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From:   Ian Pilcher <arequipeno@gmail.com>
To:     pavel@ucw.cz
Cc:     linux-leds@vger.kernel.org, linux-kernel@vger.kernel.org,
        gregkh@linuxfoundation.org, kabel@kernel.org, hch@infradead.org
Subject: [PATCH v5 2/2] leds: trigger: Add block device LED trigger
Date:   Mon, 20 Sep 2021 15:56:26 -0500
Message-Id: <20210920205626.288366-3-arequipeno@gmail.com>
In-Reply-To: <20210920205626.288366-1-arequipeno@gmail.com>
References: <20210920205626.288366-1-arequipeno@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Add "blkdev" LED trigger to blink LEDs in response to block device
activity.

Add LEDS_TRIGGER_BLKDEV (tristate) config option to control building of
the trigger.

Signed-off-by: Ian Pilcher <arequipeno@gmail.com>
---
 drivers/leds/trigger/Kconfig          |   9 +
 drivers/leds/trigger/Makefile         |   1 +
 drivers/leds/trigger/ledtrig-blkdev.c | 965 ++++++++++++++++++++++++++
 3 files changed, 975 insertions(+)
 create mode 100644 drivers/leds/trigger/ledtrig-blkdev.c

diff --git a/drivers/leds/trigger/Kconfig b/drivers/leds/trigger/Kconfig
index 1f1d57288085..219d15c046d7 100644
--- a/drivers/leds/trigger/Kconfig
+++ b/drivers/leds/trigger/Kconfig
@@ -153,4 +153,13 @@ config LEDS_TRIGGER_TTY
 
 	  When build as a module this driver will be called ledtrig-tty.
 
+config LEDS_TRIGGER_BLKDEV
+	tristate "LED Trigger for block devices"
+	depends on BLOCK
+	help
+	  The blkdev LED trigger allows LEDs to be controlled by block device
+	  activity (reads and writes).
+
+	  See Documentation/leds/ledtrig-blkdev.rst.
+
 endif # LEDS_TRIGGERS
diff --git a/drivers/leds/trigger/Makefile b/drivers/leds/trigger/Makefile
index 25c4db97cdd4..d53bab5d93f1 100644
--- a/drivers/leds/trigger/Makefile
+++ b/drivers/leds/trigger/Makefile
@@ -16,3 +16,4 @@ obj-$(CONFIG_LEDS_TRIGGER_NETDEV)	+= ledtrig-netdev.o
 obj-$(CONFIG_LEDS_TRIGGER_PATTERN)	+= ledtrig-pattern.o
 obj-$(CONFIG_LEDS_TRIGGER_AUDIO)	+= ledtrig-audio.o
 obj-$(CONFIG_LEDS_TRIGGER_TTY)		+= ledtrig-tty.o
+obj-$(CONFIG_LEDS_TRIGGER_BLKDEV)	+= ledtrig-blkdev.o
diff --git a/drivers/leds/trigger/ledtrig-blkdev.c b/drivers/leds/trigger/ledtrig-blkdev.c
new file mode 100644
index 000000000000..feead76c2e9d
--- /dev/null
+++ b/drivers/leds/trigger/ledtrig-blkdev.c
@@ -0,0 +1,965 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+/*
+ *	Block device LED trigger
+ *
+ *	Copyright 2021 Ian Pilcher <arequipeno@gmail.com>
+ */
+
+#include <linux/blkdev.h>
+#include <linux/leds.h>
+#include <linux/module.h>
+#include <linux/part_stat.h>
+#include <linux/xarray.h>
+
+/* Default blink time & check interval (milliseconds) */
+#define LED_BDEV_BLINK_MSEC	75
+#define LED_BDEV_INTERVAL	100
+
+/* Minimum blink time & check interval (milliseconds) */
+#define LED_BDEV_MIN_BLINK	10
+#define LED_BDEV_MIN_INT	25
+
+/* Mode for blkdev_get_by_path() & blkdev_put() */
+#define LED_BDEV_FMODE		0
+
+/* Device activity type that will make LED blink */
+enum led_bdev_led_mode {
+	LED_BDEV_MODE_RO	= 0,
+	LED_BDEV_MODE_WO	= 1,
+	LED_BDEV_MODE_RW	= 2
+};
+
+/* When unlinking a block device from an LED, is the blkdev being released? */
+enum led_bdev_unlink_mode {
+	LED_BDEV_RELEASING,
+	LED_BDEV_NOT_RELEASING
+};
+
+/* An "LED block device" (LBD) - 1 per blkdev linked to at least 1 LED */
+struct led_bdev_lbd {
+	unsigned long		index;
+	struct block_device	*bdev;
+	struct xarray		linked_leds;
+	unsigned long		read_ios;
+	unsigned long		write_ios;
+	unsigned int		generation;
+	bool			read_act;
+	bool			write_act;
+};
+
+/* Every LED associated with the blkdev trigger gets one of these */
+struct led_bdev_led {
+	unsigned long		index;
+	struct led_classdev	*led_cdev;
+	unsigned int		blink_msec;
+	struct xarray		linked_lbds;
+	struct hlist_node	all_leds_node;
+	enum led_bdev_led_mode	mode;
+};
+
+/* Forward declarations to make this file compile in a more readable order */
+static void led_bdev_process(struct work_struct *work);
+static struct led_bdev_lbd *led_bdev_get_lbd(const char *buf, size_t size);
+static struct block_device *led_bdev_get_bdev(const char *buf, size_t size,
+					      fmode_t mode);
+static int led_bdev_link(struct led_bdev_led *led, struct led_bdev_lbd *lbd);
+static void led_bdev_put_lbd(struct led_bdev_lbd *lbd);
+static void led_bdev_lbd_release(struct device *dev, void *res);
+static void led_bdev_unlink(struct led_bdev_led *led,
+			    struct led_bdev_lbd *lbd,
+			    enum led_bdev_unlink_mode unlink_mode);
+static void led_bdev_update_lbd(struct led_bdev_lbd *lbd);
+static bool led_bdev_blink(const struct led_bdev_led *led,
+			   const struct led_bdev_lbd *lbd);
+
+/* Index for next LBD or LED */
+static unsigned long led_bdev_next_index;
+
+/* Protects everything except atomic sysfs attributes */
+static DEFINE_MUTEX(led_bdev_mutex);
+
+/* All LEDs associated with the trigger */
+static HLIST_HEAD(led_bdev_all_leds);
+
+/* sysfs class for "global" trigger attributes (interval) */
+static struct class *led_bdev_class;
+
+/* Delayed work to periodically check for activity & blink LEDs */
+static DECLARE_DELAYED_WORK(led_bdev_work, led_bdev_process);
+
+/* How often to run the delayed work - in jiffies */
+static unsigned int led_bdev_interval;
+
+/* Incremented every time the delayed work runs */
+static unsigned int led_bdev_generation;
+
+/* Total number of device-to-LED associations (links) */
+static unsigned int led_bdev_link_count;
+
+/* Empty attribute list for the linked_leds & linked_devices "groups" */
+static struct attribute *led_bdev_attrs_empty[] = { NULL };
+
+/* linked_leds sysfs directory for block devs linked to 1 or more LEDs */
+static const struct attribute_group led_bdev_linked_leds = {
+	.name	= "linked_leds",
+	.attrs	= led_bdev_attrs_empty,
+};
+
+/* linked_devices sysfs directory for each LED associated with the trigger */
+static const struct attribute_group led_bdev_linked_devs = {
+	.name	= "linked_devices",
+	.attrs	= led_bdev_attrs_empty,
+};
+
+/**
+ * led_bdev_activate() - Called when an LED is associated with the trigger.
+ * @led_cdev:	The LED
+ *
+ * Allocates & initializes the @led_bdev_led structure, adds it to the
+ * @led_bdev_all_leds list, and sets the LED's trigger data.
+ *
+ * Context:	Process context.  Takes and releases @led_bdev_mutex.
+ * Return:	``0`` on success, ``-errno`` on error.
+ */
+static int led_bdev_activate(struct led_classdev *const led_cdev)
+{
+	struct led_bdev_led *led;
+	int ret;
+
+	if (led_bdev_next_index == ULONG_MAX) {
+		ret = -EOVERFLOW;
+		goto exit_return;
+	}
+
+	led = kmalloc(sizeof(*led), GFP_KERNEL);
+	if (led == NULL) {
+		ret = -ENOMEM;
+		goto exit_return;
+	}
+
+	led->index = led_bdev_next_index++;
+	led->led_cdev = led_cdev;
+	led->blink_msec = LED_BDEV_BLINK_MSEC;
+	led->mode = LED_BDEV_MODE_RW;
+	xa_init(&led->linked_lbds);
+
+	ret = mutex_lock_interruptible(&led_bdev_mutex);
+	if (ret != 0)
+		goto exit_free;
+
+	hlist_add_head(&led->all_leds_node, &led_bdev_all_leds);
+	led_set_trigger_data(led_cdev, led);
+
+	mutex_unlock(&led_bdev_mutex);
+
+exit_free:
+	if (ret != 0)
+		kfree(led);
+exit_return:
+	return ret;
+}
+
+/**
+ * link_device_store() - ``link_device`` device attribute store function.
+ * @dev:	The LED device
+ * @attr:	The ``link_device`` attribute (@dev_attr_link_device)
+ * @buf:	The value written to the attribute, which should be the path to
+ *		the special file that represents the block device to be linked
+ *		to the LED (e.g. /dev/sda)
+ * @count:	The number of characters in @buf
+ *
+ * Calls led_bdev_get_lbd() to find or create the LBD for the block device,
+ * checks that the device isn't already linked to this LED, and calls
+ * led_bdev_link() to create the link.
+ *
+ * Context:	Process context.  Takes and releases @led_bdev_mutex.
+ * Return:	@count on success, ``-errno`` on error.
+ */
+static ssize_t link_device_store(struct device *const dev,
+				 struct device_attribute *const attr,
+				 const char *const buf, const size_t count)
+{
+	struct led_bdev_led *const led = led_trigger_get_drvdata(dev);
+	struct led_bdev_lbd *lbd;
+	int ret;
+
+	ret = mutex_lock_interruptible(&led_bdev_mutex);
+	if (ret != 0)
+		goto exit_return;
+
+	lbd = led_bdev_get_lbd(buf, count);
+	if (IS_ERR(lbd)) {
+		ret = PTR_ERR(lbd);
+		goto exit_unlock;
+	}
+
+	if (xa_load(&lbd->linked_leds, led->index) != NULL) {
+		ret = -EEXIST;
+		goto exit_put_lbd;
+	}
+
+	ret = led_bdev_link(led, lbd);
+
+exit_put_lbd:
+	if (ret != 0)
+		led_bdev_put_lbd(lbd);
+exit_unlock:
+	mutex_unlock(&led_bdev_mutex);
+exit_return:
+	return (ret == 0) ? count : ret;
+}
+
+/**
+ * led_bdev_get_lbd() - Find or create the LBD for a block device.
+ * @buf:	The value written to the ``link_device`` attribute, which should
+ *		be the path to a special file that represents a block device
+ * @count:	The number of characters in @buf
+ *
+ * Calls led_bdev_get() to get the block device represented by the path in @buf.
+ * If the device already has an LBD (because it is already linked to an LED),
+ * simply returns the existing LBD.
+ *
+ * Otherwise, allocates a new LBD (as a device resource), creates the block
+ * device's ``linked_leds`` directory (attribute group), calls
+ * led_bdev_update_lbd() to set the LBD's activity counters, and adds the LBD
+ * resource to the block device.
+ *
+ * Context:	Process context.  Caller must hold @led_bdev_mutex.
+ * Return:	Pointer to the LBD, error pointer on error.
+ */
+static struct led_bdev_lbd *led_bdev_get_lbd(const char *const buf,
+					     const size_t count)
+{
+	struct block_device *bdev;
+	struct led_bdev_lbd *lbd;
+	int ret;
+
+	bdev = led_bdev_get_bdev(buf, count, LED_BDEV_FMODE);
+	if (IS_ERR(bdev)) {
+		ret = PTR_ERR(bdev);
+		goto exit_return;
+	}
+
+	lbd = devres_find(&bdev->bd_device, led_bdev_lbd_release, NULL, NULL);
+	if (lbd != NULL) {
+		ret = 0;
+		goto exit_put_bdev;
+	}
+
+	if (led_bdev_next_index == ULONG_MAX) {
+		ret = -EOVERFLOW;
+		goto exit_put_bdev;
+	}
+
+	lbd = devres_alloc(led_bdev_lbd_release, sizeof(*lbd), GFP_KERNEL);
+	if (lbd == NULL) {
+		ret = -ENOMEM;
+		goto exit_put_bdev;
+	}
+
+	ret = sysfs_create_group(bdev_kobj(bdev), &led_bdev_linked_leds);
+	if (ret != 0)
+		goto exit_free_lbd;
+
+	lbd->index = led_bdev_next_index++;
+	lbd->bdev = bdev;
+	xa_init(&lbd->linked_leds);
+
+	/* Ensure that led_bdev_update_lbd() updates the LBD */
+	lbd->generation = led_bdev_generation - 1;
+	led_bdev_update_lbd(lbd);
+	/* Don't blink for activity that may have happened long ago */
+	lbd->read_act = false;
+	lbd->write_act = false;
+
+	devres_add(&bdev->bd_device, lbd);
+
+exit_free_lbd:
+	if (ret != 0)
+		devres_free(lbd);
+exit_put_bdev:
+	blkdev_put(bdev, LED_BDEV_FMODE);  /* Allow the device to be released */
+exit_return:
+	return (ret == 0) ? lbd : ERR_PTR(ret);
+}
+
+/**
+ * led_bdev_get_bdev() - Get a block device by path.
+ * @buf:	The value written to the ``link_device`` or ``unlink_device``
+ *		attribute, which should be the path to a special file that
+ *		represents a block device
+ * @count:	The number of characters in @buf (not including its terminating
+ *		null)
+ *
+ * Copies @buf to a writable buffer, trims the trailing newline (if any), and
+ * calls blkdev_get_by_path() to resolve the block device.
+ *
+ * The caller must call blkdev_put() when finished with the device.
+ *
+ * Context:	Process context.
+ * Return:	The block device, or an error pointer.
+ */
+static struct block_device *led_bdev_get_bdev(const char *const buf,
+					      const size_t count,
+					      const fmode_t mode)
+{
+	struct block_device *bdev;
+	char *path;
+
+	path = kmemdup(buf, count + 1, GFP_KERNEL);  /* include null */
+	if (path == NULL)
+		return ERR_PTR(-ENOMEM);
+
+	if (path[count - 1] == '\n')
+		path[count - 1] = 0;
+
+	bdev = blkdev_get_by_path(path, mode, THIS_MODULE);
+	kfree(path);
+	return bdev;
+}
+
+/**
+ * led_bdev_update_lbd() - Update an LBD's activity counters for the current
+ *			   generation
+ * @lbd:	The LBD
+ *
+ * Does nothing if the LBD has already been updated during this generation.
+ *
+ * Otherwise, updates the LBD's activity counters (@read_ios & @write_ios) and
+ * @generation, and sets @read_act and/or @write_act if the corresponding
+ * counters have changed.
+ *
+ * Context:	Process context.  Caller must hold @led_bdev_mutex.
+ */
+static void led_bdev_update_lbd(struct led_bdev_lbd *const lbd)
+{
+	unsigned long read_ios, write_ios;
+
+	if (lbd->generation != led_bdev_generation) {
+
+		read_ios = part_stat_read(lbd->bdev, ios[STAT_READ]);
+
+		write_ios = part_stat_read(lbd->bdev, ios[STAT_WRITE])
+				+ part_stat_read(lbd->bdev, ios[STAT_DISCARD])
+				+ part_stat_read(lbd->bdev, ios[STAT_FLUSH]);
+
+		if (lbd->read_ios != read_ios) {
+			lbd->read_act = true;
+			lbd->read_ios = read_ios;
+		} else {
+			lbd->read_act = false;
+		}
+
+		if (lbd->write_ios != write_ios) {
+			lbd->write_act = true;
+			lbd->write_ios = write_ios;
+		} else {
+			lbd->write_act = false;
+		}
+
+		lbd->generation = led_bdev_generation;
+	}
+}
+
+/**
+ * led_bdev_link() - "Link" a block device to an LED.
+ * @led:	The LED
+ * @lbd:	The block device
+ *
+ * Called from link_device_store() to create the link between an LED and a
+ * block device.
+ *
+ *   * Adds block device symlink to LED's ``linked_devices`` directory.
+ *   * Adds LED symlink to block devices's ``linked_leds`` directory.
+ *   * Adds the LBD to the LED's @linked_lbds and adds the LED to the LBD's
+ *     @linked_leds.
+ *
+ * If the new link is the only one (i.e. no other block device/LED links
+ * already exist), schedule delayed work to periodically check for block
+ * device activity and blink LEDs.
+ *
+ * Context:	Process context.  Caller must hold @led_bdev_mutex.
+ * Return:	0 on success, ``-errno`` on error.
+ */
+static int led_bdev_link(struct led_bdev_led *const led,
+			 struct led_bdev_lbd *const lbd)
+{
+	unsigned long delay;
+	int ret;
+
+	ret = xa_insert(&lbd->linked_leds, led->index, led, GFP_KERNEL);
+	if (ret != 0)
+		goto error_return;
+
+	ret = xa_insert(&led->linked_lbds, lbd->index, lbd, GFP_KERNEL);
+	if (ret != 0)
+		goto error_erase_led;
+
+	/* /sys/class/block/<bdev>/linked_leds/<led> */
+	ret = sysfs_add_link_to_group(bdev_kobj(lbd->bdev),
+				      led_bdev_linked_leds.name,
+				      &led->led_cdev->dev->kobj,
+				      led->led_cdev->name);
+	if (ret != 0)
+		goto error_erase_lbd;
+
+	/* /sys/class/leds/<led>/linked_devices/<bdev> */
+	ret = sysfs_add_link_to_group(&led->led_cdev->dev->kobj,
+				      led_bdev_linked_devs.name,
+				      bdev_kobj(lbd->bdev),
+				      dev_name(&lbd->bdev->bd_device));
+	if (ret != 0)
+		goto error_remove_symlink;
+
+	if (led_bdev_link_count == 0) {
+		delay = READ_ONCE(led_bdev_interval);
+		WARN_ON(!schedule_delayed_work(&led_bdev_work, delay));
+	}
+
+	++led_bdev_link_count;
+
+	return 0;
+
+error_remove_symlink:
+	sysfs_remove_link_from_group(bdev_kobj(lbd->bdev),
+				     led_bdev_linked_leds.name,
+				     led->led_cdev->name);
+error_erase_lbd:
+	xa_erase(&led->linked_lbds, lbd->index);
+error_erase_led:
+	xa_erase(&lbd->linked_leds, led->index);
+error_return:
+	return ret;
+}
+
+/**
+ * unlink_device_store() - ``unlink_device`` device attribute store function.
+ * @dev:	The LED device
+ * @attr:	The ``unlink_device`` attribute (@dev_attr_unlink_device)
+ * @buf:	The value written to the attribute, which should be the path to
+ *		the special file that represents the block device to be unlinked
+ *		from the LED (e.g. /dev/sda)
+ * @count:	The number of characters in @buf
+ *
+ * Block device name is written to the attribute to "unlink" the block device
+ * from the LED.  I.e. the LED will no longer blink to show activity on that
+ * block device.
+ *
+ * Calls led_bdev_get() to get the block device represented by the path in @buf.
+ * If the device has an LBD, searches the LBD's list of LEDs for a link to this
+ * LED and (if found) calls led_bdev_unlink() to destroy the link.
+ *
+ * Context:	Process context.  Takes and releases @led_bdev_mutex.
+ * Return:	@count on success, ``-errno`` on error.
+ */
+static ssize_t unlink_device_store(struct device *const dev,
+				   struct device_attribute *const attr,
+				   const char *const buf, const size_t count)
+{
+	struct led_bdev_led *const led = led_trigger_get_drvdata(dev);
+	struct block_device *bdev;
+	struct led_bdev_lbd *lbd;
+	int ret;
+
+	bdev = led_bdev_get_bdev(buf, count, LED_BDEV_FMODE);
+	if (IS_ERR(bdev)) {
+		ret = PTR_ERR(bdev);
+		goto exit_return;
+	}
+
+	ret = mutex_lock_interruptible(&led_bdev_mutex);
+	if (ret != 0)
+		goto exit_put_bdev;
+
+	lbd = devres_find(&bdev->bd_device, led_bdev_lbd_release, NULL, NULL);
+	if (lbd == NULL) {
+		ret = -EUNATCH;  /* bdev isn't linked to any LED */
+		goto exit_unlock;
+	}
+
+	if (xa_load(&lbd->linked_leds, led->index) == NULL) {
+		ret = -EUNATCH;  /* bdev isn't linked to this LED */
+		goto exit_unlock;
+	}
+
+	led_bdev_unlink(led, lbd, LED_BDEV_NOT_RELEASING);
+
+exit_unlock:
+	mutex_unlock(&led_bdev_mutex);
+exit_put_bdev:
+	blkdev_put(bdev, LED_BDEV_FMODE);
+exit_return:
+	return (ret == 0) ? count : ret;
+}
+
+/**
+ * led_bdev_unlink() - "Unlink" a block device from an LED.
+ * @led:		The LED
+ * @lbd:		The block device
+ * @unlink_mode:	Indicates whether the LBD is being released (because the
+ *			block device has been removed)
+ *
+ * Removes the link between an LED and a block device.
+ *
+ *   * Removes the LBD from the LED's @linked_lbds and removes the LED from the
+ *     LBD's @linked_leds.
+ *   * Removes the block device symlink from the LED's ``linked_devices``
+ *     directory.
+ *
+ * If the block device is **not** being released:
+ *
+ *   * Removes the LED symlink from the block device's ``linked_leds``
+ *     directory.
+ *   * Calls led_bdev_put_lbd() to clean up the LBD, if required.
+ *
+ * If the removed link was the only one (i.e. there are no existing block
+ * device/LED links after its removal), cancels the periodic delayed work
+ * which checks for device activity.
+ *
+ * This function is called from multiple locations.
+ *
+ *   * unlink_device_store() calls this function when a block device is unlinked
+ *     from an LED via the ``unlink_device`` sysfs attribute.   (@unlink_mode ==
+ *     ``LED_BDEV_NOT_RELEASING``)
+ *   * led_bdev_deactivate() calls this function for each block device linked to
+ *     an LED that is being deactivated (disassociated from the trigger).
+ *     (@unlink_mode == ``LED_BDEV_NOT_RELEASING``).
+ *   * led_bdev_lbd_release() calls this function for each LED linked to a block
+ *     device that has been removed from the system.  (@unlink_mode ==
+ *     ``LED_BDEV_RELEASING).
+ *
+ * Context:	Process context.  Caller must hold @led_bdev_mutex.
+ */
+static void led_bdev_unlink(struct led_bdev_led *const led,
+			    struct led_bdev_lbd *const lbd,
+			    const enum led_bdev_unlink_mode unlink_mode)
+{
+	--led_bdev_link_count;
+
+	if (led_bdev_link_count == 0)
+		WARN_ON(!cancel_delayed_work_sync(&led_bdev_work));
+
+	xa_erase(&lbd->linked_leds, led->index);
+	xa_erase(&led->linked_lbds, lbd->index);
+
+	/* /sys/class/leds/<led>/linked_devices/<bdev> */
+	sysfs_remove_link_from_group(&led->led_cdev->dev->kobj,
+				     led_bdev_linked_devs.name,
+				     dev_name(&lbd->bdev->bd_device));
+
+	/*
+	 * If the LBD is being released, the device's attribute groups have
+	 * already been removed, and the LBD will be freed automatically.
+	 */
+	if (unlink_mode == LED_BDEV_NOT_RELEASING) {
+
+		/* /sys/class/block/<bdev>/linked_leds/<led> */
+		sysfs_remove_link_from_group(bdev_kobj(lbd->bdev),
+					     led_bdev_linked_leds.name,
+					     led->led_cdev->name);
+		led_bdev_put_lbd(lbd);
+	}
+}
+
+/**
+ * led_bdev_put_lbd() - Remove and free an LBD, if it is no longer needed.
+ * @lbd:	The LBD
+ *
+ * Does nothing if the LBD (block device) is still linked to at least one LED.
+ *
+ * If the LBD is no longer linked to any LEDs, removes the block device's
+ * ``linked_leds`` directory (attribute group), removes the LBD from the
+ * block device's resource list, and frees the LBD.
+ *
+ * Called from led_bdev_unlink() (and in the link_device_store() error path).
+ *
+ * Context:	Process context.  Caller must hold @led_bdev_mutex.
+ */
+static void led_bdev_put_lbd(struct led_bdev_lbd *const lbd)
+{
+	struct block_device *const bdev = lbd->bdev;
+	int ret;
+
+	if (xa_empty(&lbd->linked_leds)) {
+
+		sysfs_remove_group(bdev_kobj(bdev), &led_bdev_linked_leds);
+		ret = devres_destroy(&bdev->bd_device, led_bdev_lbd_release,
+				     NULL, NULL);
+		WARN_ON(ret != 0);
+	}
+}
+
+/**
+ * led_bdev_deactivate() - Called when an LED is disassociated from the
+ *			   trigger.
+ * @led_cdev:	The LED
+ *
+ * Calls led_bdev_unlink() for each block device linked to the LED, removes
+ * the LED from the @led_bdev_all_leds list, and frees the @led_bdev_led.
+ *
+ * Context:	Process context.  Takes and releases @led_bdev_mutex.
+ */
+static void led_bdev_deactivate(struct led_classdev *const led_cdev)
+{
+	struct led_bdev_led *const led = led_get_trigger_data(led_cdev);
+	struct led_bdev_lbd *lbd;
+	unsigned long index;
+
+	mutex_lock(&led_bdev_mutex);
+
+	xa_for_each (&led->linked_lbds, index, lbd)
+		led_bdev_unlink(led, lbd, LED_BDEV_NOT_RELEASING);
+
+	hlist_del(&led->all_leds_node);
+	kfree(led);
+
+	mutex_unlock(&led_bdev_mutex);
+}
+
+/**
+ * led_bdev_lbd_release() - LBD device resource release function.
+ * @dev:	The block device
+ * @res:	The LBD
+ *
+ * Called by the driver core when a block device with an LBD is removed from the
+ * system.  Calls led_bdev_unlink() for each LED linked to the block device.
+ *
+ * Context:	Process context.  Takes and releases @led_bdev_mutex.
+ */
+static void led_bdev_lbd_release(struct device *const dev, void *const res)
+{
+	struct led_bdev_lbd *const lbd = res;
+	struct led_bdev_led *led;
+	unsigned long index;
+
+	mutex_lock(&led_bdev_mutex);
+
+	xa_for_each (&lbd->linked_leds, index, led)
+		led_bdev_unlink(led, lbd, LED_BDEV_RELEASING);
+
+	mutex_unlock(&led_bdev_mutex);
+}
+
+/**
+ * led_bdev_process() - Check linked devices for activity and blink LEDs.
+ * @work:	Delayed work (@led_bdev_work)
+ *
+ * Iterates through block devices linked to LEDs and calls led_bdev_update_lbd()
+ * to update the LBD's activity counters.  If a particular LED hasn't already
+ * been blinked, calls led_bdev_blink() to blink the LED if appropriate for that
+ * device's activity.
+ *
+ * When finished, schedules itself to run again after @led_bdev_interval
+ * jiffies.
+ *
+ * Context:	Process context.  Takes and releases @led_bdev_mutex.
+ */
+static void led_bdev_process(struct work_struct *const work)
+{
+	struct led_bdev_led *led;
+	unsigned long delay;
+
+	if (!mutex_trylock(&led_bdev_mutex))
+		goto exit_reschedule;
+
+	hlist_for_each_entry (led, &led_bdev_all_leds, all_leds_node) {
+
+		struct led_bdev_lbd *lbd;
+		unsigned long index;
+		bool blinked = false;
+
+		xa_for_each (&led->linked_lbds, index, lbd) {
+
+			led_bdev_update_lbd(lbd);
+			if (!blinked)
+				blinked = led_bdev_blink(led, lbd);
+		}
+	}
+
+	++led_bdev_generation;
+
+	mutex_unlock(&led_bdev_mutex);
+
+exit_reschedule:
+	delay = READ_ONCE(led_bdev_interval);
+	WARN_ON_ONCE(!schedule_delayed_work(&led_bdev_work, delay));
+}
+
+/**
+ * led_bdev_blink() - Blink an LED, if the correct type of activity has occurred
+ *		      on the block device.
+ * @led:	The LED
+ * @lbd:	The block device
+ *
+ * Context:	Process context.  Caller must hold @led_bdev_mutex.
+ * Return:	``true`` if the LED is blinked, ``false`` if not.
+ */
+static bool led_bdev_blink(const struct led_bdev_led *const led,
+			   const struct led_bdev_lbd *const lbd)
+{
+	unsigned long delay_on, delay_off;
+
+	if ((lbd->read_act && led->mode != LED_BDEV_MODE_WO)
+			|| (lbd->write_act && led->mode != LED_BDEV_MODE_RO)) {
+
+		delay_on = READ_ONCE(led->blink_msec);
+		delay_off = 1;	/* 0 leaves LED turned on */
+
+		led_blink_set_oneshot(led->led_cdev, &delay_on, &delay_off, 0);
+		return true;
+	}
+
+	return false;
+}
+
+/**
+ * blink_time_show() - ``blink_time`` device attribute show function.
+ * @dev:	The LED device
+ * @attr:	The ``blink_time`` attribute (@dev_attr_blink_time)
+ * @buf:	Output buffer
+ *
+ * Writes the current value of the LED's @blink_msec to @buf.
+ *
+ * Context:	Process context.
+ * Return:	The number of characters written to @buf.
+ */
+static ssize_t blink_time_show(struct device *const dev,
+			       struct device_attribute *const attr,
+			       char *const buf)
+{
+	const struct led_bdev_led *const led = led_trigger_get_drvdata(dev);
+
+	return sprintf(buf, "%u\n", READ_ONCE(led->blink_msec));
+}
+
+/**
+ * blink_time_store() - ``blink_time`` device attribute store function.
+ * @dev:	The LED device
+ * @attr:	The ``blink_time`` attribute (@dev_attr_blink_time)
+ * @buf:	The new value (as written to the sysfs attribute)
+ * @count:	The number of characters in @buf
+ *
+ * Sets the LED's @blink_msec (the duration in milliseconds of one blink).
+ *
+ * Context:	Process context.
+ * Return:	@count on success, ``-errno`` on error.
+ */
+static ssize_t blink_time_store(struct device *const dev,
+				struct device_attribute *const attr,
+				const char *const buf, const size_t count)
+{
+	struct led_bdev_led *const led = led_trigger_get_drvdata(dev);
+	unsigned int value;
+	int ret;
+
+	ret = kstrtouint(buf, 0, &value);
+	if (ret != 0)
+		return ret;
+
+	if (value < LED_BDEV_MIN_BLINK)
+		return -ERANGE;
+
+	WRITE_ONCE(led->blink_msec, value);
+	return count;
+}
+
+/**
+ * mode_show() - ``mode`` device attribute show function.
+ * @dev:	The LED device
+ * @attr:	The ``mode`` attribute (@dev_attr_mode)
+ * @buf:	Output buffer
+ *
+ * Writes the current value of the LED's @mode to @buf.
+ *
+ * Context:	Process context.
+ * Return:	The number of characters written to @buf.
+ */
+static ssize_t mode_show(struct device *const dev,
+			 struct device_attribute *const attr, char *const buf)
+{
+	static const char modes[][sizeof("[read] write rw\n")] = {
+		[LED_BDEV_MODE_RO] = "[read] write rw\n",
+		[LED_BDEV_MODE_WO] = "read [write] rw\n",
+		[LED_BDEV_MODE_RW] = "read write [rw]\n"
+	};
+
+	const struct led_bdev_led *const led = led_trigger_get_drvdata(dev);
+
+	return sprintf(buf, modes[READ_ONCE(led->mode)]);
+}
+
+/**
+ * mode_store() - ``mode`` device attribute store function.
+ * @dev:	The LED device
+ * @attr:	The ``mode`` attribute (@dev_attr_mode)
+ * @buf:	The new value (as written to the sysfs attribute)
+ * @count:	The number of characters in @buf
+ *
+ * Sets the LED's @mode (``read``, ``write``, or ``rw``).
+ *
+ * Context:	Process context.
+ * Return:	@count on success, ``-errno`` on error.
+ */
+static ssize_t mode_store(struct device *const dev,
+			  struct device_attribute *const attr,
+			  const char *const buf, const size_t count)
+{
+	static const char modes[][sizeof("write")] = {
+		[LED_BDEV_MODE_RO] = "read",
+		[LED_BDEV_MODE_WO] = "write",  /* longest */
+		[LED_BDEV_MODE_RW] = "rw"
+	};
+
+	struct led_bdev_led *const led = led_trigger_get_drvdata(dev);
+	enum led_bdev_led_mode mode;
+
+	for (mode = LED_BDEV_MODE_RO; mode <= LED_BDEV_MODE_RW; ++mode) {
+
+		if (sysfs_streq(modes[mode], buf)) {
+			WRITE_ONCE(led->mode, mode);
+			return count;
+		}
+	}
+
+	return -EINVAL;
+}
+
+/**
+ * interval_show() - ``interval`` class attribute show function.
+ * @class:	The ``ledtrig_blkdev`` class (@led_bdev_class)
+ * @attr:	The ``interval`` attribute (@class_attr_interval)
+ * @buf:	Output buffer
+ *
+ * Writes the current value of @led_bdev_interval to @buf.
+ *
+ * Context:	Process context.
+ * Return:	The number of characters written to @buf.
+ */
+static ssize_t interval_show(struct class *const class,
+			     struct class_attribute *const attr,
+			     char *const buf)
+{
+	return sprintf(buf, "%u\n",
+		       jiffies_to_msecs(READ_ONCE(led_bdev_interval)));
+}
+
+/**
+ * interval_store() - ``interval`` class attribute store function
+ * @class:	The ``ledtrig_blkdev`` class (@led_bdev_class)
+ * @attr:	The ``interval`` attribute (@class_attr_interval)
+ * @buf:	The new value (as written to the sysfs attribute)
+ * @count:	The number of characters in @buf
+ *
+ * Sets @led_bdev_interval (after converting to jiffies).
+ *
+ * Context:	Process context.
+ * Return:	@count on success, ``-errno`` on error.
+ */
+static ssize_t interval_store(struct class *const class,
+			      struct class_attribute *const attr,
+			      const char *const buf, const size_t count)
+{
+	unsigned int value;
+	int ret;
+
+	ret = kstrtouint(buf, 0, &value);
+	if (ret != 0)
+		return ret;
+
+	if (value < LED_BDEV_MIN_INT)
+		return -ERANGE;
+
+	WRITE_ONCE(led_bdev_interval, msecs_to_jiffies(value));
+
+	return count;
+}
+
+/* Device and class attributes */
+static DEVICE_ATTR_WO(link_device);
+static DEVICE_ATTR_WO(unlink_device);
+static DEVICE_ATTR_RW(blink_time);
+static DEVICE_ATTR_RW(mode);
+static CLASS_ATTR_RW(interval);
+
+/* Device attributes in LED directory (/sys/class/leds/<led>/...) */
+static struct attribute *led_bdev_attrs[] = {
+	&dev_attr_link_device.attr,
+	&dev_attr_unlink_device.attr,
+	&dev_attr_blink_time.attr,
+	&dev_attr_mode.attr,
+	NULL
+};
+
+/* Unnamed attribute group == no subdirectory */
+static const struct attribute_group led_bdev_attr_group = {
+	.attrs	= led_bdev_attrs,
+};
+
+/* Attribute groups for the trigger */
+static const struct attribute_group *led_bdev_attr_groups[] = {
+	&led_bdev_attr_group,   /* /sys/class/leds/<led>/... */
+	&led_bdev_linked_devs,  /* /sys/class/leds/<led>/linked_devices/ */
+	NULL
+};
+
+/* Trigger registration data */
+static struct led_trigger led_bdev_trigger = {
+	.name		= "blkdev",
+	.activate	= led_bdev_activate,
+	.deactivate	= led_bdev_deactivate,
+	.groups		= led_bdev_attr_groups,
+};
+
+/**
+ * led_bdev_init() - Block device LED trigger initialization.
+ *
+ * Converts default @led_bdev_interval from milliseconds to jiffies, creates
+ * the ``/sys/class/ledtrig_blkdev/interval`` attribute, and registers the LED
+ * trigger.
+ *
+ * Return:	0 on success, ``-errno`` on failure.
+ */
+static int __init led_bdev_init(void)
+{
+	int ret;
+
+	WRITE_ONCE(led_bdev_interval,
+		   msecs_to_jiffies(LED_BDEV_INTERVAL));
+
+	led_bdev_class = class_create(THIS_MODULE, "ledtrig_blkdev");
+	if (IS_ERR(led_bdev_class)) {
+		ret = PTR_ERR(led_bdev_class);
+		goto exit_return;
+	}
+
+	ret = class_create_file(led_bdev_class, &class_attr_interval);
+	if (ret != 0)
+		goto exit_destroy_class;
+
+	ret = led_trigger_register(&led_bdev_trigger);
+
+exit_destroy_class:
+	if (ret != 0)
+		class_destroy(led_bdev_class);  /* removes the attribute file */
+exit_return:
+	return ret;
+}
+module_init(led_bdev_init);
+
+/**
+ * led_bdev_exit() - Block device LED trigger module exit.
+ *
+ * Unregisters the LED trigger and removes the
+ * ``/sys/class/ledtrig_blkdev/interval`` attribute.
+ */
+static void __exit led_bdev_exit(void)
+{
+	led_trigger_unregister(&led_bdev_trigger);
+	class_destroy(led_bdev_class);
+}
+module_exit(led_bdev_exit);
+
+MODULE_DESCRIPTION("Block device LED trigger");
+MODULE_AUTHOR("Ian Pilcher <arequipeno@gmail.com>");
+MODULE_LICENSE("GPL v2");
-- 
2.31.1