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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.
Németh Márton0013b232008-03-09 20:54:37 +000022The timer trigger will periodically change the LED brightness between
23LED_OFF and the current brightness setting. The "on" and "off" time can
24be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds.
25You can change the brightness value of a LED independently of the timer
26trigger. However, if you set the brightness value to LED_OFF it will
27also disable the timer trigger.
Richard Purdie75c1d312006-03-31 02:31:03 -080028
29You can change triggers in a similar manner to the way an IO scheduler
30is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
31parameters can appear in /sys/class/leds/<device> once a given trigger is
32selected.
33
34
35Design Philosophy
36=================
37
38The underlying design philosophy is simplicity. LEDs are simple devices
39and the aim is to keep a small amount of code giving as much functionality
40as possible. Please keep this in mind when suggesting enhancements.
41
42
43LED Device Naming
44=================
45
46Is currently of the form:
47
Richard Purdie6c152be2007-10-31 15:00:07 +010048"devicename:colour:function"
Richard Purdie75c1d312006-03-31 02:31:03 -080049
50There have been calls for LED properties such as colour to be exported as
51individual led class attributes. As a solution which doesn't incur as much
52overhead, I suggest these become part of the device name. The naming scheme
Richard Purdie6c152be2007-10-31 15:00:07 +010053above leaves scope for further attributes should they be needed. If sections
54of the name don't apply, just leave that section blank.
Richard Purdie75c1d312006-03-31 02:31:03 -080055
56
Márton Németh4c791412007-10-31 15:07:12 +010057Hardware accelerated blink of LEDs
58==================================
59
60Some LEDs can be programmed to blink without any CPU interaction. To
61support this feature, a LED driver can optionally implement the
62blink_set() function (see <linux/leds.h>). If implemeted, triggers can
63attempt to use it before falling back to software timers. The blink_set()
64function should return 0 if the blink setting is supported, or -EINVAL
65otherwise, which means that LED blinking will be handled by software.
66
67The blink_set() function should choose a user friendly blinking
68value if it is called with *delay_on==0 && *delay_off==0 parameters. In
69this case the driver should give back the chosen value through delay_on
70and delay_off parameters to the leds subsystem.
71
Németh Márton0013b232008-03-09 20:54:37 +000072Setting the brightness to zero with brightness_set() callback function
73should completely turn off the LED and cancel the previously programmed
74hardware blinking function, if any.
Márton Németh4c791412007-10-31 15:07:12 +010075
76
Richard Purdie75c1d312006-03-31 02:31:03 -080077Known Issues
78============
79
80The LED Trigger core cannot be a module as the simple trigger functions
81would cause nightmare dependency issues. I see this as a minor issue
82compared to the benefits the simple trigger functionality brings. The
83rest of the LED subsystem can be modular.
84
Richard Purdie75c1d312006-03-31 02:31:03 -080085
86Future Development
87==================
88
89At the moment, a trigger can't be created specifically for a single LED.
90There are a number of cases where a trigger might only be mappable to a
91particular LED (ACPI?). The addition of triggers provided by the LED driver
92should cover this option and be possible to add without breaking the
93current interface.
94