1 files changed, 41 insertions, 6 deletions

diff --git a/Documentation/laptops/thinkpad-acpi.txt b/Documentation/laptops/thinkpad-acpi.txt
index af1f2bcb6f66..73b80a7e9b03 100644
--- a/Documentation/laptops/thinkpad-acpi.txt
+++ b/Documentation/laptops/thinkpad-acpi.txt
@@ -876,28 +876,63 @@ The cmos command interface is prone to firmware split-brain problems, as
 in newer ThinkPads it is just a compatibility layer.  Do not use it, it is
 exported just as a debug tool.
 
-LED control -- /proc/acpi/ibm/led
----------------------------------
+LED control
+-----------
 
-Some of the LED indicators can be controlled through this feature. The
-available commands are:
+procfs: /proc/acpi/ibm/led
+sysfs attributes: as per led class, see below for names
+
+Some of the LED indicators can be controlled through this feature.  On
+some older ThinkPad models, it is possible to query the status of the
+LED indicators as well.  Newer ThinkPads cannot query the real status
+of the LED indicators.
+
+procfs notes:
+
+The available commands are:
 
         echo '<led number> on' >/proc/acpi/ibm/led
         echo '<led number> off' >/proc/acpi/ibm/led
         echo '<led number> blink' >/proc/acpi/ibm/led
 
 The <led number> range is 0 to 7. The set of LEDs that can be
-controlled varies from model to model. Here is the mapping on the X40:
+controlled varies from model to model. Here is the common ThinkPad
+mapping:
 
         0 - power
         1 - battery (orange)
         2 - battery (green)
-        3 - UltraBase
+        3 - UltraBase/dock
         4 - UltraBay
+        5 - UltraBase battery slot
+        6 - (unknown)
         7 - standby
 
 All of the above can be turned on and off and can be made to blink.
 
+sysfs notes:
+
+The ThinkPad LED sysfs interface is described in detail by the led class
+documentation, in Documentation/leds-class.txt.
+
+The leds are named (in LED ID order, from 0 to 7):
+"tpacpi::power", "tpacpi:orange:batt", "tpacpi:green:batt",
+"tpacpi::dock_active", "tpacpi::bay_active", "tpacpi::dock_batt",
+"tpacpi::unknown_led", "tpacpi::standby".
+
+Due to limitations in the sysfs led class, if the status of the LED
+indicators cannot be read due to an error, thinkpad-acpi will report it as
+a brightness of zero (same as LED off).
+
+If the thinkpad firmware doesn't support reading the current status,
+trying to read the current LED brightness will just return whatever
+brightness was last written to that attribute.
+
+These LEDs can blink using hardware acceleration.  To request that a
+ThinkPad indicator LED should blink in hardware accelerated mode, use the
+"timer" trigger, and leave the delay_on and delay_off parameters set to
+zero (to request hardware acceleration autodetection).
+
 ACPI sounds -- /proc/acpi/ibm/beep
 ----------------------------------