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Corentin Chary5f634c62009-08-28 12:56:45 +00001
Richard Purdie75c1d312006-03-31 02:31:03 -08002LED 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
Corentin Chary5f634c62009-08-28 12:56:45 +00009userspace. 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.
Richard Purdie75c1d312006-03-31 02:31:03 -080013
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.
Németh Márton0013b232008-03-09 20:54:37 +000023The 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.
Richard Purdie75c1d312006-03-31 02:31:03 -080029
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
Richard Purdie6c152be2007-10-31 15:00:07 +010049"devicename:colour:function"
Richard Purdie75c1d312006-03-31 02:31:03 -080050
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
Richard Purdie6c152be2007-10-31 15:00:07 +010054above leaves scope for further attributes should they be needed. If sections
55of the name don't apply, just leave that section blank.
Richard Purdie75c1d312006-03-31 02:31:03 -080056
57
Márton Németh4c791412007-10-31 15:07:12 +010058Hardware 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
Johannes Berg5ada28b2010-11-11 14:05:21 -080063blink_set() function (see <linux/leds.h>). To set an LED to blinking,
64however, it is better to use use the API function led_blink_set(),
65as it will check and implement software fallback if necessary.
Márton Németh4c791412007-10-31 15:07:12 +010066
Johannes Berg5ada28b2010-11-11 14:05:21 -080067To 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.
Márton Németh4c791412007-10-31 15:07:12 +010075
Németh Márton0013b232008-03-09 20:54:37 +000076Setting 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.
Márton Németh4c791412007-10-31 15:07:12 +010079
80
Richard Purdie75c1d312006-03-31 02:31:03 -080081Known 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
Richard Purdie75c1d312006-03-31 02:31:03 -080089
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