Commit | Line | Data |
---|---|---|
75c1d31d RP |
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. | |
0013b23d NM |
22 | The timer trigger will periodically change the LED brightness between |
23 | LED_OFF and the current brightness setting. The "on" and "off" time can | |
24 | be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds. | |
25 | You can change the brightness value of a LED independently of the timer | |
26 | trigger. However, if you set the brightness value to LED_OFF it will | |
27 | also disable the timer trigger. | |
75c1d31d RP |
28 | |
29 | You can change triggers in a similar manner to the way an IO scheduler | |
30 | is chosen (via /sys/class/leds/<device>/trigger). Trigger specific | |
31 | parameters can appear in /sys/class/leds/<device> once a given trigger is | |
32 | selected. | |
33 | ||
34 | ||
35 | Design Philosophy | |
36 | ================= | |
37 | ||
38 | The underlying design philosophy is simplicity. LEDs are simple devices | |
39 | and the aim is to keep a small amount of code giving as much functionality | |
40 | as possible. Please keep this in mind when suggesting enhancements. | |
41 | ||
42 | ||
43 | LED Device Naming | |
44 | ================= | |
45 | ||
46 | Is currently of the form: | |
47 | ||
6c152bee | 48 | "devicename:colour:function" |
75c1d31d RP |
49 | |
50 | There have been calls for LED properties such as colour to be exported as | |
51 | individual led class attributes. As a solution which doesn't incur as much | |
52 | overhead, I suggest these become part of the device name. The naming scheme | |
6c152bee RP |
53 | above leaves scope for further attributes should they be needed. If sections |
54 | of the name don't apply, just leave that section blank. | |
75c1d31d RP |
55 | |
56 | ||
4c79141d MN |
57 | Hardware accelerated blink of LEDs |
58 | ================================== | |
59 | ||
60 | Some LEDs can be programmed to blink without any CPU interaction. To | |
61 | support this feature, a LED driver can optionally implement the | |
62 | blink_set() function (see <linux/leds.h>). If implemeted, triggers can | |
63 | attempt to use it before falling back to software timers. The blink_set() | |
64 | function should return 0 if the blink setting is supported, or -EINVAL | |
65 | otherwise, which means that LED blinking will be handled by software. | |
66 | ||
67 | The blink_set() function should choose a user friendly blinking | |
68 | value if it is called with *delay_on==0 && *delay_off==0 parameters. In | |
69 | this case the driver should give back the chosen value through delay_on | |
70 | and delay_off parameters to the leds subsystem. | |
71 | ||
0013b23d NM |
72 | Setting the brightness to zero with brightness_set() callback function |
73 | should completely turn off the LED and cancel the previously programmed | |
74 | hardware blinking function, if any. | |
4c79141d MN |
75 | |
76 | ||
75c1d31d RP |
77 | Known Issues |
78 | ============ | |
79 | ||
80 | The LED Trigger core cannot be a module as the simple trigger functions | |
81 | would cause nightmare dependency issues. I see this as a minor issue | |
82 | compared to the benefits the simple trigger functionality brings. The | |
83 | rest of the LED subsystem can be modular. | |
84 | ||
75c1d31d RP |
85 | |
86 | Future Development | |
87 | ================== | |
88 | ||
89 | At the moment, a trigger can't be created specifically for a single LED. | |
90 | There are a number of cases where a trigger might only be mappable to a | |
91 | particular LED (ACPI?). The addition of triggers provided by the LED driver | |
92 | should cover this option and be possible to add without breaking the | |
93 | current interface. | |
94 |