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-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 maximum brightness of the
-LED is defined in max_brightness file. The brightness file will set the brightness
-of the LED (taking a value 0-max_brightness). 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.
-The timer trigger will periodically change the LED brightness between
-LED_OFF and the current brightness setting. The "on" and "off" time can
-be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds.
-You can change the brightness value of a LED independently of the timer
-trigger. However, if you set the brightness value to LED_OFF it will
-also disable the timer trigger.
-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
-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:
-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. If sections
-of the name don't apply, just leave that section blank.
-Hardware accelerated blink of LEDs
-Some LEDs can be programmed to blink without any CPU interaction. To
-support this feature, a LED driver can optionally implement the
-blink_set() function (see <linux/leds.h>). To set an LED to blinking,
-however, it is better to use the API function led_blink_set(), as it
-will check and implement software fallback if necessary.
-To turn off blinking again, use the API function led_brightness_set()
-as that will not just set the LED brightness but also stop any software
-timers that may have been required for blinking.
-The blink_set() function should choose a user friendly blinking value
-if it is called with *delay_on==0 && *delay_off==0 parameters. In this
-case the driver should give back the chosen value through delay_on and
-delay_off parameters to the leds subsystem.
-Setting the brightness to zero with brightness_set() callback function
-should completely turn off the LED and cancel the previously programmed
-hardware blinking function, if any.
-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.
-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.