1 2LED handling under Linux 3======================== 4 5If you're reading this and thinking about keyboard leds, these are 6handled by the input subsystem and the led class is *not* needed. 7 8In its simplest form, the LED class just allows control of LEDs from 9userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the 10LED is defined in max_brightness file. The brightness file will set the brightness 11of the LED (taking a value 0-max_brightness). Most LEDs don't have hardware 12brightness support so will just be turned on for non-zero brightness settings. 13 14The class also introduces the optional concept of an LED trigger. A trigger 15is a kernel based source of led events. Triggers can either be simple or 16complex. A simple trigger isn't configurable and is designed to slot into 17existing subsystems with minimal additional code. Examples are the ide-disk, 18nand-disk and sharpsl-charge triggers. With led triggers disabled, the code 19optimises away. 20 21Complex triggers whilst available to all LEDs have LED specific 22parameters and work on a per LED basis. The timer trigger is an example. 23The timer trigger will periodically change the LED brightness between 24LED_OFF and the current brightness setting. The "on" and "off" time can 25be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds. 26You can change the brightness value of a LED independently of the timer 27trigger. However, if you set the brightness value to LED_OFF it will 28also disable the timer trigger. 29 30You can change triggers in a similar manner to the way an IO scheduler 31is chosen (via /sys/class/leds/<device>/trigger). Trigger specific 32parameters can appear in /sys/class/leds/<device> once a given trigger is 33selected. 34 35 36Design Philosophy 37================= 38 39The underlying design philosophy is simplicity. LEDs are simple devices 40and the aim is to keep a small amount of code giving as much functionality 41as possible. Please keep this in mind when suggesting enhancements. 42 43 44LED Device Naming 45================= 46 47Is currently of the form: 48 49"devicename:colour:function" 50 51There have been calls for LED properties such as colour to be exported as 52individual led class attributes. As a solution which doesn't incur as much 53overhead, I suggest these become part of the device name. The naming scheme 54above leaves scope for further attributes should they be needed. If sections 55of the name don't apply, just leave that section blank. 56 57 58Hardware accelerated blink of LEDs 59================================== 60 61Some LEDs can be programmed to blink without any CPU interaction. To 62support this feature, a LED driver can optionally implement the 63blink_set() function (see <linux/leds.h>). To set an LED to blinking, 64however, it is better to use the API function led_blink_set(), as it 65will check and implement software fallback if necessary. 66 67To turn off blinking again, use the API function led_brightness_set() 68as that will not just set the LED brightness but also stop any software 69timers that may have been required for blinking. 70 71The blink_set() function should choose a user friendly blinking value 72if it is called with *delay_on==0 && *delay_off==0 parameters. In this 73case the driver should give back the chosen value through delay_on and 74delay_off parameters to the leds subsystem. 75 76Setting the brightness to zero with brightness_set() callback function 77should completely turn off the LED and cancel the previously programmed 78hardware blinking function, if any. 79 80 81Known Issues 82============ 83 84The LED Trigger core cannot be a module as the simple trigger functions 85would cause nightmare dependency issues. I see this as a minor issue 86compared to the benefits the simple trigger functionality brings. The 87rest of the LED subsystem can be modular. 88 89 90Future Development 91================== 92 93At the moment, a trigger can't be created specifically for a single LED. 94There are a number of cases where a trigger might only be mappable to a 95particular LED (ACPI?). The addition of triggers provided by the LED driver 96should cover this option and be possible to add without breaking the 97current interface. 98