1
  2LED handling under Linux
  3========================
  4
  5In its simplest form, the LED class just allows control of LEDs from
  6userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the
  7LED is defined in max_brightness file. The brightness file will set the brightness
  8of the LED (taking a value 0-max_brightness). Most LEDs don't have hardware
  9brightness support so will just be turned on for non-zero brightness settings.
 10
 11The class also introduces the optional concept of an LED trigger. A trigger
 12is a kernel based source of led events. Triggers can either be simple or
 13complex. A simple trigger isn't configurable and is designed to slot into
 14existing subsystems with minimal additional code. Examples are the ide-disk,
 15nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
 16optimises away.
 17
 18Complex triggers whilst available to all LEDs have LED specific
 19parameters and work on a per LED basis. The timer trigger is an example.
 20The timer trigger will periodically change the LED brightness between
 21LED_OFF and the current brightness setting. The "on" and "off" time can
 22be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds.
 23You can change the brightness value of a LED independently of the timer
 24trigger. However, if you set the brightness value to LED_OFF it will
 25also disable the timer trigger.
 26
 27You can change triggers in a similar manner to the way an IO scheduler
 28is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
 29parameters can appear in /sys/class/leds/<device> once a given trigger is
 30selected.
 31
 32
 33Design Philosophy
 34=================
 35
 36The underlying design philosophy is simplicity. LEDs are simple devices
 37and the aim is to keep a small amount of code giving as much functionality
 38as possible.  Please keep this in mind when suggesting enhancements.
 39
 40
 41LED Device Naming
 42=================
 43
 44Is currently of the form:
 45
 46"devicename:colour:function"
 47
 48There have been calls for LED properties such as colour to be exported as
 49individual led class attributes. As a solution which doesn't incur as much
 50overhead, I suggest these become part of the device name. The naming scheme
 51above leaves scope for further attributes should they be needed. If sections
 52of the name don't apply, just leave that section blank.
 53
 54
 55Brightness setting API
 56======================
 57
 58LED subsystem core exposes following API for setting brightness:
 59
 60    - led_set_brightness : it is guaranteed not to sleep, passing LED_OFF stops
 61		blinking,
 62    - led_set_brightness_sync : for use cases when immediate effect is desired -
 63		it can block the caller for the time required for accessing
 64		device registers and can sleep, passing LED_OFF stops hardware
 65		blinking, returns -EBUSY if software blink fallback is enabled.
 66
 67
 68Hardware accelerated blink of LEDs
 69==================================
 70
 71Some LEDs can be programmed to blink without any CPU interaction. To
 72support this feature, a LED driver can optionally implement the
 73blink_set() function (see <linux/leds.h>). To set an LED to blinking,
 74however, it is better to use the API function led_blink_set(), as it
 75will check and implement software fallback if necessary.
 76
 77To turn off blinking again, use the API function led_brightness_set()
 78as that will not just set the LED brightness but also stop any software
 79timers that may have been required for blinking.
 80
 81The blink_set() function should choose a user friendly blinking value
 82if it is called with *delay_on==0 && *delay_off==0 parameters. In this
 83case the driver should give back the chosen value through delay_on and
 84delay_off parameters to the leds subsystem.
 85
 86Setting the brightness to zero with brightness_set() callback function
 87should completely turn off the LED and cancel the previously programmed
 88hardware blinking function, if any.
 89
 90
 91Known Issues
 92============
 93
 94The LED Trigger core cannot be a module as the simple trigger functions
 95would cause nightmare dependency issues. I see this as a minor issue
 96compared to the benefits the simple trigger functionality brings. The
 97rest of the LED subsystem can be modular.
 98
 99
100Future Development
101==================
102
103At the moment, a trigger can't be created specifically for a single LED.
104There are a number of cases where a trigger might only be mappable to a
105particular LED (ACPI?). The addition of triggers provided by the LED driver
106should cover this option and be possible to add without breaking the
107current interface.