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LED handling under Linux

========================

If you're reading this and thinking about keyboard leds, these are

handled by the input subsystem and the led class is *not* needed.

In its simplest form, the LED class just allows control of LEDs from

userspace. LEDs appear in /sys/class/leds/. The brightness file will

set the brightness of the LED (taking a value 0-255). Most LEDs don't

have hardware brightness support so will just be turned on for non-zero

brightness settings.

The class also introduces the optional concept of an LED trigger. A trigger

is a kernel based source of led events. Triggers can either be simple or

complex. A simple trigger isn't configurable and is designed to slot into

existing subsystems with minimal additional code. Examples are the ide-disk,

nand-disk and sharpsl-charge triggers. With led triggers disabled, the code

optimises away.

Complex triggers whilst available to all LEDs have LED specific

parameters and work on a per LED basis. The timer trigger is an example.

You can change triggers in a similar manner to the way an IO scheduler

is chosen (via /sys/class/leds//trigger). Trigger specific

parameters can appear in /sys/class/leds/ once a given trigger is

selected.

Design Philosophy

=================

The underlying design philosophy is simplicity. LEDs are simple devices

and the aim is to keep a small amount of code giving as much functionality

as possible.  Please keep this in mind when suggesting enhancements.

LED Device Naming

=================

Is currently of the form:

"devicename:colour"

There have been calls for LED properties such as colour to be exported as

individual led class attributes. As a solution which doesn't incur as much

overhead, I suggest these become part of the device name. The naming scheme

above leaves scope for further attributes should they be needed.

Known Issues

============

The LED Trigger core cannot be a module as the simple trigger functions

would cause nightmare dependency issues. I see this as a minor issue

compared to the benefits the simple trigger functionality brings. The

rest of the LED subsystem can be modular.

Some leds can be programmed to flash in hardware. As this isn't a generic

LED device property, this should be exported as a device specific sysfs

attribute rather than part of the class if this functionality is required.

Future Development

==================

At the moment, a trigger can't be created specifically for a single LED.

There are a number of cases where a trigger might only be mappable to a

particular LED (ACPI?). The addition of triggers provided by the LED driver

should cover this option and be possible to add without breaking the

current interface.