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f60c8ba77d
This change creates a new input handler called "leds" that exports LEDs on input devices as standard LED class devices in sysfs and allows controlling their state via sysfs or via any of the standard LED triggers. This allows to re-purpose and reassign LDEs on the keyboards to represent states other than the standard keyboard states (CapsLock, NumLock, etc). The old API of controlling input LEDs by writing into /dev/input/eventX devices is still present and will take precedence over accessing via LEDs subsystem (i.e. it may override state set by a trigger). If input device is "grabbed" then requests coming through LED subsystem will be ignored. Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org> Tested-by: Pavel Machek <pavel@ucw.cz> Acked-by: Pavel Machek <pavel@ucw.cz> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
95 lines
3.9 KiB
Plaintext
95 lines
3.9 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 ide-disk,
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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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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 again, use the API function led_brightness_set()
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as that will not just set the LED brightness but also 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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