• Richard Purdie's avatar
    [PATCH] LED: class documentation · 75c1d31d
    Richard Purdie authored
    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
    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
    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: default avatarRichard Purdie <rpurdie@rpsys.net>
    Cc: Russell King <rmk@arm.linux.org.uk>
    Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
leds-class.txt 2.66 KB