path: root/Documentation/leds
diff options
authorDavid Lechner <>2016-09-16 14:16:48 -0500
committerJacek Anaszewski <>2016-11-22 12:07:02 +0100
commite381322b0190c1253d347de3f28b5c37756fb651 (patch)
tree792a2bc1695cae581e152565e95127405424f5c6 /Documentation/leds
parent1001354ca34179f3db924eb66672442a173147dc (diff)
leds: Introduce userspace LED class driver
This driver creates a userspace leds driver similar to uinput. New LEDs are created by opening /dev/uleds and writing a uleds_user_dev struct. A new LED class device is registered with the name given in the struct. Reading will return a single byte that is the current brightness. The poll() syscall is also supported. It will be triggered whenever the brightness changes. Closing the file handle to /dev/uleds will remove the leds class device. Signed-off-by: David Lechner <> Signed-off-by: Jacek Anaszewski <>
Diffstat (limited to 'Documentation/leds')
1 files changed, 36 insertions, 0 deletions
diff --git a/Documentation/leds/uleds.txt b/Documentation/leds/uleds.txt
new file mode 100644
index 000000000000..13e375a580f9
--- /dev/null
+++ b/Documentation/leds/uleds.txt
@@ -0,0 +1,36 @@
+Userspace LEDs
+The uleds driver supports userspace LEDs. This can be useful for testing
+triggers and can also be used to implement virtual LEDs.
+When the driver is loaded, a character device is created at /dev/uleds. To
+create a new LED class device, open /dev/uleds and write a uleds_user_dev
+structure to it (found in kernel public header file linux/uleds.h).
+ #define LED_MAX_NAME_SIZE 64
+ struct uleds_user_dev {
+ char name[LED_MAX_NAME_SIZE];
+ };
+A new LED class device will be created with the name given. The name can be
+any valid sysfs device node name, but consider using the LED class naming
+convention of "devicename:color:function".
+The current brightness is found by reading a single byte from the character
+device. Values are unsigned: 0 to 255. Reading will block until the brightness
+changes. The device node can also be polled to notify when the brightness value
+The LED class device will be removed when the open file handle to /dev/uleds
+is closed.
+Multiple LED class devices are created by opening additional file handles to
+See tools/leds/uledmon.c for an example userspace program.