1======================== 2LED handling under Linux 3======================== 4 5In its simplest form, the LED class just allows control of LEDs from 6userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the 7LED is defined in max_brightness file. The brightness file will set the brightness 8of the LED (taking a value 0-max_brightness). Most LEDs don't have hardware 9brightness 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 trigger 12is a kernel based source of led events. Triggers can either be simple or 13complex. A simple trigger isn't configurable and is designed to slot into 14existing subsystems with minimal additional code. Examples are the disk-activity, 15nand-disk and sharpsl-charge triggers. With led triggers disabled, the code 16optimises away. 17 18Complex triggers while available to all LEDs have LED specific 19parameters and work on a per LED basis. The timer trigger is an example. 20The timer trigger will periodically change the LED brightness between 21LED_OFF and the current brightness setting. The "on" and "off" time can 22be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds. 23You can change the brightness value of a LED independently of the timer 24trigger. However, if you set the brightness value to LED_OFF it will 25also disable the timer trigger. 26 27You can change triggers in a similar manner to the way an IO scheduler 28is chosen (via /sys/class/leds/<device>/trigger). Trigger specific 29parameters can appear in /sys/class/leds/<device> once a given trigger is 30selected. 31 32 33Design Philosophy 34================= 35 36The underlying design philosophy is simplicity. LEDs are simple devices 37and the aim is to keep a small amount of code giving as much functionality 38as possible. Please keep this in mind when suggesting enhancements. 39 40 41LED Device Naming 42================= 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, rather 51 than to the hardware; the information related to the product and the bus 52 to which given device is hooked is available in sysfs and can be 53 retrieved using get_led_device_info.sh script from tools/leds; generally 54 this section is expected mostly for LEDs that are somehow associated with 55 other devices. 56 57- color: 58 one of LED_COLOR_ID_* definitions from the header 59 include/dt-bindings/leds/common.h. 60 61- function: 62 one of LED_FUNCTION_* definitions from the header 63 include/dt-bindings/leds/common.h. 64 65If required color or function is missing, please submit a patch 66to linux-leds@vger.kernel.org. 67 68It is possible that more than one LED with the same color and function will 69be 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 use 72function-enumerator property for that and then the concatenation will be handled 73automatically by the LED core upon LED class device registration. 74 75LED subsystem has also a protection against name clash, that may occur 76when LED class device is created by a driver of hot-pluggable device and 77it doesn't provide unique devicename section. In this case numerical 78suffix (e.g. "_1", "_2", "_3" etc.) is added to the requested LED class 79device name. 80 81There might be still LED class drivers around using vendor or product name 82for devicename, but this approach is now deprecated as it doesn't convey 83any added value. Product information can be found in other places in sysfs 84(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 name 102meets the requirements pointed out here. It performs validation of the LED class 103devicename sections and gives hints on expected value for a section in case 104the validation fails for it. So far the script supports validation 105of associations between LEDs and following types of devices: 106 107 - input devices 108 - ieee80211 compliant USB devices 109 110The script is open to extensions. 111 112There have been calls for LED properties such as color to be exported as 113individual led class attributes. As a solution which doesn't incur as much 114overhead, I suggest these become part of the device name. The naming scheme 115above leaves scope for further attributes should they be needed. If sections 116of the name don't apply, just leave that section blank. 117 118 119Brightness setting API 120====================== 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 stops 126 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 accessing 131 device registers and can sleep, passing LED_OFF stops hardware 132 blinking, returns -EBUSY if software blink fallback is enabled. 133 134 135LED registration API 136==================== 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 call 140`[devm_]led_classdev_register`. If the non devm version is used the driver 141must call led_classdev_unregister from its remove function before 142free-ing the led_classdev struct. 143 144If the driver can detect hardware initiated brightness changes and thus 145wants to have a brightness_hw_changed attribute then the LED_BRIGHT_HW_CHANGED 146flag must be set in flags before registering. Calling 147led_classdev_notify_brightness_hw_changed on a classdev not registered with 148the LED_BRIGHT_HW_CHANGED flag is a bug and will trigger a WARN_ON. 149 150Hardware accelerated blink of LEDs 151================================== 152 153Some LEDs can be programmed to blink without any CPU interaction. To 154support this feature, a LED driver can optionally implement the 155blink_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 it 157will 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 software 161timers that may have been required for blinking. 162 163The blink_set() function should choose a user friendly blinking value 164if it is called with `*delay_on==0` && `*delay_off==0` parameters. In this 165case the driver should give back the chosen value through delay_on and 166delay_off parameters to the leds subsystem. 167 168Setting the brightness to zero with brightness_set() callback function 169should completely turn off the LED and cancel the previously programmed 170hardware blinking function, if any. 171 172Hardware driven LEDs 173==================== 174 175Some LEDs can be programmed to be driven by hardware. This is not 176limited to blink but also to turn off or on autonomously. 177To support this feature, a LED needs to implement various additional 178ops 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 control 186 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 can 191 be parsed and activate hw control on the LED. 192 193 Return 0 if the passed flags mask is supported and 194 can be set with hw_control_set(). 195 196 If the passed flags mask is not supported -EOPNOTSUPP 197 must be returned, the LED trigger will use software 198 fallback in this case. 199 200 Return a negative error in case of any other error like 201 device not ready or timeouts. 202 203 - hw_control_set: 204 activate hw control. LED driver will use the provided 205 flags passed from the supported trigger, parse them to 206 a set of mode and setup the LED to be driven by hardware 207 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 to 212 apply flags. 213 214 - hw_control_get: 215 get active modes from a LED already in hw control, parse 216 them and set in flags the current active flags for the 217 supported trigger. 218 219 Return 0 on success, a negative error number on failing 220 parsing the initial mode. 221 Error from this function is NOT FATAL as the device may 222 be in a not supported initial state by the attached LED 223 trigger. 224 225 - hw_control_get_device: 226 return the device associated with the LED driver in 227 hw control. A trigger might use this to match the 228 returned device from this function with a configured 229 device for the trigger as the source for blinking 230 events and correctly enable hw control. 231 (example a netdev trigger configured to blink for a 232 particular dev match the returned dev from get_device 233 to set hw control) 234 235 Returns a pointer to a struct device or NULL if nothing 236 is currently attached. 237 238LED driver can activate additional modes by default to workaround the 239impossibility of supporting each different mode on the supported trigger. 240Examples are hardcoding the blink speed to a set interval, enable special 241feature like bypassing blink if some requirements are not met. 242 243A trigger should first check if the hw control API are supported by the LED 244driver 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 at 246the 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 control 249and init their flags. 250 251When the LED is in hw control, no software blink is possible and doing so 252will effectively disable hw control. 253 254Known Issues 255============ 256 257The LED Trigger core cannot be a module as the simple trigger functions 258would cause nightmare dependency issues. I see this as a minor issue 259compared to the benefits the simple trigger functionality brings. The 260rest of the LED subsystem can be modular. 261