[v3,1/3] docs: Add block device (blkdev) LED trigger documentation

diff --git a/Documentation/ABI/testing/sysfs-block b/Documentation/ABI/testing/sysfs-block
index a0ed87386639..80d4becc4e6d 100644
--- a/Documentation/ABI/testing/sysfs-block
+++ b/Documentation/ABI/testing/sysfs-block
@@ -328,3 +328,12 @@  Description:
 		does not complete in this time then the block driver timeout
 		handler is invoked. That timeout handler can decide to retry
 		the request, to fail it or to start a device recovery strategy.
+
+What:		/sys/block/<disk>/linked_leds
+Date:		September 2021
+Contact:	Ian Pilcher <arequipeno@gmail.com>
+Description:
+		Directory containing links to all LEDs that are associated
+		with this block device through the blkdev LED trigger.  Only
+		present when at least one LED is associated.  (See
+		Documentation/leds/ledtrig-blkdev.rst.)
diff --git a/Documentation/ABI/testing/sysfs-class-led-trigger-blkdev b/Documentation/ABI/testing/sysfs-class-led-trigger-blkdev
new file mode 100644
index 000000000000..2deee0e42212
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-led-trigger-blkdev
@@ -0,0 +1,43 @@ 
+What:		/sys/class/leds/<led>/blink_time
+Date:		September 2021
+Contact:	Ian Pilcher <arequipeno@gmail.com>
+Description:
+		Time (in milliseconds) that the LED will be on during a single
+		"blink".
+
+What:		/sys/class/leds/<led>/mode
+Date:		September 2021
+Contact:	Ian Pilcher <arequipeno@gmail.com>
+Description:
+		Type of events for which LED will blink - read, write,
+		or rw (both).  Note that any activity that changes the state of
+		the device's non-volatile storage (including discards and cache
+		flushes) is considered to be a write.
+
+What:		/sys/class/leds/<led>/link_device
+Date:		September 2021
+Contact:	Ian Pilcher <arequipeno@gmail.com>
+Description:
+		Associate a block device with this LED by writing its kernel
+		name (as shown in /sys/class/block) to this attribute.
+
+What:		/sys/class/leds/<led>/unlink_device
+Date:		September 2021
+Contact:	Ian Pilcher <arequipeno@gmail.com>
+Description:
+		Remove the association between this LED and a block device by
+		writing the device's kernel name to this attribute.
+
+What:		/sys/class/leds/<led>/linked_devices
+Date:		September 2021
+Contact:	Ian Pilcher <arequipeno@gmail.com>
+Description:
+		Directory containing links to all block devices that are
+		associated with this LED.
+
+What:		/sys/class/ledtrig_blkdev/interval
+Date:		September 2021
+Contact:	Ian Pilcher <arequipeno@gmail.com>
+Description:
+		Frequency (in milliseconds) with which block devices associated
+		with the blkdev LED trigger will be checked for activity.
diff --git a/Documentation/leds/index.rst b/Documentation/leds/index.rst
index e5d63b940045..e3c24e468cbc 100644
--- a/Documentation/leds/index.rst
+++ b/Documentation/leds/index.rst
@@ -10,6 +10,7 @@  LEDs
    leds-class
    leds-class-flash
    leds-class-multicolor
+   ledtrig-blkdev
    ledtrig-oneshot
    ledtrig-transient
    ledtrig-usbport
diff --git a/Documentation/leds/ledtrig-blkdev.rst b/Documentation/leds/ledtrig-blkdev.rst
new file mode 100644
index 000000000000..04bb47ec38b6
--- /dev/null
+++ b/Documentation/leds/ledtrig-blkdev.rst
@@ -0,0 +1,144 @@ 
+.. SPDX-License-Identifier: GPL-2.0
+
+=================================
+Block Device (blkdev) LED Trigger
+=================================
+
+Available when ``CONFIG_LEDS_TRIGGER_BLKDEV=y`` or
+``CONFIG_LEDS_TRIGGER_BLKDEV=m``.
+
+See also:
+
+* ``Documentation/ABI/testing/sysfs-class-led-trigger-blkdev``
+* ``Documentation/ABI/testing/sysfs-block`` (``/sys/block/<disk>/linked_leds``)
+
+Overview
+========
+
+.. note::
+	The examples below use ``<LED>`` to refer to the name of a
+	system-specific LED.  If no suitable LED is available on a test
+	system (in a virtual machine, for example), it is possible to
+	use a userspace LED.  (See ``Documentation/leds/uleds.rst``.)
+
+Verify that the ``blkdev`` LED trigger is available::
+
+	# grep blkdev /sys/class/leds/<LED>/trigger
+	... rfkill-none blkdev
+
+(If the previous command produces no output, you may need to load the trigger
+module - ``modprobe ledtrig_blkdev``.  If the module is not available, check
+the value of ``CONFIG_LEDS_TRIGGER_BLKDEV`` in your kernel configuration.)
+
+Associate the LED with the ``blkdev`` LED trigger::
+
+	# echo blkdev > /sys/class/leds/<LED>/trigger
+
+	# cat /sys/class/leds/<LED>/trigger
+	... rfkill-none [blkdev]
+
+Note that several new device attributes are available in the
+``/sys/class/leds/<LED>`` directory.
+
+* ``link_device`` and ``unlink_device`` are used to manage the set of block
+  devices associated with this LED.  The LED will blink in response to read or
+  write activity on its linked devices.
+
+* ``mode`` is used to control the type of device activity that will cause this
+  LED to blink - read activity, write activity, or both.  (Note that any
+  activity that changes the state of a device's non-volatile storage is
+  considered to be a write.  This includes discard and cache flush requests.)
+
+* ``blink_time`` is the duration (in milliseconds) of each blink of this LED.
+  (The minimum value is 10 milliseconds.)
+
+* The ``linked_devices`` directory will contain a symbolic link to every device
+  that is associated with this LED.
+
+Link a block device to the LED::
+
+	# echo sda > /sys/class/leds/<LED>/link_device
+
+	# ls /sys/class/leds/<LED>/linked_devices
+	sda
+
+Read and write activity on the device should cause the LED to blink.  The
+duration of each blink (in milliseconds) can be adjusted by setting
+``/sys/class/leds/<LED>/blink_time``.  (But see **interval and blink_time**
+below.)
+
+Associate a second device with the LED::
+
+	# echo sdb > /sys/class/leds/<LED>/link_device
+
+	# ls /sys/class/leds/<LED>/linked_devices
+	sda  sdb
+
+When a block device is linked to one or more LEDs, the LEDs are linked from
+the device's ``linked_leds`` directory::
+
+	# ls /sys/block/sd{a,b}/linked_leds
+	/sys/block/sda/linked_leds:
+	<LED>
+
+	/sys/block/sdb/linked_leds:
+	<LED>
+
+(The ``linked_leds`` directory only exists when the block device is linked to
+at least one LED.)
+
+``interval`` and ``blink_time``
+===============================
+
+* By default, linked block devices are checked for activity every 100
+  milliseconds.  This frequency can be changed via the
+  ``/sys/class/ledtrig_blkdev/interval`` attribute.  (The minimum value is 25
+  milliseconds.)
+
+* All associated devices are checked for activity every ``interval``
+  milliseconds, and a blink is triggered on appropriate LEDs.  The duration
+  of an LED's blink is determined by its ``blink_time`` attribute.  Thus
+  (assuming that activity of the relevant type has occurred on one of an LED's
+  linked devices), the LED will be on for ``blink_time`` milliseconds and off
+  for ``interval - blink_time`` milliseconds.
+
+* The LED subsystem ignores new blink requests for an LED that is already in
+  in the process of blinking, so setting a ``blink_time`` greater than or equal
+  to ``interval`` will cause some blinks to be missed.
+
+* Because of processing times, scheduling latencies, etc., avoiding missed
+  blinks actually requires a difference of at least a few milliseconds between
+  the ``blink_time`` and ``interval``.  The required difference is likely to
+  vary from system to system.  As a  reference, a Thecus N5550 NAS requires a
+  difference of 7 milliseconds (``interval == 100``, ``blink_time == 93``).
+
+* The default values (``interval == 100``, ``blink_time == 75``) cause the LED
+  associated with a continuously active device to blink rapidly.  For a more
+  "always on" effect, increase the ``blink_time`` (but not too much; see the
+  previous bullet).
+
+Other Notes
+===========
+
+* Many (possibly all) types of block devices work with this trigger, including:
+
+  * SCSI (including SATA and USB) hard disk drives and SSDs
+  * SCSI (including SATA and USB) optical drives
+  * NVMe SSDs
+  * SD cards
+  * loopback block devices (``/dev/loop*``)
+  * device mapper devices, such as LVM logical volumes
+  * MD RAID devices
+  * zRAM compressed RAM-disks
+  * partitions on block devics that support them
+
+* The device names written to the ``link_device`` and ``unlink_device``
+  attributes must be kernel names (the names shown in
+  ``/sys/class/block``).  Symbolic link names, such as the names commonly
+  used to access logical volumes will not work.  (Kernel names of logical
+  volumes and other device mapper devices can be seen as link targets in the
+  ``/dev/mapper`` directory.)
+
+* The ``blkdev`` LED trigger supports many-to-many device/LED associations.
+  A device can be associated with multiple LEDs, and an LED can be associated
+  with multiple devices.