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![Hans de Goede](/assets/img/avatar_default.png)
Some LEDs may have their brightness level changed autonomously (outside of kernel control) by hardware / firmware. This commit adds support for an optional brightness_hw_changed attribute to signal such changes to userspace (if a driver can detect them): What: /sys/class/leds/<led>/brightness_hw_changed Date: January 2017 KernelVersion: 4.11 Description: Last hardware set brightness level for this LED. Some LEDs may be changed autonomously by hardware/firmware. Only LEDs where this happens and the driver can detect this, will have this file. This file supports poll() to detect when the hardware changes the brightness. Reading this file will return the last brightness level set by the hardware, this may be different from the current brightness. Drivers which want to support this, simply add LED_BRIGHT_HW_CHANGED to their flags field and call led_classdev_notify_brightness_hw_changed() with the hardware set brightness when they detect a hardware / firmware triggered brightness change. Signed-off-by: Hans de Goede <hdegoede@redhat.com> Acked-by: Pavel Machek <pavel@ucw.cz> Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
122 lines
5 KiB
Text
122 lines
5 KiB
Text
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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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