tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
1LED 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:function"
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. If sections
48of the name don't apply, just leave that section blank.
49
50
51Hardware accelerated blink of LEDs
52==================================
53
54Some LEDs can be programmed to blink without any CPU interaction. To
55support this feature, a LED driver can optionally implement the
56blink_set() function (see <linux/leds.h>). If implemeted, triggers can
57attempt to use it before falling back to software timers. The blink_set()
58function should return 0 if the blink setting is supported, or -EINVAL
59otherwise, which means that LED blinking will be handled by software.
60
61The blink_set() function should choose a user friendly blinking
62value if it is called with *delay_on==0 && *delay_off==0 parameters. In
63this case the driver should give back the chosen value through delay_on
64and delay_off parameters to the leds subsystem.
65
66Any call to the brightness_set() callback function should cancel the
67previously programmed hardware blinking function so setting the brightness
68to 0 can also cancel the blinking of the LED.
69
70
71Known Issues
72============
73
74The LED Trigger core cannot be a module as the simple trigger functions
75would cause nightmare dependency issues. I see this as a minor issue
76compared to the benefits the simple trigger functionality brings. The
77rest of the LED subsystem can be modular.
78
79
80Future Development
81==================
82
83At the moment, a trigger can't be created specifically for a single LED.
84There are a number of cases where a trigger might only be mappable to a
85particular LED (ACPI?). The addition of triggers provided by the LED driver
86should cover this option and be possible to add without breaking the
87current interface.
88