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.
The timer trigger will periodically change the LED brightness between
LED_OFF and the current brightness setting. The "on" and "off" time can
be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds.
You can change the brightness value of a LED independently of the timer
trigger. However, if you set the brightness value to LED_OFF it will
also disable the timer trigger.

You can change triggers in a similar manner to the way an IO scheduler
is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
parameters can appear in /sys/class/leds/<device> 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:function"

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. If sections
of the name don't apply, just leave that section blank.


Hardware accelerated blink of LEDs
==================================

Some LEDs can be programmed to blink without any CPU interaction. To
support this feature, a LED driver can optionally implement the
blink_set() function (see <linux/leds.h>). If implemented, triggers can
attempt to use it before falling back to software timers. The blink_set()
function should return 0 if the blink setting is supported, or -EINVAL
otherwise, which means that LED blinking will be handled by software.

The blink_set() function should choose a user friendly blinking
value if it is called with *delay_on==0 && *delay_off==0 parameters. In
this case the driver should give back the chosen value through delay_on
and delay_off parameters to the leds subsystem.

Setting the brightness to zero with brightness_set() callback function
should completely turn off the LED and cancel the previously programmed
hardware blinking function, if any.


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.


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.