[PATCH] LED: class documentation

The LED class/subsystem takes John Lenz's work and extends and alters it 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, spitz and tosa Zaurus models
* LED device driver for locomo LEDs
* LED device driver for ARM ixp4xx LEDs
* Zaurus charging LED trigger
* IDE disk activity LED trigger
* NAND MTD activity LED trigger

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.

A CPU activity trigger similar to that found in the arm led implementation
should be trivial to add.

This patch:

Add some brief documentation of the design decisions behind the LED class and
how it appears to users.

Signed-off-by: Richard Purdie <rpurdie@rpsys.net>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/Documentation/leds-class.txt b/Documentation/leds-class.txt
new file mode 100644 (file)
index 0000000..8c35c04
--- /dev/null
+++ b/Documentation/leds-class.txt
@@ -0,0 +1,71 @@
+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/<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"
+
+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.
+