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1========================2LED handling under Linux3========================4 5In its simplest form, the LED class just allows control of LEDs from6userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the7LED is defined in max_brightness file. The brightness file will set the brightness8of the LED (taking a value 0-max_brightness). Most LEDs don't have hardware9brightness support so will just be turned on for non-zero brightness settings.10 11The class also introduces the optional concept of an LED trigger. A trigger12is a kernel based source of led events. Triggers can either be simple or13complex. A simple trigger isn't configurable and is designed to slot into14existing subsystems with minimal additional code. Examples are the disk-activity,15nand-disk and sharpsl-charge triggers. With led triggers disabled, the code16optimises away.17 18Complex triggers while available to all LEDs have LED specific19parameters and work on a per LED basis. The timer trigger is an example.20The timer trigger will periodically change the LED brightness between21LED_OFF and the current brightness setting. The "on" and "off" time can22be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds.23You can change the brightness value of a LED independently of the timer24trigger. However, if you set the brightness value to LED_OFF it will25also disable the timer trigger.26 27You can change triggers in a similar manner to the way an IO scheduler28is chosen (via /sys/class/leds/<device>/trigger). Trigger specific29parameters can appear in /sys/class/leds/<device> once a given trigger is30selected.31 32 33Design Philosophy34=================35 36The underlying design philosophy is simplicity. LEDs are simple devices37and the aim is to keep a small amount of code giving as much functionality38as possible. Please keep this in mind when suggesting enhancements.39 40 41LED Device Naming42=================43 44Is currently of the form:45 46 "devicename:color:function"47 48- devicename:49 it should refer to a unique identifier created by the kernel,50 like e.g. phyN for network devices or inputN for input devices, rather51 than to the hardware; the information related to the product and the bus52 to which given device is hooked is available in sysfs and can be53 retrieved using get_led_device_info.sh script from tools/leds; generally54 this section is expected mostly for LEDs that are somehow associated with55 other devices.56 57- color:58 one of LED_COLOR_ID_* definitions from the header59 include/dt-bindings/leds/common.h.60 61- function:62 one of LED_FUNCTION_* definitions from the header63 include/dt-bindings/leds/common.h.64 65If required color or function is missing, please submit a patch66to linux-leds@vger.kernel.org.67 68It is possible that more than one LED with the same color and function will69be required for given platform, differing only with an ordinal number.70In this case it is preferable to just concatenate the predefined LED_FUNCTION_*71name with required "-N" suffix in the driver. fwnode based drivers can use72function-enumerator property for that and then the concatenation will be handled73automatically by the LED core upon LED class device registration.74 75LED subsystem has also a protection against name clash, that may occur76when LED class device is created by a driver of hot-pluggable device and77it doesn't provide unique devicename section. In this case numerical78suffix (e.g. "_1", "_2", "_3" etc.) is added to the requested LED class79device name.80 81There might be still LED class drivers around using vendor or product name82for devicename, but this approach is now deprecated as it doesn't convey83any added value. Product information can be found in other places in sysfs84(see tools/leds/get_led_device_info.sh).85 86Examples of proper LED names:87 88 - "red:disk"89 - "white:flash"90 - "red:indicator"91 - "phy1:green:wlan"92 - "phy3::wlan"93 - ":kbd_backlight"94 - "input5::kbd_backlight"95 - "input3::numlock"96 - "input3::scrolllock"97 - "input3::capslock"98 - "mmc1::status"99 - "white:status"100 101get_led_device_info.sh script can be used for verifying if the LED name102meets the requirements pointed out here. It performs validation of the LED class103devicename sections and gives hints on expected value for a section in case104the validation fails for it. So far the script supports validation105of associations between LEDs and following types of devices:106 107 - input devices108 - ieee80211 compliant USB devices109 110The script is open to extensions.111 112There have been calls for LED properties such as color to be exported as113individual led class attributes. As a solution which doesn't incur as much114overhead, I suggest these become part of the device name. The naming scheme115above leaves scope for further attributes should they be needed. If sections116of the name don't apply, just leave that section blank.117 118 119Brightness setting API120======================121 122LED subsystem core exposes following API for setting brightness:123 124 - led_set_brightness:125 it is guaranteed not to sleep, passing LED_OFF stops126 blinking,127 128 - led_set_brightness_sync:129 for use cases when immediate effect is desired -130 it can block the caller for the time required for accessing131 device registers and can sleep, passing LED_OFF stops hardware132 blinking, returns -EBUSY if software blink fallback is enabled.133 134 135LED registration API136====================137 138A driver wanting to register a LED classdev for use by other drivers /139userspace needs to allocate and fill a led_classdev struct and then call140`[devm_]led_classdev_register`. If the non devm version is used the driver141must call led_classdev_unregister from its remove function before142free-ing the led_classdev struct.143 144If the driver can detect hardware initiated brightness changes and thus145wants to have a brightness_hw_changed attribute then the LED_BRIGHT_HW_CHANGED146flag must be set in flags before registering. Calling147led_classdev_notify_brightness_hw_changed on a classdev not registered with148the LED_BRIGHT_HW_CHANGED flag is a bug and will trigger a WARN_ON.149 150Hardware accelerated blink of LEDs151==================================152 153Some LEDs can be programmed to blink without any CPU interaction. To154support this feature, a LED driver can optionally implement the155blink_set() function (see <linux/leds.h>). To set an LED to blinking,156however, it is better to use the API function led_blink_set(), as it157will check and implement software fallback if necessary.158 159To turn off blinking, use the API function led_brightness_set()160with brightness value LED_OFF, which should stop any software161timers that may have been required for blinking.162 163The blink_set() function should choose a user friendly blinking value164if it is called with `*delay_on==0` && `*delay_off==0` parameters. In this165case the driver should give back the chosen value through delay_on and166delay_off parameters to the leds subsystem.167 168Setting the brightness to zero with brightness_set() callback function169should completely turn off the LED and cancel the previously programmed170hardware blinking function, if any.171 172Hardware driven LEDs173====================174 175Some LEDs can be programmed to be driven by hardware. This is not176limited to blink but also to turn off or on autonomously.177To support this feature, a LED needs to implement various additional178ops and needs to declare specific support for the supported triggers.179 180With hw control we refer to the LED driven by hardware.181 182LED driver must define the following value to support hw control:183 184 - hw_control_trigger:185 unique trigger name supported by the LED in hw control186 mode.187 188LED driver must implement the following API to support hw control:189 - hw_control_is_supported:190 check if the flags passed by the supported trigger can191 be parsed and activate hw control on the LED.192 193 Return 0 if the passed flags mask is supported and194 can be set with hw_control_set().195 196 If the passed flags mask is not supported -EOPNOTSUPP197 must be returned, the LED trigger will use software198 fallback in this case.199 200 Return a negative error in case of any other error like201 device not ready or timeouts.202 203 - hw_control_set:204 activate hw control. LED driver will use the provided205 flags passed from the supported trigger, parse them to206 a set of mode and setup the LED to be driven by hardware207 following the requested modes.208 209 Set LED_OFF via the brightness_set to deactivate hw control.210 211 Return 0 on success, a negative error number on failing to212 apply flags.213 214 - hw_control_get:215 get active modes from a LED already in hw control, parse216 them and set in flags the current active flags for the217 supported trigger.218 219 Return 0 on success, a negative error number on failing220 parsing the initial mode.221 Error from this function is NOT FATAL as the device may222 be in a not supported initial state by the attached LED223 trigger.224 225 - hw_control_get_device:226 return the device associated with the LED driver in227 hw control. A trigger might use this to match the228 returned device from this function with a configured229 device for the trigger as the source for blinking230 events and correctly enable hw control.231 (example a netdev trigger configured to blink for a232 particular dev match the returned dev from get_device233 to set hw control)234 235 Returns a pointer to a struct device or NULL if nothing236 is currently attached.237 238LED driver can activate additional modes by default to workaround the239impossibility of supporting each different mode on the supported trigger.240Examples are hardcoding the blink speed to a set interval, enable special241feature like bypassing blink if some requirements are not met.242 243A trigger should first check if the hw control API are supported by the LED244driver and check if the trigger is supported to verify if hw control is possible,245use hw_control_is_supported to check if the flags are supported and only at246the end use hw_control_set to activate hw control.247 248A trigger can use hw_control_get to check if a LED is already in hw control249and init their flags.250 251When the LED is in hw control, no software blink is possible and doing so252will effectively disable hw control.253 254Known Issues255============256 257The LED Trigger core cannot be a module as the simple trigger functions258would cause nightmare dependency issues. I see this as a minor issue259compared to the benefits the simple trigger functionality brings. The260rest of the LED subsystem can be modular.261