[RFC PATCH 0/8] LED Class, Triggers and Drivers

[Richard Purdie <rpurdie@rpsys.net>]

I've implemented the LED related ideas which were mentioned recently and
this patch series is a proposed LED subsystem implementation. It takes
John Lenz's work and extends and alters it a bit to give what I think
should be a fairly universal LED implementation.

The series consists of several logical units:

* LED Core + Class implementation
* LED Trigger Core implementation:
* LED Timer trigger (example of a complex trigger)
* LED Device drivers for corgi and spitz Zaurus models
* LED Device driver for locomo LEDs
* Zaurus Charging LED trigger
* IDE disk activity LED trigger
* NAND MTD activity LED trigger:


(Note: The Zaurus charging trigger patch applies after
http://www.rpsys.net/openzaurus/patches/sharpsl_pm_move-r0.patch but is
included as example code for completeness.)


Why?
====

LEDs are really simple devices usually amounting to a GPIO that can be
turned on and off so why do we need all this code? On handheld or
embedded devices they're an important part of an often limited user
interface. Both users and developers want to be able to control and
configure what the LED does and the number of different things they'd
potentially want the LED to show is large. 

A subsystem is needed to try and provide all this different
functionality in an architecture independent, simple but complete,
generic and scalable manner.

The alternative is for everyone to implement just what they need hidden
away in different corners of the kernel source tree and to provide an
inconsistent interface to userspace.


Other Implementations
=====================

I'm aware of the existing arm led implementation. Currently the new
subsystem and the arm code can coexist quite happily. Its up to the arm
community to decide whether this new interface is acceptable to them. As
far as I can see, the new interface can do everything the existing arm
implementation can with the advantage that the new code is architecture
independent and much more generic configurable and scalable.

I'm prepared to make the conversion to the LED subsystem (or assist with
it) if appropriate.


Implementation Details
======================

I've stripped a lot of code out of John's original LED class. Colours
were removed as LED colour is now part of the device name. Multiple
colours are to be handled as multiple led devices. This means you get
full control over each colour. I also removed the LED hardware timer
code as the generic timer isn't going to add much overhead and is just
as useful. I also decided to have the LED core track the current LED
status (to ease suspend/resume handling) removing the need for
brightness_get implementations in the LED drivers.

An underlying design philosophy is simplicity. The aim is to keep a
small amount of code giving as much functionality as possible.

The major new idea is the led "trigger". A trigger is a 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
zaurus-charging triggers. With leds disabled, the code optimises away.
Examples are nand-disk and ide-disk.

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/somedevice/trigger).

So far there are only a handful of examples but it should easy to add
further LED triggers without too much interference into other
subsystems.


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, I think this should be exported as a device
specific sysfs attribute rather than part of the class if this
functionality is required (eg. to keep the led flashing whilst the
device is suspended).


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. The addition of triggers provided by the LED driver
should cover this option and be possible to add without breaking the
current interface.

The need for a timer with a duty cycle other than 50% has been
mentioned. Due to the added complexity, I think this should be in the
form of a new complex trigger similar to the existing timer trigger. 


Richard

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