm_kfree(&client->dev, drv_data);
+end:
+	return ret;
+}
+
+/*
+ * Callback for bus_find_device() used in ddcci_remove()
+ *
+ * Find next device on i2c_client not flagged with
+ * DDCCI_FLAG_REMOVED and flag it.
+ */
+static int ddcci_remove_helper(struct device *dev, const void *p)
+{
+	struct ddcci_device *device;
+
+	device = ddcci_verify_device(dev);
+	if (!device || device->flags & DDCCI_FLAG_REMOVED)
+		return 0;
+
+	if (!p || (dev->parent == p)) {
+		device->flags |= DDCCI_FLAG_REMOVED;
+		/* Memory Barrier */
+		wmb();
+		return 1;
+	}
+
+	return 0;
+}
+
+/* I2C driver remove callback: unregister all subdevices */
+static int ddcci_remove(struct i2c_client *client)
+{
+	struct ddcci_bus_drv_data *drv_data = i2c_get_clientdata(client);
+	struct device *dev;
+
+	mutex_lock(&drv_data->mut);
+	while (1) {
+		dev = bus_find_device(&ddcci_bus_type, NULL, client,
+				      ddcci_remove_helper);
+		if (!dev)
+			break;
+		device_unregister(dev);
+		put_device(dev);
+	}
+	mutex_unlock(&drv_data->mut);
+	return 0;
+}
+
+/*
+ * I2C driver device identification table.
+ */
+static const struct i2c_device_id ddcci_idtable[] = {
+	{ "ddcci", 0 },
+	{ "ddcci-dependent", 1 },
+	{}
+};
+MODULE_DEVICE_TABLE(i2c, ddcci_idtable);
+
+/*
+ * I2C driver description structure
+ */
+static struct i2c_driver ddcci_driver = {
+	.driver = {
+		.name	= "ddcci",
+		.owner	= THIS_MODULE,
+	},
+
+	.id_table	= ddcci_idtable,
+	.probe		= ddcci_probe,
+	.remove		= ddcci_remove,
+	.class		= I2C_CLASS_DDC,
+	.detect		= ddcci_detect,
+	.address_list	= I2C_ADDRS(
+		DDCCI_DEFAULT_DEVICE_ADDR>>1
+	),
+};
+
+/*
+ * Module initialization function. Called when the module is inserted or
+ * (if builtin) at boot time.
+ */
+static int __init ddcci_module_init(void)
+{
+	int ret;
+
+	pr_debug("initializing ddcci driver\n");
+	/* Allocate a device number region for the character devices */
+	ret = alloc_chrdev_region(&ddcci_cdev_first, 0, 128, DEVICE_NAME);
+	if (ret < 0) {
+		pr_err("failed to register device region: error %d\n", ret);
+		goto err_chrdevreg;
+	}
+	ddcci_cdev_next = ddcci_cdev_first;
+	ddcci_cdev_end = MKDEV(MAJOR(ddcci_cdev_first), MINOR(ddcci_cdev_first) + 128);
+
+	/* Register bus */
+	ret = bus_register(&ddcci_bus_type);
+	if (ret) {
+		pr_err("failed to register bus 'ddcci'\n");
+		goto err_busreg;
+	}
+
+	/* Register I2C driver */
+	ret = i2c_add_driver(&ddcci_driver);
+	if (ret) {
+		pr_err("failed to register i2c driver\n");
+		goto err_drvreg;
+	}
+
+	pr_debug("ddcci driver initialized\n");
+
+	return 0;
+
+err_drvreg:
+	bus_unregister(&ddcci_bus_type);
+err_busreg:
+	unregister_chrdev_region(ddcci_cdev_first, 128);
+err_chrdevreg:
+	return ret;
+}
+
+/*
+ * Module clean-up function. Called when the module is removed.
+ */
+static void __exit ddcci_module_exit(void)
+{
+	struct device *dev;
+
+	while (1) {
+		dev = bus_find_device(&ddcci_bus_type, NULL, NULL, ddcci_remove_helper);
+		if (!dev)
+			break;
+		device_unregister(dev);
+		put_device(dev);
+	}
+
+	i2c_del_driver(&ddcci_driver);
+	bus_unregister(&ddcci_bus_type);
+	unregister_chrdev_region(ddcci_cdev_first, 128);
+}
+
+/* Let the kernel know the calls for module init and exit */
+module_init(ddcci_module_init);
+module_exit(ddcci_module_exit);
+
+/* Module parameter description */
+module_param(delay, uint, 0644);
+MODULE_PARM_DESC(delay, "default delay after bus writes (in ms, default 60)");
+module_param_array(autoprobe_addrs, ushort, &autoprobe_addr_count, 0644);
+MODULE_PARM_DESC(autoprobe_addrs, "internal dependent device addresses to autoprobe");
+
+/* Module description */
+MODULE_AUTHOR("Christoph Grenz <webmaster@grenz-bonn.de>");
+MODULE_DESCRIPTION("DDC/CI bus driver");
+MODULE_VERSION("0.4.2");
+MODULE_LICENSE("GPL");
diff --git a/drivers/video/backlight/Kconfig b/drivers/video/backlight/Kconfig
index e32694c13da5..7a26088c3c3f 100644
--- a/drivers/video/backlight/Kconfig
+++ b/drivers/video/backlight/Kconfig
@@ -289,6 +289,17 @@ config BACKLIGHT_QCOM_WLED
 	  If you have the Qualcomm PMIC, say Y to enable a driver for the
 	  WLED block. Currently it supports PM8941 and PMI8998.
 
+config BACKLIGHT_DDCCI
+	tristate "DDCCI Backlight Driver"
+	depends on DDCCI
+	help
+	  If you have a DDC/CI supporing monitor, say Y to enable a driver
+	  to control its backlight using DDC/CI. This could be useful if
+	  your monitor does not include a backlight driver. For this to be
+	  useful you need to enable DDCCI support which can be found in
+	  Device Drivers -> Character devices and that further depends on
+	  I2C.
+
 config BACKLIGHT_RT4831
 	tristate "Richtek RT4831 Backlight Driver"
 	depends on MFD_RT4831
diff --git a/drivers/video/backlight/Makefile b/drivers/video/backlight/Makefile
index cae2c83422ae..7bfb6e506b35 100644
--- a/drivers/video/backlight/Makefile
+++ b/drivers/video/backlight/Makefile
@@ -58,3 +58,4 @@ obj-$(CONFIG_BACKLIGHT_WM831X)		+= wm831x_bl.o
 obj-$(CONFIG_BACKLIGHT_ARCXCNN) 	+= arcxcnn_bl.o
 obj-$(CONFIG_BACKLIGHT_RAVE_SP)		+= rave-sp-backlight.o
 obj-$(CONFIG_BACKLIGHT_LED)		+= led_bl.o
+obj-$(CONFIG_BACKLIGHT_DDCCI)		+= ddcci-backlight.o
diff --git a/drivers/video/backlight/ddcci-backlight.c b/drivers/video/backlight/ddcci-backlight.c
new file mode 100644
index 000000000000..eda812a595fe
--- /dev/null
+++ b/drivers/video/backlight/ddcci-backlight.c
@@ -0,0 +1,411 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  DDC/CI monitor backlight driver
+ *
+ *  Copyright (c) 2015 Christoph Grenz
+ */
+
+/*
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+#include <linux/backlight.h>
+#include <linux/module.h>
+#include <linux/fb.h>
+#include <linux/sysfs.h>
+
+#include <linux/ddcci.h>
+
+
+#define DDCCI_COMMAND_READ	0x01	/* read ctrl value */
+#define DDCCI_REPLY_READ	0x02	/* read ctrl value reply */
+#define DDCCI_COMMAND_WRITE	0x03	/* write ctrl value */
+#define DDCCI_COMMAND_SAVE	0x0c	/* save current settings */
+
+#define DDCCI_MONITOR_LUMINANCE	0x10
+#define DDCCI_MONITOR_BACKLIGHT	0x13
+#define DDCCI_MONITOR_BL_WHITE		0x6B
+
+static bool convenience_symlink = true;
+
+struct ddcci_monitor_drv_data {
+	struct ddcci_device *device;
+	struct backlight_device *bl_dev;
+	struct device *fb_dev;
+	unsigned char used_vcp;
+};
+
+static int ddcci_monitor_writectrl(struct ddcci_device *device,
+				   unsigned char ctrl, unsigned short value)
+{
+	unsigned char buf[4];
+	int ret;
+
+	buf[0] = DDCCI_COMMAND_WRITE;
+	buf[1] = ctrl;
+	buf[2] = (value >> 8);
+	buf[3] = (value & 255);
+
+	ret = ddcci_device_write(device, true, buf, sizeof(buf));
+
+	return ret;
+}
+
+static int ddcci_monitor_readctrl(struct ddcci_device *device,
+				  unsigned char ctrl, unsigned short *value,
+				  unsigned short *maximum)
+{
+	int ret;
+	unsigned char buf[10];
+
+	buf[0] = DDCCI_COMMAND_READ;
+	buf[1] = ctrl;
+
+	ret = ddcci_device_writeread(device, true, buf, 2, sizeof(buf));
+	if (ret < 0)
+		return ret;
+
+	if (ret == 0)
+		return -ENOTSUPP;
+
+	if (ret == 8 && buf[0] == DDCCI_REPLY_READ && buf[2] == ctrl) {
+		if (value)
+			*value = buf[6] * 256 + buf[7];
+
+		if (maximum)
+			*maximum = buf[4] * 256 + buf[5];
+
+		if (buf[1] == 1)
+			return -ENOTSUPP;
+		if (buf[1] != 0)
+			return -EIO;
+		return 0;
+	}
+
+	return -EIO;
+}
+
+static int ddcci_backlight_check_fb(struct backlight_device *bl,
+				   struct fb_info *info)
+{
+	struct ddcci_monitor_drv_data *drv_data = bl_get_data(bl);
+
+	return drv_data->fb_dev == NULL || drv_data->fb_dev == info->dev;
+}
+
+static int ddcci_backlight_update_status(struct backlight_device *bl)
+{
+	struct ddcci_monitor_drv_data *drv_data = bl_get_data(bl);
+	int brightness = bl->props.brightness;
+	int ret;
+
+	if (bl->props.power != FB_BLANK_UNBLANK ||
+	    bl->props.state & BL_CORE_FBBLANK)
+		brightness = 0;
+
+	ret = ddcci_monitor_writectrl(drv_data->device, drv_data->used_vcp,
+				      brightness);
+	if (ret > 0)
+		ret = 0;
+	return ret;
+}
+
+static int ddcci_backlight_get_brightness(struct backlight_device *bl)
+{
+	unsigned short value = 0, maxval = 0;
+	int ret;
+	struct ddcci_monitor_drv_data *drv_data = bl_get_data(bl);
+
+	ret = ddcci_monitor_readctrl(drv_data->device, drv_data->used_vcp,
+				     &value, &maxval);
+	if (ret < 0)
+		return ret;
+
+	bl->props.brightness = value;
+	bl->props.max_brightness = maxval;
+	ret = value;
+
+	return ret;
+}
+
+static const struct backlight_ops ddcci_backlight_ops = {
+	.options	= 0,
+	.update_status	= ddcci_backlight_update_status,
+	.get_brightness = ddcci_backlight_get_brightness,
+	.check_fb	= ddcci_backlight_check_fb,
+};
+
+static const char *ddcci_monitor_vcp_name(unsigned char vcp)
+{
+	switch (vcp) {
+	case DDCCI_MONITOR_BL_WHITE:
+		return "backlight";
+	case DDCCI_MONITOR_LUMINANCE:
+		return "luminance";
+	default:
+		return "???";
+	}
+}
+
+static const char *ddcci_monitor_next_vcp_item(const char *ptr)
+{
+	int depth = 0;
+
+	/* Sanity check */
+	if (unlikely(ptr == NULL || ptr[0] == '\0'))
+		return NULL;
+
+	/* Find next white space outside of parentheses */
+	while ((ptr = strpbrk(ptr, " ()"))) {
+		if (!ptr || depth == INT_MAX)
+			return NULL;
+		else if (*ptr == '(')
+			depth++;
+		else if (depth > 0) {
+			if (*ptr == ')')
+				depth--;
+		} else
+			break;
+		++ptr;
+	}
+
+	/* Skip over whitespace */
+	ptr = skip_spaces(ptr);
+
+	/* Check if we're now at the end of the list */
+	if (unlikely(*ptr == '\0' || *ptr == ')'))
+		return NULL;
+
+	return ptr;
+}
+
+static bool ddcci_monitor_find_vcp(unsigned char vcp, const char *s)
+{
+	const char *ptr = s;
+	char vcp_hex[3];
+
+	/* Sanity check */
+	if (unlikely(s == NULL || s[0] == '\0'))
+		return false;
+
+	/* Create hex representation of VCP */
+	if (unlikely(snprintf(vcp_hex, 3, "%02hhX", vcp) != 2)) {
+		pr_err("snprintf failed to convert to hex. This should not happen.\n");
+		return false;
+	}
+
+	/* Search for it */
+	do {
+		if (strncasecmp(vcp_hex, ptr, 2) == 0) {
+			if (ptr[2] == ' ' || ptr[2] == '(' || ptr[2] == ')')
+				return true;
+		}
+	} while ((ptr = ddcci_monitor_next_vcp_item(ptr)));
+
+	return false;
+}
+
+static int ddcci_backlight_create_symlink(struct ddcci_device *ddcci_dev)
+{
+	int i, result;
+	struct device *dev = &ddcci_dev->dev;
+	struct kernfs_node *dirent;
+
+	for (i = 0; i < 3; ++i) {
+		dev = dev->parent;
+		if (!dev) {
+			dev_dbg(&ddcci_dev->dev, "failed to create convenience symlink: ancestor device not found\n");
+			return -ENOENT;
+		}
+	}
+	dirent = sysfs_get_dirent(dev->kobj.sd, "ddcci_backlight");
+	if (dirent) {
+		sysfs_put(dirent);
+		dev_dbg(&ddcci_dev->dev, "failed to create convenience symlink: %s/ddcci_backlight already exists\n", dev_name(dev));
+		return -EEXIST;
+	}
+
+	result = sysfs_create_link(&dev->kobj, &ddcci_dev->dev.kobj, "ddcci_backlight");
+	if (result == 0)
+		dev_dbg(&ddcci_dev->dev, "created symlink %s/ddcci_backlight\n", dev_name(dev));
+	else
+		dev_info(&ddcci_dev->dev, "failed to create convenience symlink: %d\n", result);
+	return result;
+}
+
+static int ddcci_backlight_remove_symlink(struct ddcci_device *ddcci_dev)
+{
+	int i;
+	struct device *dev = &ddcci_dev->dev;
+	struct kernfs_node *dirent;
+
+	for (i = 0; i < 3; ++i) {
+		dev = dev->parent;
+		if (!dev)
+			return -ENOENT;
+	}
+	dirent = sysfs_get_dirent(dev->kobj.sd, "ddcci_backlight");
+	if (!dirent)
+		return -ENOENT;
+
+	if ((dirent->flags & KERNFS_LINK) == 0) {
+		sysfs_put(dirent);
+		dev_dbg(&ddcci_dev->dev, "won't remove %s/ddcci_backlight: not a symlink\n", dev_name(dev));
+		return -EINVAL;
+	}
+
+	if (dirent->symlink.target_kn != ddcci_dev->dev.kobj.sd) {
+		sysfs_put(dirent);
+		dev_dbg(&ddcci_dev->dev, "won't remove %s/ddcci_backlight: we are not the link target\n", dev_name(dev));
+		return -EINVAL;
+	}
+
+	sysfs_put(dirent);
+
+	sysfs_remove_link(&dev->kobj, "ddcci_backlight");
+	dev_dbg(&ddcci_dev->dev, "removed symlink %s/ddcci_backlight\n", dev_name(dev));
+	return 0;
+}
+
+static int ddcci_monitor_probe(struct ddcci_device *dev,
+			       const struct ddcci_device_id *id)
+{
+	struct ddcci_monitor_drv_data *drv_data;
+	struct backlight_properties props;
+	struct backlight_device *bl = NULL;
+	int ret = 0;
+	bool support_luminance, support_bl_white;
+	unsigned short brightness = 0, max_brightness = 0;
+	const char *vcps;
+
+	dev_dbg(&dev->dev, "probing monitor backlight device\n");
+
+	/* Get VCP list */
+	vcps = ddcci_find_capstr_item(dev->capabilities, "vcp", NULL);
+	if (IS_ERR(vcps)) {
+		dev_info(&dev->dev,
+			 "monitor doesn't provide a list of supported controls.\n");
+		support_bl_white = support_luminance = true;
+	} else {
+		/* Check VCP list for supported VCPs */
+		support_bl_white = ddcci_monitor_find_vcp(DDCCI_MONITOR_BL_WHITE, vcps);
+		support_luminance = ddcci_monitor_find_vcp(DDCCI_MONITOR_LUMINANCE, vcps);
+		/* Fallback to trying if no support is found */
+		if (!support_bl_white && !support_luminance) {
+			dev_info(&dev->dev,
+				 "monitor doesn't announce support for backlight or luminance controls.\n");
+			support_bl_white = support_luminance = true;
+		}
+	}
+
+	/* Initialize driver data structure */
+	drv_data = devm_kzalloc(&dev->dev, sizeof(struct ddcci_monitor_drv_data),
+				GFP_KERNEL);
+	if (!drv_data)
+		return -ENOMEM;
+	drv_data->device = dev;
+
+	if (support_bl_white) {
+		/* Try getting backlight level */
+		dev_dbg(&dev->dev,
+			"trying to access \"backlight level white\" control\n");
+		ret = ddcci_monitor_readctrl(drv_data->device, DDCCI_MONITOR_BL_WHITE,
+						&brightness, &max_brightness);
+		if (ret < 0) {
+			if (ret == -ENOTSUPP)
+				dev_info(&dev->dev,
+					"monitor does not support reading backlight level\n");
+			else
+				goto err_free;
+		} else {
+			drv_data->used_vcp = DDCCI_MONITOR_BL_WHITE;
+		}
+	}
+
+	if (support_luminance && !drv_data->used_vcp) {
+		/* Try getting luminance */
+		dev_dbg(&dev->dev,
+			"trying to access \"luminance\" control\n");
+		ret = ddcci_monitor_readctrl(drv_data->device, DDCCI_MONITOR_LUMINANCE,
+					     &brightness, &max_brightness);
+		if (ret < 0) {
+			if (ret == -ENOTSUPP)
+				dev_info(&dev->dev,
+					"monitor does not support reading luminance\n");
+			else
+				goto err_free;
+		} else {
+			drv_data->used_vcp = DDCCI_MONITOR_LUMINANCE;
+		}
+		drv_data->used_vcp = DDCCI_MONITOR_LUMINANCE;
+	}
+
+	if (!drv_data->used_vcp)
+		goto err_free;
+
+	/* Create brightness device */
+	memset(&props, 0, sizeof(props));
+	props.type = BACKLIGHT_RAW;
+	props.max_brightness = max_brightness;
+	props.brightness = brightness;
+	bl = devm_backlight_device_register(&dev->dev, dev_name(&dev->dev),
+					    &dev->dev, drv_data,
+					    &ddcci_backlight_ops, &props);
+	drv_data->bl_dev = bl;
+	if (IS_ERR(bl)) {
+		dev_err(&dev->dev, "failed to register backlight\n");
+		return PTR_ERR(bl);
+	}
+	dev_info(&dev->dev, "registered %s as backlight device %s\n",
+		 ddcci_monitor_vcp_name(drv_data->used_vcp),
+		 dev_name(&dev->dev));
+
+	if (convenience_symlink)
+		ddcci_backlight_create_symlink(dev);
+
+	goto end;
+err_free:
+	devm_kfree(&dev->dev, drv_data);
+end:
+	return ret;
+}
+
+static int ddcci_monitor_remove(struct ddcci_device *dev)
+{
+	dev_dbg(&dev->dev, "removing device\n");
+	ddcci_backlight_remove_symlink(dev);
+	return 0;
+}
+
+static struct ddcci_device_id ddcci_monitor_idtable[] = {
+	{ "monitor", DDCCI_ANY_ID, DDCCI_ANY_ID, DDCCI_ANY_ID, DDCCI_ANY_ID, 0 },
+	{}
+};
+
+static struct ddcci_driver ddcci_backlight_driver = {
+	.driver = {
+		.name	= "ddcci-backlight",
+		.owner	= THIS_MODULE,
+	},
+
+	.id_table	= ddcci_monitor_idtable,
+	.probe		= ddcci_monitor_probe,
+	.remove		= ddcci_monitor_remove,
+};
+
+module_ddcci_driver(ddcci_backlight_driver);
+
+/* Module parameter description */
+module_param(convenience_symlink, bool, 0644);
+MODULE_PARM_DESC(convenience_symlink, "add convenience symlink \"ddcci_backlight\" to ancestor device in sysfs (default true)");
+
+MODULE_AUTHOR("Christoph Grenz <webmaster@grenz-bonn.de>");
+MODULE_DESCRIPTION("DDC/CI generic monitor backlight driver");
+MODULE_VERSION("0.4.2");
+MODULE_LICENSE("GPL");
+
+MODULE_ALIAS("ddcci:monitor-*-*-*-*");
diff --git a/include/linux/ddcci.h b/include/linux/ddcci.h
new file mode 100644
index 000000000000..a219f031e584
--- /dev/null
+++ b/include/linux/ddcci.h
@@ -0,0 +1,164 @@
+/*
+ *  DDC/CI bus driver
+ *
+ *  Copyright (c) 2015 Christoph Grenz
+ */
+
+/*
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ */
+
+#ifndef _DDCCI_H
+#define _DDCCI_H
+
+#include <linux/mod_devicetable.h>
+#include <linux/device.h>
+#include <linux/cdev.h>
+
+#define DDCCI_MODULE_PREFIX "ddcci:"
+
+/* Special addresses */
+
+/* default device address (even) */
+#define DDCCI_DEFAULT_DEVICE_ADDR	0x6E
+/* receiving host address for communication with default device address */
+#define DDCCI_HOST_ADDR_EVEN	0x50
+/* sending host address for communication with default device address */
+#define DDCCI_HOST_ADDR_ODD	0x51
+
+/* Command codes */
+
+/* Identification Request */
+#define DDCCI_COMMAND_ID	0xf1
+/* Identification Reply */
+#define DDCCI_REPLY_ID	0xe1
+/* Capabilities Request */
+#define DDCCI_COMMAND_CAPS	0xf3
+/* Capabilities Reply */
+#define DDCCI_REPLY_CAPS	0xe3
+
+/* Quirks */
+
+/* Device always responds with unset protocol flag */
+#define DDCCI_QUIRK_NO_PFLAG BIT(1)
+/* Device needs writing one byte at a time  */
+#define DDCCI_QUIRK_WRITE_BYTEWISE BIT(2)
+/* Device repeats first byte on read */
+#define DDCCI_QUIRK_SKIP_FIRST_BYTE BIT(3)
+
+/* Flags */
+
+#define DDCCI_FLAG_REMOVED BIT(1)
+#define DDCCI_FLAG_DEPENDENT BIT(2)
+#define DDCCI_FLAG_EXTERNAL BIT(3)
+
+extern struct bus_type ddcci_bus_type;
+
+struct ddcci_bus_drv_data;
+
+/* struct ddcci_device_id - identifies DDC/CI devices for probing */
+struct ddcci_device_id {
+	char prot[9];
+	char type[9];
+	char model[9];
+	char vendor[9];
+	char module[9];
+	kernel_ulong_t driver_data;	/* Data private to the driver */
+};
+#define DDCCI_ANY_ID "\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF"
+
+/**
+ * struct ddcci_device - represent an DDC/CI device
+ * @outer_addr: Outer device address (I2C address << 1).
+ * @inner_addr: Inner device address.
+ * @flags: Device flags.
+ * @capabilities: Device capability string.
+ * @capabilities_len: Length of capability string.
+ * @i2c_client: Parent I2C device.
+ * @bus_drv_data: Driver internal data structure.
+ * @dev: Driver model device node for the slave.
+ * @cdev: Character device structure
+ * @cdev_sem: RW semaphore for exclusive access on character device.
+ * @prot: Device class ("protocol", from capability string)
+ * @type: Device subclass ("type", from capability string)
+ * @model: Device model (from capability string)
+ * @vendor: Device vendor (from identification command response)
+ * @module: Device module (from identification command response)
+ * @device_number: Device serial (from identification command response)
+ */
+struct ddcci_device {
+	unsigned short outer_addr;
+	unsigned short inner_addr;
+	int flags;
+	char *capabilities;
+	size_t capabilities_len;
+	struct i2c_client *i2c_client;
+	struct ddcci_bus_drv_data *bus_drv_data;
+	struct device dev;
+	struct cdev cdev;
+	struct rw_semaphore cdev_sem;
+	char prot[9];
+	char type[9];
+	char model[9];
+	char vendor[9];
+	char module[9];
+	int device_number;
+};
+#define to_ddcci_device(d) container_of(d, struct ddcci_device, dev)
+
+/**
+ * struct ddcci_driver - represent an DDC/CI device driver
+ * @probe: Callback for device binding
+ * @remove: Callback for device unbinding
+ * @driver: Device driver model driver
+ * @id_table: List of DDC/CI devices supported by this driver
+ *
+ * The driver.owner field should be set to the module owner of this driver.
+ * The driver.name field should be set to the name of this driver.
+ */
+struct ddcci_driver {
+	int (*probe)(struct ddcci_device *, const struct ddcci_device_id *);
+	int (*remove)(struct ddcci_device *);
+	struct device_driver driver;
+	struct ddcci_device_id *id_table;
+};
+#define to_ddcci_driver(d) container_of(d, struct ddcci_driver, driver)
+
+int ddcci_register_driver(struct module *owner, struct ddcci_driver *driver);
+#define ddcci_add_driver(driver) \
+	ddcci_register_driver(THIS_MODULE, driver)
+void ddcci_del_driver(struct ddcci_driver *driver);
+
+struct ddcci_device *ddcci_verify_device(struct device *dev);
+
+#define module_ddcci_driver(__ddcci_driver) \
+	module_driver(__ddcci_driver, ddcci_add_driver, \
+			ddcci_del_driver)
+
+int ddcci_device_write(struct ddcci_device *, bool p_flag, unsigned char *data,
+		       unsigned char length);
+int ddcci_device_read(struct ddcci_device *, bool p_flag, unsigned char *buffer,
+		      unsigned char length);
+int ddcci_device_writeread(struct ddcci_device *, bool p_flag,
+			   unsigned char *buffer, unsigned char length,
+			   unsigned char maxlength);
+
+static inline void *ddcci_get_drvdata(const struct ddcci_device *dev)
+{
+	return dev_get_drvdata(&dev->dev);
+}
+
+static inline void ddcci_set_drvdata(struct ddcci_device *dev, void *data)
+{
+	dev_set_drvdata(&dev->dev, data);
+}
+
+unsigned long ddcci_quirks(struct ddcci_device *dev);
+
+const char *ddcci_find_capstr_item(const char *capabilities, const char *tag,
+				   size_t *length);
+
+#endif
-- 
2.25.1