123 lines
5 KiB
Plaintext
123 lines
5 KiB
Plaintext
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LED handling under Linux
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========================
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In its simplest form, the LED class just allows control of LEDs from
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userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the
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LED is defined in max_brightness file. The brightness file will set the brightness
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of the LED (taking a value 0-max_brightness). Most LEDs don't have hardware
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brightness support so will just be turned on for non-zero brightness settings.
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The class also introduces the optional concept of an LED trigger. A trigger
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is a kernel based source of led events. Triggers can either be simple or
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complex. A simple trigger isn't configurable and is designed to slot into
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existing subsystems with minimal additional code. Examples are the disk-activity,
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nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
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optimises away.
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Complex triggers whilst available to all LEDs have LED specific
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parameters and work on a per LED basis. The timer trigger is an example.
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The timer trigger will periodically change the LED brightness between
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LED_OFF and the current brightness setting. The "on" and "off" time can
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be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds.
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You can change the brightness value of a LED independently of the timer
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trigger. However, if you set the brightness value to LED_OFF it will
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also disable the timer trigger.
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You can change triggers in a similar manner to the way an IO scheduler
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is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
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parameters can appear in /sys/class/leds/<device> once a given trigger is
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selected.
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Design Philosophy
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=================
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The underlying design philosophy is simplicity. LEDs are simple devices
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and the aim is to keep a small amount of code giving as much functionality
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as possible. Please keep this in mind when suggesting enhancements.
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LED Device Naming
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=================
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Is currently of the form:
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"devicename:colour:function"
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There have been calls for LED properties such as colour to be exported as
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individual led class attributes. As a solution which doesn't incur as much
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overhead, I suggest these become part of the device name. The naming scheme
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above leaves scope for further attributes should they be needed. If sections
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of the name don't apply, just leave that section blank.
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Brightness setting API
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======================
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LED subsystem core exposes following API for setting brightness:
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- led_set_brightness : it is guaranteed not to sleep, passing LED_OFF stops
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blinking,
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- led_set_brightness_sync : for use cases when immediate effect is desired -
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it can block the caller for the time required for accessing
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device registers and can sleep, passing LED_OFF stops hardware
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blinking, returns -EBUSY if software blink fallback is enabled.
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LED registration API
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====================
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A driver wanting to register a LED classdev for use by other drivers /
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userspace needs to allocate and fill a led_classdev struct and then call
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[devm_]led_classdev_register. If the non devm version is used the driver
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must call led_classdev_unregister from its remove function before
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free-ing the led_classdev struct.
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If the driver can detect hardware initiated brightness changes and thus
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wants to have a brightness_hw_changed attribute then the LED_BRIGHT_HW_CHANGED
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flag must be set in flags before registering. Calling
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led_classdev_notify_brightness_hw_changed on a classdev not registered with
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the LED_BRIGHT_HW_CHANGED flag is a bug and will trigger a WARN_ON.
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Hardware accelerated blink of LEDs
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==================================
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Some LEDs can be programmed to blink without any CPU interaction. To
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support this feature, a LED driver can optionally implement the
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blink_set() function (see <linux/leds.h>). To set an LED to blinking,
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however, it is better to use the API function led_blink_set(), as it
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will check and implement software fallback if necessary.
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To turn off blinking, use the API function led_brightness_set()
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with brightness value LED_OFF, which should stop any software
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timers that may have been required for blinking.
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The blink_set() function should choose a user friendly blinking value
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if it is called with *delay_on==0 && *delay_off==0 parameters. In this
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case the driver should give back the chosen value through delay_on and
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delay_off parameters to the leds subsystem.
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Setting the brightness to zero with brightness_set() callback function
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should completely turn off the LED and cancel the previously programmed
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hardware blinking function, if any.
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Known Issues
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============
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The LED Trigger core cannot be a module as the simple trigger functions
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would cause nightmare dependency issues. I see this as a minor issue
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compared to the benefits the simple trigger functionality brings. The
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rest of the LED subsystem can be modular.
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Future Development
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==================
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At the moment, a trigger can't be created specifically for a single LED.
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There are a number of cases where a trigger might only be mappable to a
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particular LED (ACPI?). The addition of triggers provided by the LED driver
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should cover this option and be possible to add without breaking the
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current interface.
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