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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
Subject: [RESEND PATCH v9 2/2] leds: trigger: Add block device LED trigger
Date:   Thu,  2 Jun 2022 11:12:24 -0500
Message-Id: <20220602161224.495943-3-arequipeno@gmail.com>
In-Reply-To: <20220602161224.495943-1-arequipeno@gmail.com>
References: <20220602161224.495943-1-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 | 1139 +++++++++++++++++++++++++
 3 files changed, 1149 insertions(+)
 create mode 100644 drivers/leds/trigger/ledtrig-blkdev.c

diff --git a/drivers/leds/trigger/Kconfig b/drivers/leds/trigger/Kconfig
index dc6816d36d06..bda249068182 100644
--- a/drivers/leds/trigger/Kconfig
+++ b/drivers/leds/trigger/Kconfig
@@ -154,4 +154,13 @@ 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..999c48a7a45a
--- /dev/null
+++ b/drivers/leds/trigger/ledtrig-blkdev.c
@@ -0,0 +1,1139 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+/*
+ *	Block device LED trigger
+ *
+ *	Copyright 2021-2022 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, minimum & maximum blink duration (milliseconds) */
+#define BLKDEV_TRIG_BLINK_DEF	75
+#define BLKDEV_TRIG_BLINK_MIN	10
+#define BLKDEV_TRIG_BLINK_MAX	86400000  /* 24 hours */
+
+/* Default, minimum & maximum activity check interval (milliseconds) */
+#define BLKDEV_TRIG_CHECK_DEF	100
+#define BLKDEV_TRIG_CHECK_MIN	25
+#define BLKDEV_TRIG_CHECK_MAX	86400000
+
+/*
+ * If blkdev_trig_check() can't lock the mutex, how long to wait before trying
+ * again (milliseconds)
+ */
+#define BLKDEV_TRIG_CHECK_RETRY	5
+
+/* Mode for blkdev_get_by_path() & blkdev_put() */
+#define BLKDEV_TRIG_FMODE	0
+
+/* When unlinking a block device from an LED, is the blkdev being released? */
+enum blkdev_trig_unlink_mode {
+	BLKDEV_TRIG_RELEASING,
+	BLKDEV_TRIG_NOT_RELEASING
+};
+
+/* Every block device linked to at least one LED gets a "BTB" */
+struct blkdev_trig_bdev {
+	unsigned long		last_checked;
+	unsigned long		last_activity[NR_STAT_GROUPS];
+	unsigned long		ios[NR_STAT_GROUPS];
+	unsigned long		index;
+	struct block_device	*bdev;
+	struct xarray		linked_leds;
+};
+
+/* Every LED associated with the blkdev trigger gets one of these */
+struct blkdev_trig_led {
+	unsigned long		last_checked;
+	unsigned long		index;
+	unsigned long		mode;  /* must be ulong for atomic bit ops */
+	struct led_classdev	*led_cdev;
+	unsigned int		blink_msec;
+	unsigned int		check_jiffies;
+	struct xarray		linked_btbs;
+	struct hlist_node	all_leds_node;
+};
+
+/* Forward declarations to make this file compile in a more readable order */
+static void blkdev_trig_check(struct work_struct *work);
+static struct blkdev_trig_bdev *blkdev_trig_get_btb(const char *buf,
+						    size_t size);
+static struct block_device *blkdev_trig_get_bdev(const char *buf, size_t size,
+						 fmode_t mode);
+static int blkdev_trig_link(struct blkdev_trig_led *led,
+			    struct blkdev_trig_bdev *btb);
+static void blkdev_trig_put_btb(struct blkdev_trig_bdev *btb);
+static void blkdev_trig_btb_release(struct device *dev, void *res);
+static void blkdev_trig_unlink(struct blkdev_trig_led *led,
+			       struct blkdev_trig_bdev *btb,
+			       enum blkdev_trig_unlink_mode unlink_mode);
+static void blkdev_trig_update_btb(struct blkdev_trig_bdev *btb,
+				   unsigned long now);
+static bool blkdev_trig_blink(const struct blkdev_trig_led *led,
+			      const struct blkdev_trig_bdev *btb);
+static void blkdev_trig_sched_led(const struct blkdev_trig_led *led);
+
+/* Index for next BTB or LED */
+static unsigned long blkdev_trig_next_index;
+
+/* Protects everything except sysfs attributes */
+static DEFINE_MUTEX(blkdev_trig_mutex);
+
+/* All LEDs associated with the trigger */
+static HLIST_HEAD(blkdev_trig_all_leds);
+
+/* Delayed work to periodically check for activity & blink LEDs */
+static DECLARE_DELAYED_WORK(blkdev_trig_work, blkdev_trig_check);
+
+/* When is the delayed work scheduled to run next (jiffies) */
+static unsigned long blkdev_trig_next_check;
+
+/* Total number of device-to-LED associations (links) */
+static unsigned int blkdev_trig_link_count;
+
+/* Empty attribute list, for next 2 declarations */
+static struct attribute *blkdev_trig_attrs_empty[] = { NULL };
+
+/* linked_leds sysfs directory for block devs linked to 1 or more LEDs */
+static const struct attribute_group blkdev_trig_linked_leds = {
+	.name	= "linked_leds",
+	.attrs	= blkdev_trig_attrs_empty,
+};
+
+/* linked_devices sysfs directory for each LED associated with the trigger */
+static const struct attribute_group blkdev_trig_linked_devs = {
+	.name	= "linked_devices",
+	.attrs	= blkdev_trig_attrs_empty,
+};
+
+/**
+ * blkdev_trig_activate() - Called when an LED is associated with the trigger.
+ * @led_cdev:	The LED
+ *
+ * Allocates & initializes the @blkdev_trig_led structure, adds it to the
+ * @blkdev_trig_all_leds list, and sets the LED's trigger data.
+ *
+ * Context:	Process context.  Takes and releases @blkdev_trig_mutex.
+ * Return:	``0`` on success, ``-errno`` on error.
+ */
+static int blkdev_trig_activate(struct led_classdev *led_cdev)
+{
+	struct blkdev_trig_led *led;
+	int err;
+
+	led = kzalloc(sizeof(*led), GFP_KERNEL);
+	if (led == NULL)
+		return -ENOMEM;
+
+	err = mutex_lock_interruptible(&blkdev_trig_mutex);
+	if (err)
+		goto exit_free;
+
+	if (blkdev_trig_next_index == ULONG_MAX) {
+		err = -EOVERFLOW;
+		goto exit_unlock;
+	}
+
+	led->index = blkdev_trig_next_index++;
+	led->last_checked = jiffies;
+	led->mode = -1;  /* set all bits */
+	led->led_cdev = led_cdev;
+	led->blink_msec = BLKDEV_TRIG_BLINK_DEF;
+	led->check_jiffies = msecs_to_jiffies(BLKDEV_TRIG_CHECK_DEF);
+	xa_init(&led->linked_btbs);
+
+	hlist_add_head(&led->all_leds_node, &blkdev_trig_all_leds);
+	led_set_trigger_data(led_cdev, led);
+
+exit_unlock:
+	mutex_unlock(&blkdev_trig_mutex);
+exit_free:
+	if (err)
+		kfree(led);
+	return err;
+}
+
+/**
+ * link_dev_by_path_store() - ``link_dev_by_path`` device attribute store
+ *			      function.
+ * @dev:	The LED device
+ * @attr:	The ``link_dev_by_path`` attribute (@dev_attr_link_dev_by_path)
+ * @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 blkdev_trig_get_btb() to find or create the BTB for the block device,
+ * checks that the device isn't already linked to this LED, and calls
+ * blkdev_trig_link() to create the link.
+ *
+ * Context:	Process context.  Takes and releases @blkdev_trig_mutex.
+ * Return:	@count on success, ``-errno`` on error.
+ */
+static ssize_t link_dev_by_path_store(struct device *dev,
+				      struct device_attribute *attr,
+				      const char *buf, size_t count)
+{
+	struct blkdev_trig_led *led = led_trigger_get_drvdata(dev);
+	struct blkdev_trig_bdev *btb;
+	int err;
+
+	err = mutex_lock_interruptible(&blkdev_trig_mutex);
+	if (err)
+		return err;
+
+	btb = blkdev_trig_get_btb(buf, count);
+	if (IS_ERR(btb)) {
+		err = PTR_ERR(btb);
+		goto exit_unlock;
+	}
+
+	if (xa_load(&btb->linked_leds, led->index) != NULL) {
+		err = -EEXIST;
+		goto exit_put_btb;
+	}
+
+	err = blkdev_trig_link(led, btb);
+
+exit_put_btb:
+	if (err)
+		blkdev_trig_put_btb(btb);
+exit_unlock:
+	mutex_unlock(&blkdev_trig_mutex);
+	return err ? : count;
+}
+
+/**
+ * blkdev_trig_get_btb() - Find or create the BTB for a block device.
+ * @buf:	The value written to the ``link_dev_by_path`` attribute, which
+ *		should be the path to a special file that represents a block
+ *		device
+ * @count:	The number of characters in @buf
+ *
+ * Calls blkdev_trig_get_bdev() to get the block device represented by the path
+ * in @buf.  If the device already has a BTB (because it is already linked to
+ * an LED), simply returns the existing BTB.
+ *
+ * Otherwise, allocates a new BTB (as a device resource), creates the block
+ * device's ``linked_leds`` directory (attribute group), calls
+ * blkdev_trig_update_btb() to set the BTB's activity counters, and adds the
+ * BTB resource to the block device.
+ *
+ * Context:	Process context.  Caller must hold @blkdev_trig_mutex.
+ * Return:	Pointer to the BTB, error pointer on error.
+ */
+static struct blkdev_trig_bdev *blkdev_trig_get_btb(const char *buf,
+						    size_t count)
+{
+	struct block_device *bdev;
+	struct blkdev_trig_bdev *btb;
+	int err;
+
+	bdev = blkdev_trig_get_bdev(buf, count, BLKDEV_TRIG_FMODE);
+	if (IS_ERR(bdev))
+		return ERR_CAST(bdev);
+
+	btb = devres_find(&bdev->bd_device, blkdev_trig_btb_release,
+			  NULL, NULL);
+	if (btb != NULL) {
+		err = 0;
+		goto exit_put_bdev;
+	}
+
+	if (blkdev_trig_next_index == ULONG_MAX) {
+		err = -EOVERFLOW;
+		goto exit_put_bdev;
+	}
+
+	btb = devres_alloc(blkdev_trig_btb_release, sizeof(*btb), GFP_KERNEL);
+	if (btb == NULL) {
+		err = -ENOMEM;
+		goto exit_put_bdev;
+	}
+
+	err = sysfs_create_group(bdev_kobj(bdev), &blkdev_trig_linked_leds);
+	if (err)
+		goto exit_free_btb;
+
+	btb->index = blkdev_trig_next_index++;
+	btb->bdev = bdev;
+	xa_init(&btb->linked_leds);
+	blkdev_trig_update_btb(btb, jiffies);
+
+	devres_add(&bdev->bd_device, btb);
+
+exit_free_btb:
+	if (err)
+		devres_free(btb);
+exit_put_bdev:
+	blkdev_put(bdev, BLKDEV_TRIG_FMODE);
+	return err ? ERR_PTR(err) : btb;
+}
+
+/**
+ * blkdev_trig_get_bdev() - Get a block device by path.
+ * @buf:	The value written to the ``link_dev_by_path`` or
+ *		``unlink_dev_by_path`` 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 *blkdev_trig_get_bdev(const char *buf, size_t count,
+						 fmode_t mode)
+{
+	struct block_device *bdev;
+	char *path;
+
+	path = kmemdup(buf, count + 1, GFP_KERNEL);  /* +1 to 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;
+}
+
+/**
+ * blkdev_trig_update_btb() - Update a BTB's activity counters.
+ * @btb:	The BTB
+ *
+ * Checks each of the BTB's block device's I/O counters.  If the counter has
+ * changed since the last check, updates the counter and its timestamp in the
+ * BTB.
+ *
+ * Context:	Process context.  Caller must hold @blkdev_trig_mutex.
+ */
+static void blkdev_trig_update_btb(struct blkdev_trig_bdev *btb,
+				   unsigned long now)
+{
+	unsigned long new_ios;
+	enum stat_group i;
+
+	for (i = STAT_READ; i <= STAT_FLUSH; ++i) {
+
+		new_ios = part_stat_read(btb->bdev, ios[i]);
+
+		if (new_ios != btb->ios[i]) {
+			btb->ios[i] = new_ios;
+			btb->last_activity[i] = now;
+		}
+	}
+
+	btb->last_checked = now;
+}
+
+/**
+ * blkdev_trig_link() - "Link" a block device to an LED.
+ * @led:	The LED
+ * @btb:	The block device
+ *
+ * Called from link_dev_by_path_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 BTB to the LED's @linked_btbs and adds the LED to the BTB's
+ *     @linked_leds.
+ *   * If this is the first block device linked to this LED, calls
+ *     blkdev_trig_new_sched() to (if needed) schedule or reschedule the delayed
+ *     work which periodically checks for block device activity and blinks LEDs.
+ *
+ * Context:	Process context.  Caller must hold @blkdev_trig_mutex.
+ * Return:	0 on success, ``-errno`` on error.
+ */
+static int blkdev_trig_link(struct blkdev_trig_led *led,
+			    struct blkdev_trig_bdev *btb)
+{
+	bool led_first_link;
+	int err;
+
+	led_first_link = xa_empty(&led->linked_btbs);
+
+	err = xa_insert(&btb->linked_leds, led->index, led, GFP_KERNEL);
+	if (err)
+		return err;
+
+	err = xa_insert(&led->linked_btbs, btb->index, btb, GFP_KERNEL);
+	if (err)
+		goto error_erase_led;
+
+	/* Create /sys/class/block/<bdev>/linked_leds/<led> symlink */
+	err = sysfs_add_link_to_group(bdev_kobj(btb->bdev),
+				      blkdev_trig_linked_leds.name,
+				      &led->led_cdev->dev->kobj,
+				      led->led_cdev->name);
+	if (err)
+		goto error_erase_btb;
+
+	/* Create /sys/class/leds/<led>/linked_devices/<bdev> symlink */
+	err = sysfs_add_link_to_group(&led->led_cdev->dev->kobj,
+				      blkdev_trig_linked_devs.name,
+				      bdev_kobj(btb->bdev),
+				      dev_name(&btb->bdev->bd_device));
+	if (err)
+		goto error_remove_symlink;
+
+	/*
+	 * If this isn't the first block device linked to this LED, then the
+	 * delayed work schedule already reflects this LED.
+	 */
+	if (led_first_link)
+		blkdev_trig_sched_led(led);
+
+	++blkdev_trig_link_count;
+
+	return 0;
+
+error_remove_symlink:
+	sysfs_remove_link_from_group(bdev_kobj(btb->bdev),
+				     blkdev_trig_linked_leds.name,
+				     led->led_cdev->name);
+error_erase_btb:
+	xa_erase(&led->linked_btbs, btb->index);
+error_erase_led:
+	xa_erase(&btb->linked_leds, led->index);
+	return err;
+}
+
+/**
+ * blkdev_trig_sched_led() - Set the schedule of the delayed work when a new
+ *			     LED is added to the schedule.
+ * @led:	The LED
+ *
+ * Called from blkdev_trig_link() to set or adjust the schedule of the delayed
+ * work which periodically checks block devices for activity and blinks LEDs,
+ * if necessary.
+ *
+ *   * If no other links exist, the delayed work is scheduled.
+ *   * If the delayed work is already scheduled to run soon enough to
+ *     accommodate the newly linked LED's @check_jiffies, no change is made to
+ *     the delayed work's schedule.
+ *   * If the delayed work is already scheduled, but it isn't scheduled to
+ *     run soon enough, the schedule is modified.
+ *
+ * Context:	Process context.  Caller must hold @blkdev_trig_mutex.
+ */
+static void blkdev_trig_sched_led(const struct blkdev_trig_led *led)
+{
+	unsigned long delay = READ_ONCE(led->check_jiffies);
+	unsigned long check_by = jiffies + delay;
+
+	if (blkdev_trig_link_count == 0) {
+		WARN_ON(!schedule_delayed_work(&blkdev_trig_work, delay));
+		blkdev_trig_next_check = check_by;
+		return;
+	}
+
+	if (time_after_eq(check_by, blkdev_trig_next_check))
+		return;
+
+	WARN_ON(!mod_delayed_work(system_wq, &blkdev_trig_work, delay));
+	blkdev_trig_next_check = check_by;
+}
+
+/**
+ * unlink_dev_by_path_store() - ``unlink_dev_by_path`` device attribute store
+ *				function.
+ * @dev:	The LED device
+ * @attr:	The ``unlink_dev_by_path`` attribute
+ *		(@dev_attr_unlink_dev_by_path)
+ * @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 blkdev_trig_get_bdev() to get the block device represented by the path
+ * in @buf.  If the device has a BTB, searches the BTB's list of LEDs for a
+ * link to this LED and (if found) calls blkdev_trig_unlink() to destroy the
+ * link.
+ *
+ * Context:	Process context.  Takes and releases @blkdev_trig_mutex.
+ * Return:	@count on success, ``-errno`` on error.
+ */
+static ssize_t unlink_dev_by_path_store(struct device *dev,
+					struct device_attribute *attr,
+					const char *buf, size_t count)
+{
+	struct blkdev_trig_led *led = led_trigger_get_drvdata(dev);
+	struct block_device *bdev;
+	struct blkdev_trig_bdev *btb;
+	int err;
+
+	bdev = blkdev_trig_get_bdev(buf, count, BLKDEV_TRIG_FMODE);
+	if (IS_ERR(bdev))
+		return PTR_ERR(bdev);
+
+	err = mutex_lock_interruptible(&blkdev_trig_mutex);
+	if (err)
+		goto exit_put_bdev;
+
+	btb = devres_find(&bdev->bd_device, blkdev_trig_btb_release,
+			  NULL, NULL);
+	if (btb == NULL) {
+		err = -EUNATCH;  /* bdev isn't linked to any LED */
+		goto exit_unlock;
+	}
+
+	if (xa_load(&btb->linked_leds, led->index) == NULL) {
+		err = -EUNATCH;  /* bdev isn't linked to this LED */
+		goto exit_unlock;
+	}
+
+	blkdev_trig_unlink(led, btb, BLKDEV_TRIG_NOT_RELEASING);
+
+exit_unlock:
+	mutex_unlock(&blkdev_trig_mutex);
+exit_put_bdev:
+	blkdev_put(bdev, BLKDEV_TRIG_FMODE);
+	return err ? : count;
+}
+
+/**
+ * blkdev_trig_unlink() - "Unlink" a block device from an LED.
+ * @led:		The LED
+ * @btb:		The block device
+ * @unlink_mode:	Indicates whether the BTB is being released (because
+ *			the block device has been removed)
+ *
+ * Removes the link between an LED and a block device.
+ *
+ *   * Removes the BTB from the LED's @linked_btbs and removes the LED from
+ *     the BTB'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 blkdev_trig_put_btb() to clean up the BTB, 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_dev_by_path_store() calls this function when a block device is
+ *     unlinked from an LED via the ``unlink_dev_by_path`` sysfs attribute.
+ *     (@unlink_mode == ``BLKDEV_TRIG_NOT_RELEASING``)
+ *   * blkdev_trig_deactivate() calls this function for each block device linked
+ *     to an LED that is being deactivated (disassociated from the trigger).
+ *     (@unlink_mode == ``BLKDEV_TRIG_NOT_RELEASING``).
+ *   * blkdev_trig_btb_release() calls this function for each LED linked to a
+ *     block device that has been removed from the system.  (@unlink_mode ==
+ *     ``BLKDEV_TRIG_RELEASING).
+ *
+ * Context:	Process context.  Caller must hold @blkdev_trig_mutex.
+ */
+static void blkdev_trig_unlink(struct blkdev_trig_led *led,
+			       struct blkdev_trig_bdev *btb,
+			       enum blkdev_trig_unlink_mode unlink_mode)
+{
+	--blkdev_trig_link_count;
+
+	if (blkdev_trig_link_count == 0)
+		WARN_ON(!cancel_delayed_work_sync(&blkdev_trig_work));
+
+	xa_erase(&btb->linked_leds, led->index);
+	xa_erase(&led->linked_btbs, btb->index);
+
+	/* Remove /sys/class/leds/<led>/linked_devices/<bdev> symlink */
+	sysfs_remove_link_from_group(&led->led_cdev->dev->kobj,
+				     blkdev_trig_linked_devs.name,
+				     dev_name(&btb->bdev->bd_device));
+
+	/*
+	 * If the BTB is being released, the driver core has already removed the
+	 * device's attribute groups, and the BTB will be freed automatically,
+	 * so only do these steps if the BTB is not being released.
+	 */
+	if (unlink_mode == BLKDEV_TRIG_NOT_RELEASING) {
+
+		/* Remove /sys/class/block/<bdev>/linked_leds/<led> symlink */
+		sysfs_remove_link_from_group(bdev_kobj(btb->bdev),
+					     blkdev_trig_linked_leds.name,
+					     led->led_cdev->name);
+		blkdev_trig_put_btb(btb);
+	}
+}
+
+/**
+ * blkdev_trig_put_btb() - Remove and free a BTB, if it is no longer needed.
+ * @btb:	The BTB
+ *
+ * Does nothing if the BTB (block device) is still linked to at least one LED.
+ *
+ * If the BTB is no longer linked to any LEDs, removes the block device's
+ * ``linked_leds`` directory (attribute group), removes the BTB from the
+ * block device's resource list, and frees the BTB.
+ *
+ * Called from blkdev_trig_unlink() (and in the link_dev_by_path_store() error
+ * path).
+ *
+ * Context:	Process context.  Caller must hold @blkdev_trig_mutex.
+ */
+static void blkdev_trig_put_btb(struct blkdev_trig_bdev *btb)
+{
+	struct block_device *bdev = btb->bdev;
+	int err;
+
+	if (xa_empty(&btb->linked_leds)) {
+
+		sysfs_remove_group(bdev_kobj(bdev), &blkdev_trig_linked_leds);
+		err = devres_destroy(&bdev->bd_device, blkdev_trig_btb_release,
+				     NULL, NULL);
+		WARN_ON(err);
+	}
+}
+
+/**
+ * blkdev_trig_deactivate() - Called when an LED is disassociated from the
+ *			      trigger.
+ * @led_cdev:	The LED
+ *
+ * Calls blkdev_trig_unlink() for each block device linked to the LED, removes
+ * the LED from the @blkdevtrig_all_leds list, and frees the @blkdev_trig_led.
+ *
+ * Context:	Process context.  Takes and releases @blkdev_trig_mutex.
+ */
+static void blkdev_trig_deactivate(struct led_classdev *led_cdev)
+{
+	struct blkdev_trig_led *led = led_get_trigger_data(led_cdev);
+	struct blkdev_trig_bdev *btb;
+	unsigned long index;
+
+	mutex_lock(&blkdev_trig_mutex);
+
+	xa_for_each (&led->linked_btbs, index, btb)
+		blkdev_trig_unlink(led, btb, BLKDEV_TRIG_NOT_RELEASING);
+
+	hlist_del(&led->all_leds_node);
+	kfree(led);
+
+	mutex_unlock(&blkdev_trig_mutex);
+}
+
+/**
+ * blkdev_trig_btb_release() - BTB device resource release function.
+ * @dev:	The block device
+ * @res:	The BTB
+ *
+ * Called by the driver core when a block device with a BTB is removed from
+ * the system.  Calls blkdev_trig_unlink() for each LED linked to the block
+ * device.
+ *
+ * Context:	Process context.  Takes and releases @blkdev_trig_mutex.
+ */
+static void blkdev_trig_btb_release(struct device *dev, void *res)
+{
+	struct blkdev_trig_bdev *btb = res;
+	struct blkdev_trig_led *led;
+	unsigned long index;
+
+	mutex_lock(&blkdev_trig_mutex);
+
+	xa_for_each (&btb->linked_leds, index, led)
+		blkdev_trig_unlink(led, btb, BLKDEV_TRIG_RELEASING);
+
+	mutex_unlock(&blkdev_trig_mutex);
+}
+
+/**
+ * blkdev_trig_check() - Check linked devices for activity and blink LEDs.
+ * @work:	Delayed work (@blkdev_trig_work)
+ *
+ * Called periodically (as delayed work) to check linked block devices for
+ * activity and blink LEDs.
+ *
+ *   * Iterates through all LEDs associated with the trigger.
+ *   * If an LED is due to be checked, iterates through the block devices (BTBs)
+ *     linked to the LED.
+ *   * If a block device has not already been checked during this pass, calls
+ *     blkdev_trig_update_btb() to update the BTB's activity counters and
+ *     timestamps.
+ *   * If the LED has not already been blinked during this pass, calls
+ *     blkdev_trig_blink() to blink it if the correct type of activity has
+ *     occurred since the LED was last checked.
+ *
+ * When finished, schedules itself to run again when the next LED is due to be
+ * checked.
+ *
+ * Context:	Process context.  Takes and releases @blkdev_trig_mutex.
+ */
+static void blkdev_trig_check(struct work_struct *work)
+{
+	struct blkdev_trig_led *led;
+	struct blkdev_trig_bdev *btb;
+	unsigned long index, delay, now, led_check, led_delay;
+	bool blinked;
+
+	if (!mutex_trylock(&blkdev_trig_mutex)) {
+		delay = msecs_to_jiffies(BLKDEV_TRIG_CHECK_RETRY);
+		goto exit_reschedule;
+	}
+
+	now = jiffies;
+	delay = ULONG_MAX;
+
+	hlist_for_each_entry (led, &blkdev_trig_all_leds, all_leds_node) {
+
+		led_check = led->last_checked + led->check_jiffies;
+
+		if (time_before_eq(led_check, now)) {
+
+			blinked = false;
+
+			xa_for_each (&led->linked_btbs, index, btb) {
+
+				if (btb->last_checked != now)
+					blkdev_trig_update_btb(btb, now);
+				if (!blinked)
+					blinked = blkdev_trig_blink(led, btb);
+			}
+
+			led->last_checked = now;
+			led_delay = led->check_jiffies;
+
+		} else {
+			led_delay = led_check - now;
+		}
+
+		if (led_delay < delay)
+			delay = led_delay;
+	}
+
+	mutex_unlock(&blkdev_trig_mutex);
+
+exit_reschedule:
+	WARN_ON_ONCE(delay == ULONG_MAX);
+	WARN_ON_ONCE(!schedule_delayed_work(&blkdev_trig_work, delay));
+}
+
+/**
+ * blkdev_trig_blink() - Blink an LED, if the correct type of activity has
+ *			 occurred on the block device.
+ * @led:	The LED
+ * @btb:	The block device
+ *
+ * Context:	Process context.  Caller must hold @blkdev_trig_mutex.
+ * Return:	``true`` if the LED is blinked, ``false`` if not.
+ */
+static bool blkdev_trig_blink(const struct blkdev_trig_led *led,
+			      const struct blkdev_trig_bdev *btb)
+{
+	unsigned long delay_on, delay_off;
+	enum stat_group i;
+	unsigned long mode, mask;
+
+	mode = READ_ONCE(led->mode);
+
+	for (i = STAT_READ, mask = 1; i <= STAT_FLUSH; ++i, mask <<= 1) {
+
+		if (!(mode & mask))
+			continue;
+
+		if (time_before_eq(btb->last_activity[i], led->last_checked))
+			continue;
+
+		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 *dev,
+			       struct device_attribute *attr, char *buf)
+{
+	const struct blkdev_trig_led *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 *dev,
+				struct device_attribute *attr,
+				const char *buf, size_t count)
+{
+	struct blkdev_trig_led *led = led_trigger_get_drvdata(dev);
+	unsigned int value;
+	int err;
+
+	err = kstrtouint(buf, 0, &value);
+	if (err)
+		return err;
+
+	if (value < BLKDEV_TRIG_BLINK_MIN || value > BLKDEV_TRIG_BLINK_MAX)
+		return -ERANGE;
+
+	WRITE_ONCE(led->blink_msec, value);
+	return count;
+}
+
+/**
+ * check_interval_show() - ``check_interval`` device attribute show function.
+ * @dev:	The LED device
+ * @attr:	The ``check_interval`` attribute (@dev_attr_check_interval)
+ * @buf:	Output buffer
+ *
+ * Writes the current value of the LED's @check_jiffies (converted to
+ * milliseconds) to @buf.
+ *
+ * Context:	Process context.
+ * Return:	The number of characters written to @buf.
+ */
+static ssize_t check_interval_show(struct device *dev,
+				   struct device_attribute *attr, char *buf)
+{
+	struct blkdev_trig_led *led = led_trigger_get_drvdata(dev);
+
+	return sprintf(buf, "%u\n",
+		       jiffies_to_msecs(READ_ONCE(led->check_jiffies)));
+}
+
+/**
+ * check_interval_store() - ``check_interval`` device attribute store function
+ * @dev:	The LED device
+ * @attr:	The ``check_interval`` attribute (@dev_attr_check_interval)
+ * @buf:	The new value (as written to the sysfs attribute)
+ * @count:	The number of characters in @buf
+ *
+ * Sets the LED's @check_jiffies (after converting from milliseconds).
+ *
+ * Context:	Process context.
+ * Return:	@count on success, ``-errno`` on error.
+ */
+static ssize_t check_interval_store(struct device *dev,
+				    struct device_attribute *attr,
+				    const char *buf, size_t count)
+{
+	struct blkdev_trig_led *led = led_trigger_get_drvdata(dev);
+	unsigned int value;
+	int err;
+
+	err = kstrtouint(buf, 0, &value);
+	if (err)
+		return err;
+
+	if (value < BLKDEV_TRIG_CHECK_MIN || value > BLKDEV_TRIG_CHECK_MAX)
+		return -ERANGE;
+
+	WRITE_ONCE(led->check_jiffies, msecs_to_jiffies(value));
+
+	return count;
+}
+
+/**
+ * blkdev_trig_mode_show() - Helper for boolean attribute show functions.
+ * @led:	The LED
+ * @buf:	Output buffer
+ * @bit:	Which bit to show
+ *
+ * Context:	Process context.
+ * Return:	The number of characters written to @buf.
+ */
+static int blkdev_trig_mode_show(const struct blkdev_trig_led *led, char *buf,
+				 enum stat_group bit)
+{
+	return sprintf(buf, READ_ONCE(led->mode) & (1 << bit) ? "Y\n" : "N\n");
+}
+
+/**
+ * blkdev_trig_mode_store() - Helper for boolean attribute store functions.
+ * @led:	The LED
+ * @buf:	The new value (as written to the sysfs attribute)
+ * @count:	The number of characters in @buf
+ * @bit:	Which bit to set
+ *
+ * Context:	Process context.
+ * Return:	@count on success, ``-errno`` on error.
+ */
+static int blkdev_trig_mode_store(struct blkdev_trig_led *led,
+				  const char *buf, size_t count,
+				  enum stat_group bit)
+{
+	bool set;
+	int err;
+
+	err = kstrtobool(buf, &set);
+	if (err)
+		return err;
+
+	if (set)
+		set_bit(bit, &led->mode);
+	else
+		clear_bit(bit, &led->mode);
+
+	return count;
+}
+
+/**
+ * blink_on_read_show() - ``blink_on_read`` device attribute show function.
+ * @dev:	The LED device
+ * @attr:	The ``blink_on_read`` attribute (@dev_attr_blink_on_read)
+ * @buf:	Output buffer
+ *
+ * Writes the current value of the LED's @mode read bit to @buf.
+ *
+ * Context:	Process context.
+ * Return:	The number of characters written to @buf.
+ */
+static ssize_t blink_on_read_show(struct device *dev,
+				  struct device_attribute *attr, char *buf)
+{
+	return blkdev_trig_mode_show(led_trigger_get_drvdata(dev),
+				     buf, STAT_READ);
+}
+
+/**
+ * blink_on_read_store() - ``blink_on_read`` device attribute store function.
+ * @dev:	The LED device
+ * @attr:	The ``blink_on_read`` attribute (@dev_attr_blink_on_read)
+ * @buf:	The new value (as written to the sysfs attribute)
+ * @count:	The number of characters in @buf
+ *
+ * Sets or clears the LED's @mode read bit.
+ *
+ * Context:	Process context.
+ * Return:	@count on success, ``-errno`` on error.
+ */
+static ssize_t blink_on_read_store(struct device *dev,
+				   struct device_attribute *attr,
+				   const char *buf, size_t count)
+{
+	return blkdev_trig_mode_store(led_trigger_get_drvdata(dev),
+				      buf, count, STAT_READ);
+}
+
+/**
+ * blink_on_write_show() - ``blink_on_write`` device attribute show function.
+ * @dev:	The LED device
+ * @attr:	The ``blink_on_write`` attribute (@dev_attr_blink_on_write)
+ * @buf:	Output buffer
+ *
+ * Writes the current value of the LED's @mode write bit to @buf.
+ *
+ * Context:	Process context.
+ * Return:	The number of characters written to @buf.
+ */
+static ssize_t blink_on_write_show(struct device *dev,
+				   struct device_attribute *attr, char *buf)
+{
+	return blkdev_trig_mode_show(led_trigger_get_drvdata(dev),
+				     buf, STAT_WRITE);
+}
+
+/**
+ * blink_on_write_store() - ``blink_on_write`` device attribute store function.
+ * @dev:	The LED device
+ * @attr:	The ``blink_on_write`` attribute (@dev_attr_blink_on_write)
+ * @buf:	The new value (as written to the sysfs attribute)
+ * @count:	The number of characters in @buf
+ *
+ * Sets or clears the LED's @mode write bit.
+ *
+ * Context:	Process context.
+ * Return:	@count on success, ``-errno`` on error.
+ */
+static ssize_t blink_on_write_store(struct device *dev,
+				    struct device_attribute *attr,
+				    const char *buf, size_t count)
+{
+	return blkdev_trig_mode_store(led_trigger_get_drvdata(dev),
+				      buf, count, STAT_WRITE);
+}
+
+/**
+ * blink_on_flush_show() - ``blink_on_flush`` device attribute show function.
+ * @dev:	The LED device
+ * @attr:	The ``blink_on_flush`` attribute (@dev_attr_blink_on_flush)
+ * @buf:	Output buffer
+ *
+ * Writes the current value of the LED's @mode flush bit to @buf.
+ *
+ * Context:	Process context.
+ * Return:	The number of characters written to @buf.
+ */
+static ssize_t blink_on_flush_show(struct device *dev,
+				   struct device_attribute *attr, char *buf)
+{
+	return blkdev_trig_mode_show(led_trigger_get_drvdata(dev),
+				     buf, STAT_FLUSH);
+}
+
+/**
+ * blink_on_flush_store() - ``blink_on_flush`` device attribute store function.
+ * @dev:	The LED device
+ * @attr:	The ``blink_on_flush`` attribute (@dev_attr_blink_on_flush)
+ * @buf:	The new value (as written to the sysfs attribute)
+ * @count:	The number of characters in @buf
+ *
+ * Sets or clears the LED's @mode flush bit.
+ *
+ * Context:	Process context.
+ * Return:	@count on success, ``-errno`` on error.
+ */
+static ssize_t blink_on_flush_store(struct device *dev,
+				    struct device_attribute *attr,
+				    const char *buf, size_t count)
+{
+	return blkdev_trig_mode_store(led_trigger_get_drvdata(dev),
+				      buf, count, STAT_FLUSH);
+}
+
+/**
+ * blink_on_discard_show() - ``blink_on_discard`` device attribute show
+ *			     function.
+ * @dev:	The LED device
+ * @attr:	The ``blink_on_discard`` attribute (@dev_attr_blink_on_discard)
+ * @buf:	Output buffer
+ *
+ * Writes the current value of the LED's @mode discard bit to @buf.
+ *
+ * Context:	Process context.
+ * Return:	The number of characters written to @buf.
+ */
+static ssize_t blink_on_discard_show(struct device *dev,
+				     struct device_attribute *attr, char *buf)
+{
+	return blkdev_trig_mode_show(led_trigger_get_drvdata(dev),
+				     buf, STAT_DISCARD);
+}
+
+/**
+ * blink_on_discard_store() - ``blink_on_discard`` device attribute store
+ *			      function.
+ * @dev:	The LED device
+ * @attr:	The ``blink_on_discard`` attribute (@dev_attr_blink_on_discard)
+ * @buf:	The new value (as written to the sysfs attribute)
+ * @count:	The number of characters in @buf
+ *
+ * Sets or clears the LED's @mode discard bit.
+ *
+ * Context:	Process context.
+ * Return:	@count on success, ``-errno`` on error.
+ */
+static ssize_t blink_on_discard_store(struct device *dev,
+				      struct device_attribute *attr,
+				      const char *buf, size_t count)
+{
+	return blkdev_trig_mode_store(led_trigger_get_drvdata(dev),
+				      buf, count, STAT_DISCARD);
+}
+
+/* Device attributes */
+static DEVICE_ATTR_WO(link_dev_by_path);
+static DEVICE_ATTR_WO(unlink_dev_by_path);
+static DEVICE_ATTR_RW(blink_time);
+static DEVICE_ATTR_RW(check_interval);
+static DEVICE_ATTR_RW(blink_on_read);
+static DEVICE_ATTR_RW(blink_on_write);
+static DEVICE_ATTR_RW(blink_on_flush);
+static DEVICE_ATTR_RW(blink_on_discard);
+
+/* Device attributes in LED directory (/sys/class/leds/<led>/...) */
+static struct attribute *blkdev_trig_attrs[] = {
+	&dev_attr_link_dev_by_path.attr,
+	&dev_attr_unlink_dev_by_path.attr,
+	&dev_attr_blink_time.attr,
+	&dev_attr_check_interval.attr,
+	&dev_attr_blink_on_read.attr,
+	&dev_attr_blink_on_write.attr,
+	&dev_attr_blink_on_flush.attr,
+	&dev_attr_blink_on_discard.attr,
+	NULL
+};
+
+/* Unnamed attribute group == no subdirectory */
+static const struct attribute_group blkdev_trig_attr_group = {
+	.attrs	= blkdev_trig_attrs,
+};
+
+/* Attribute groups for the trigger */
+static const struct attribute_group *blkdev_trig_attr_groups[] = {
+	&blkdev_trig_attr_group,   /* /sys/class/leds/<led>/... */
+	&blkdev_trig_linked_devs,  /* /sys/class/leds/<led>/linked_devices/ */
+	NULL
+};
+
+/* Trigger registration data */
+static struct led_trigger blkdev_trig_trigger = {
+	.name		= "blkdev",
+	.activate	= blkdev_trig_activate,
+	.deactivate	= blkdev_trig_deactivate,
+	.groups		= blkdev_trig_attr_groups,
+};
+
+/**
+ * blkdev_trig_init() - Block device LED trigger initialization.
+ *
+ * Registers the LED trigger.
+ *
+ * Return:	0 on success, ``-errno`` on failure.
+ */
+static int __init blkdev_trig_init(void)
+{
+	return led_trigger_register(&blkdev_trig_trigger);
+}
+module_init(blkdev_trig_init);
+
+/**
+ * blkdev_trig_exit() - Block device LED trigger module exit.
+ *
+ * Unregisters the LED trigger.
+ */
+static void __exit blkdev_trig_exit(void)
+{
+	led_trigger_unregister(&blkdev_trig_trigger);
+}
+module_exit(blkdev_trig_exit);
+
+MODULE_DESCRIPTION("Block device LED trigger");
+MODULE_AUTHOR("Ian Pilcher <arequipeno@gmail.com>");
+MODULE_LICENSE("GPL v2");
-- 
2.36.1