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Richard Purdie75c1d312006-03-31 02:31:03 -08001LED handling under Linux
2========================
3
4If you're reading this and thinking about keyboard leds, these are
5handled by the input subsystem and the led class is *not* needed.
6
7In its simplest form, the LED class just allows control of LEDs from
8userspace. LEDs appear in /sys/class/leds/. The brightness file will
9set the brightness of the LED (taking a value 0-255). Most LEDs don't
10have hardware brightness support so will just be turned on for non-zero
11brightness settings.
12
13The class also introduces the optional concept of an LED trigger. A trigger
14is a kernel based source of led events. Triggers can either be simple or
15complex. A simple trigger isn't configurable and is designed to slot into
16existing subsystems with minimal additional code. Examples are the ide-disk,
17nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
18optimises away.
19
20Complex triggers whilst available to all LEDs have LED specific
21parameters and work on a per LED basis. The timer trigger is an example.
22
23You can change triggers in a similar manner to the way an IO scheduler
24is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
25parameters can appear in /sys/class/leds/<device> once a given trigger is
26selected.
27
28
29Design Philosophy
30=================
31
32The underlying design philosophy is simplicity. LEDs are simple devices
33and the aim is to keep a small amount of code giving as much functionality
34as possible. Please keep this in mind when suggesting enhancements.
35
36
37LED Device Naming
38=================
39
40Is currently of the form:
41
42"devicename:colour"
43
44There have been calls for LED properties such as colour to be exported as
45individual led class attributes. As a solution which doesn't incur as much
46overhead, I suggest these become part of the device name. The naming scheme
47above leaves scope for further attributes should they be needed.
48
49
50Known Issues
51============
52
53The LED Trigger core cannot be a module as the simple trigger functions
54would cause nightmare dependency issues. I see this as a minor issue
55compared to the benefits the simple trigger functionality brings. The
56rest of the LED subsystem can be modular.
57
58Some leds can be programmed to flash in hardware. As this isn't a generic
59LED device property, this should be exported as a device specific sysfs
60attribute rather than part of the class if this functionality is required.
61
62
63Future Development
64==================
65
66At the moment, a trigger can't be created specifically for a single LED.
67There are a number of cases where a trigger might only be mappable to a
68particular LED (ACPI?). The addition of triggers provided by the LED driver
69should cover this option and be possible to add without breaking the
70current interface.
71