Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 2 LED handling under Linux 3 ======================== 4 5 In its simplest form, the LED class just allows control of LEDs from 6 userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the 7 LED is defined in max_brightness file. The brightness file will set the brightness 8 of the LED (taking a value 0-max_brightness). Most LEDs don't have hardware 9 brightness support so will just be turned on for non-zero brightness settings. 10 11 The class also introduces the optional concept of an LED trigger. A trigger 12 is a kernel based source of led events. Triggers can either be simple or 13 complex. A simple trigger isn't configurable and is designed to slot into 14 existing subsystems with minimal additional code. Examples are the disk-activity, 15 nand-disk and sharpsl-charge triggers. With led triggers disabled, the code 16 optimises away. 17 18 Complex triggers whilst available to all LEDs have LED specific 19 parameters and work on a per LED basis. The timer trigger is an example. 20 The timer trigger will periodically change the LED brightness between 21 LED_OFF and the current brightness setting. The "on" and "off" time can 22 be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds. 23 You can change the brightness value of a LED independently of the timer 24 trigger. However, if you set the brightness value to LED_OFF it will 25 also disable the timer trigger. 26 27 You can change triggers in a similar manner to the way an IO scheduler 28 is chosen (via /sys/class/leds/<device>/trigger). Trigger specific 29 parameters can appear in /sys/class/leds/<device> once a given trigger is 30 selected. 31 32 33 Design Philosophy 34 ================= 35 36 The underlying design philosophy is simplicity. LEDs are simple devices 37 and the aim is to keep a small amount of code giving as much functionality 38 as possible. Please keep this in mind when suggesting enhancements. 39 40 41 LED Device Naming 42 ================= 43 44 Is currently of the form: 45 46 "devicename:colour:function" 47 48 There have been calls for LED properties such as colour to be exported as 49 individual led class attributes. As a solution which doesn't incur as much 50 overhead, I suggest these become part of the device name. The naming scheme 51 above leaves scope for further attributes should they be needed. If sections 52 of the name don't apply, just leave that section blank. 53 54 55 Brightness setting API 56 ====================== 57 58 LED subsystem core exposes following API for setting brightness: 59 60 - led_set_brightness : it is guaranteed not to sleep, passing LED_OFF stops 61 blinking, 62 - led_set_brightness_sync : for use cases when immediate effect is desired - 63 it can block the caller for the time required for accessing 64 device registers and can sleep, passing LED_OFF stops hardware 65 blinking, returns -EBUSY if software blink fallback is enabled. 66 67 68 LED registration API 69 ==================== 70 71 A driver wanting to register a LED classdev for use by other drivers / 72 userspace needs to allocate and fill a led_classdev struct and then call 73 [devm_]led_classdev_register. If the non devm version is used the driver 74 must call led_classdev_unregister from its remove function before 75 free-ing the led_classdev struct. 76 77 If the driver can detect hardware initiated brightness changes and thus 78 wants to have a brightness_hw_changed attribute then the LED_BRIGHT_HW_CHANGED 79 flag must be set in flags before registering. Calling 80 led_classdev_notify_brightness_hw_changed on a classdev not registered with 81 the LED_BRIGHT_HW_CHANGED flag is a bug and will trigger a WARN_ON. 82 83 Hardware accelerated blink of LEDs 84 ================================== 85 86 Some LEDs can be programmed to blink without any CPU interaction. To 87 support this feature, a LED driver can optionally implement the 88 blink_set() function (see <linux/leds.h>). To set an LED to blinking, 89 however, it is better to use the API function led_blink_set(), as it 90 will check and implement software fallback if necessary. 91 92 To turn off blinking, use the API function led_brightness_set() 93 with brightness value LED_OFF, which should stop any software 94 timers that may have been required for blinking. 95 96 The blink_set() function should choose a user friendly blinking value 97 if it is called with *delay_on==0 && *delay_off==0 parameters. In this 98 case the driver should give back the chosen value through delay_on and 99 delay_off parameters to the leds subsystem. 100 101 Setting the brightness to zero with brightness_set() callback function 102 should completely turn off the LED and cancel the previously programmed 103 hardware blinking function, if any. 104 105 106 Known Issues 107 ============ 108 109 The LED Trigger core cannot be a module as the simple trigger functions 110 would cause nightmare dependency issues. I see this as a minor issue 111 compared to the benefits the simple trigger functionality brings. The 112 rest of the LED subsystem can be modular. 113 114 115 Future Development 116 ================== 117 118 At the moment, a trigger can't be created specifically for a single LED. 119 There are a number of cases where a trigger might only be mappable to a 120 particular LED (ACPI?). The addition of triggers provided by the LED driver 121 should cover this option and be possible to add without breaking the 122 current interface.