linux/Documentation/leds/leds-class.txt
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   2LED handling under Linux
   3========================
   4
   5If you're reading this and thinking about keyboard leds, these are
   6handled by the input subsystem and the led class is *not* needed.
   7
   8In its simplest form, the LED class just allows control of LEDs from
   9userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the
  10LED is defined in max_brightness file. The brightness file will set the brightness
  11of the LED (taking a value 0-max_brightness). Most LEDs don't have hardware
  12brightness support so will just be turned on for non-zero brightness settings.
  13
  14The class also introduces the optional concept of an LED trigger. A trigger
  15is a kernel based source of led events. Triggers can either be simple or
  16complex. A simple trigger isn't configurable and is designed to slot into
  17existing subsystems with minimal additional code. Examples are the ide-disk,
  18nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
  19optimises away.
  20
  21Complex triggers whilst available to all LEDs have LED specific
  22parameters and work on a per LED basis. The timer trigger is an example.
  23The timer trigger will periodically change the LED brightness between
  24LED_OFF and the current brightness setting. The "on" and "off" time can
  25be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds.
  26You can change the brightness value of a LED independently of the timer
  27trigger. However, if you set the brightness value to LED_OFF it will
  28also disable the timer trigger.
  29
  30You can change triggers in a similar manner to the way an IO scheduler
  31is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
  32parameters can appear in /sys/class/leds/<device> once a given trigger is
  33selected.
  34
  35
  36Design Philosophy
  37=================
  38
  39The underlying design philosophy is simplicity. LEDs are simple devices
  40and the aim is to keep a small amount of code giving as much functionality
  41as possible.  Please keep this in mind when suggesting enhancements.
  42
  43
  44LED Device Naming
  45=================
  46
  47Is currently of the form:
  48
  49"devicename:colour:function"
  50
  51There have been calls for LED properties such as colour to be exported as
  52individual led class attributes. As a solution which doesn't incur as much
  53overhead, I suggest these become part of the device name. The naming scheme
  54above leaves scope for further attributes should they be needed. If sections
  55of the name don't apply, just leave that section blank.
  56
  57
  58Hardware accelerated blink of LEDs
  59==================================
  60
  61Some LEDs can be programmed to blink without any CPU interaction. To
  62support this feature, a LED driver can optionally implement the
  63blink_set() function (see <linux/leds.h>). To set an LED to blinking,
  64however, it is better to use the API function led_blink_set(), as it
  65will check and implement software fallback if necessary.
  66
  67To turn off blinking again, use the API function led_brightness_set()
  68as that will not just set the LED brightness but also stop any software
  69timers that may have been required for blinking.
  70
  71The blink_set() function should choose a user friendly blinking value
  72if it is called with *delay_on==0 && *delay_off==0 parameters. In this
  73case the driver should give back the chosen value through delay_on and
  74delay_off parameters to the leds subsystem.
  75
  76Setting the brightness to zero with brightness_set() callback function
  77should completely turn off the LED and cancel the previously programmed
  78hardware blinking function, if any.
  79
  80
  81Known Issues
  82============
  83
  84The LED Trigger core cannot be a module as the simple trigger functions
  85would cause nightmare dependency issues. I see this as a minor issue
  86compared to the benefits the simple trigger functionality brings. The
  87rest of the LED subsystem can be modular.
  88
  89
  90Future Development
  91==================
  92
  93At the moment, a trigger can't be created specifically for a single LED.
  94There are a number of cases where a trigger might only be mappable to a
  95particular LED (ACPI?). The addition of triggers provided by the LED driver
  96should cover this option and be possible to add without breaking the
  97current interface.
  98
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