linux/Documentation/leds-class.txt
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   1LED handling under Linux
   2========================
   3
   4If you're reading this and thinking about keyboard leds, these are
   5handled by the input subsystem and the led class is *not* needed.
   6
   7In its simplest form, the LED class just allows control of LEDs from
   8userspace. LEDs appear in /sys/class/leds/. The brightness file will
   9set the brightness of the LED (taking a value 0-255). Most LEDs don't
  10have hardware brightness support so will just be turned on for non-zero
  11brightness settings.
  12
  13The class also introduces the optional concept of an LED trigger. A trigger
  14is a kernel based source of led events. Triggers can either be simple or
  15complex. A simple trigger isn't configurable and is designed to slot into
  16existing subsystems with minimal additional code. Examples are the ide-disk,
  17nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
  18optimises away.
  19
  20Complex triggers whilst available to all LEDs have LED specific
  21parameters and work on a per LED basis. The timer trigger is an example.
  22The timer trigger will periodically change the LED brightness between
  23LED_OFF and the current brightness setting. The "on" and "off" time can
  24be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds.
  25You can change the brightness value of a LED independently of the timer
  26trigger. However, if you set the brightness value to LED_OFF it will
  27also disable the timer trigger.
  28
  29You can change triggers in a similar manner to the way an IO scheduler
  30is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
  31parameters can appear in /sys/class/leds/<device> once a given trigger is
  32selected.
  33
  34
  35Design Philosophy
  36=================
  37
  38The underlying design philosophy is simplicity. LEDs are simple devices
  39and the aim is to keep a small amount of code giving as much functionality
  40as possible.  Please keep this in mind when suggesting enhancements.
  41
  42
  43LED Device Naming
  44=================
  45
  46Is currently of the form:
  47
  48"devicename:colour:function"
  49
  50There have been calls for LED properties such as colour to be exported as
  51individual led class attributes. As a solution which doesn't incur as much
  52overhead, I suggest these become part of the device name. The naming scheme
  53above leaves scope for further attributes should they be needed. If sections
  54of the name don't apply, just leave that section blank.
  55
  56
  57Hardware accelerated blink of LEDs
  58==================================
  59
  60Some LEDs can be programmed to blink without any CPU interaction. To
  61support this feature, a LED driver can optionally implement the
  62blink_set() function (see <linux/leds.h>). If implemented, triggers can
  63attempt to use it before falling back to software timers. The blink_set()
  64function should return 0 if the blink setting is supported, or -EINVAL
  65otherwise, which means that LED blinking will be handled by software.
  66
  67The blink_set() function should choose a user friendly blinking
  68value if it is called with *delay_on==0 && *delay_off==0 parameters. In
  69this case the driver should give back the chosen value through delay_on
  70and delay_off parameters to the leds subsystem.
  71
  72Setting the brightness to zero with brightness_set() callback function
  73should completely turn off the LED and cancel the previously programmed
  74hardware blinking function, if any.
  75
  76
  77Known Issues
  78============
  79
  80The LED Trigger core cannot be a module as the simple trigger functions
  81would cause nightmare dependency issues. I see this as a minor issue
  82compared to the benefits the simple trigger functionality brings. The
  83rest of the LED subsystem can be modular.
  84
  85
  86Future Development
  87==================
  88
  89At the moment, a trigger can't be created specifically for a single LED.
  90There are a number of cases where a trigger might only be mappable to a
  91particular LED (ACPI?). The addition of triggers provided by the LED driver
  92should cover this option and be possible to add without breaking the
  93current interface.
  94
  95
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