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diff --git a/Documentation/leds/leds-class.rst b/Documentation/leds/leds-class.rst new file mode 100644 index 000000000000..a0708d3f3d0b --- /dev/null +++ b/Documentation/leds/leds-class.rst @@ -0,0 +1,189 @@ +======================== +LED handling under Linux +======================== + +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 disk-activity, +nand-disk and sharpsl-charge triggers. With led triggers disabled, the code +optimises away. + +Complex triggers while 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 +selected. + + +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: + + "devicename:color:function" + +- devicename: + it should refer to a unique identifier created by the kernel, + like e.g. phyN for network devices or inputN for input devices, rather + than to the hardware; the information related to the product and the bus + to which given device is hooked is available in sysfs and can be + retrieved using get_led_device_info.sh script from tools/leds; generally + this section is expected mostly for LEDs that are somehow associated with + other devices. + +- color: + one of LED_COLOR_ID_* definitions from the header + include/dt-bindings/leds/common.h. + +- function: + one of LED_FUNCTION_* definitions from the header + include/dt-bindings/leds/common.h. + +If required color or function is missing, please submit a patch +to linux-leds@vger.kernel.org. + +It is possible that more than one LED with the same color and function will +be required for given platform, differing only with an ordinal number. +In this case it is preferable to just concatenate the predefined LED_FUNCTION_* +name with required "-N" suffix in the driver. fwnode based drivers can use +function-enumerator property for that and then the concatenation will be handled +automatically by the LED core upon LED class device registration. + +LED subsystem has also a protection against name clash, that may occur +when LED class device is created by a driver of hot-pluggable device and +it doesn't provide unique devicename section. In this case numerical +suffix (e.g. "_1", "_2", "_3" etc.) is added to the requested LED class +device name. + +There might be still LED class drivers around using vendor or product name +for devicename, but this approach is now deprecated as it doesn't convey +any added value. Product information can be found in other places in sysfs +(see tools/leds/get_led_device_info.sh). + +Examples of proper LED names: + + - "red:disk" + - "white:flash" + - "red:indicator" + - "phy1:green:wlan" + - "phy3::wlan" + - ":kbd_backlight" + - "input5::kbd_backlight" + - "input3::numlock" + - "input3::scrolllock" + - "input3::capslock" + - "mmc1::status" + - "white:status" + +get_led_device_info.sh script can be used for verifying if the LED name +meets the requirements pointed out here. It performs validation of the LED class +devicename sections and gives hints on expected value for a section in case +the validation fails for it. So far the script supports validation +of associations between LEDs and following types of devices: + + - input devices + - ieee80211 compliant USB devices + +The script is open to extensions. + +There have been calls for LED properties such as color 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. + + +Brightness setting API +====================== + +LED subsystem core exposes following API for setting brightness: + + - led_set_brightness: + it is guaranteed not to sleep, passing LED_OFF stops + blinking, + + - led_set_brightness_sync: + for use cases when immediate effect is desired - + it can block the caller for the time required for accessing + device registers and can sleep, passing LED_OFF stops hardware + blinking, returns -EBUSY if software blink fallback is enabled. + + +LED registration API +==================== + +A driver wanting to register a LED classdev for use by other drivers / +userspace needs to allocate and fill a led_classdev struct and then call +`[devm_]led_classdev_register`. If the non devm version is used the driver +must call led_classdev_unregister from its remove function before +free-ing the led_classdev struct. + +If the driver can detect hardware initiated brightness changes and thus +wants to have a brightness_hw_changed attribute then the LED_BRIGHT_HW_CHANGED +flag must be set in flags before registering. Calling +led_classdev_notify_brightness_hw_changed on a classdev not registered with +the LED_BRIGHT_HW_CHANGED flag is a bug and will trigger a WARN_ON. + +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, use the API function led_brightness_set() +with brightness value LED_OFF, which should 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. |