linux-stable/Documentation/leds/uleds.txt
David Lechner e381322b01 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 <david@lechnology.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
2016-11-22 12:07:02 +01:00

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Userspace LEDs
==============
The uleds driver supports userspace LEDs. This can be useful for testing
triggers and can also be used to implement virtual LEDs.
Usage
=====
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
changes.
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
/dev/uleds.
See tools/leds/uledmon.c for an example userspace program.