Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 EST.
1 LED handling under Linux 2 ======================== 3 4 If you're reading this and thinking about keyboard leds, these are 5 handled by the input subsystem and the led class is *not* needed. 6 7 In its simplest form, the LED class just allows control of LEDs from 8 userspace. LEDs appear in /sys/class/leds/. The brightness file will 9 set the brightness of the LED (taking a value 0-255). Most LEDs don't 10 have hardware brightness support so will just be turned on for non-zero 11 brightness settings. 12 13 The class also introduces the optional concept of an LED trigger. A trigger 14 is a kernel based source of led events. Triggers can either be simple or 15 complex. A simple trigger isn't configurable and is designed to slot into 16 existing subsystems with minimal additional code. Examples are the ide-disk, 17 nand-disk and sharpsl-charge triggers. With led triggers disabled, the code 18 optimises away. 19 20 Complex triggers whilst available to all LEDs have LED specific 21 parameters and work on a per LED basis. The timer trigger is an example. 22 23 You can change triggers in a similar manner to the way an IO scheduler 24 is chosen (via /sys/class/leds/<device>/trigger). Trigger specific 25 parameters can appear in /sys/class/leds/<device> once a given trigger is 26 selected. 27 28 29 Design Philosophy 30 ================= 31 32 The underlying design philosophy is simplicity. LEDs are simple devices 33 and the aim is to keep a small amount of code giving as much functionality 34 as possible. Please keep this in mind when suggesting enhancements. 35 36 37 LED Device Naming 38 ================= 39 40 Is currently of the form: 41 42 "devicename:colour:function" 43 44 There have been calls for LED properties such as colour to be exported as 45 individual led class attributes. As a solution which doesn't incur as much 46 overhead, I suggest these become part of the device name. The naming scheme 47 above leaves scope for further attributes should they be needed. If sections 48 of the name don't apply, just leave that section blank. 49 50 51 Hardware accelerated blink of LEDs 52 ================================== 53 54 Some LEDs can be programmed to blink without any CPU interaction. To 55 support this feature, a LED driver can optionally implement the 56 blink_set() function (see <linux/leds.h>). If implemeted, triggers can 57 attempt to use it before falling back to software timers. The blink_set() 58 function should return 0 if the blink setting is supported, or -EINVAL 59 otherwise, which means that LED blinking will be handled by software. 60 61 The blink_set() function should choose a user friendly blinking 62 value if it is called with *delay_on==0 && *delay_off==0 parameters. In 63 this case the driver should give back the chosen value through delay_on 64 and delay_off parameters to the leds subsystem. 65 66 Any call to the brightness_set() callback function should cancel the 67 previously programmed hardware blinking function so setting the brightness 68 to 0 can also cancel the blinking of the LED. 69 70 71 Known Issues 72 ============ 73 74 The LED Trigger core cannot be a module as the simple trigger functions 75 would cause nightmare dependency issues. I see this as a minor issue 76 compared to the benefits the simple trigger functionality brings. The 77 rest of the LED subsystem can be modular. 78 79 80 Future Development 81 ================== 82 83 At the moment, a trigger can't be created specifically for a single LED. 84 There are a number of cases where a trigger might only be mappable to a 85 particular LED (ACPI?). The addition of triggers provided by the LED driver 86 should cover this option and be possible to add without breaking the 87 current interface.
