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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
Richard Purdie75c1d312006-03-31 02:31:03 -08005In its simplest form, the LED class just allows control of LEDs from
Corentin Chary5f634c62009-08-28 12:56:45 +00006userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the
7LED is defined in max_brightness file. The brightness file will set the brightness
8of the LED (taking a value 0-max_brightness). Most LEDs don't have hardware
9brightness support so will just be turned on for non-zero brightness settings.
Richard Purdie75c1d312006-03-31 02:31:03 -080010
11The class also introduces the optional concept of an LED trigger. A trigger
12is a kernel based source of led events. Triggers can either be simple or
13complex. A simple trigger isn't configurable and is designed to slot into
14existing subsystems with minimal additional code. Examples are the ide-disk,
15nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
16optimises away.
17
18Complex triggers whilst available to all LEDs have LED specific
19parameters and work on a per LED basis. The timer trigger is an example.
Németh Márton0013b232008-03-09 20:54:37 +000020The timer trigger will periodically change the LED brightness between
21LED_OFF and the current brightness setting. The "on" and "off" time can
22be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds.
23You can change the brightness value of a LED independently of the timer
24trigger. However, if you set the brightness value to LED_OFF it will
25also disable the timer trigger.
Richard Purdie75c1d312006-03-31 02:31:03 -080026
27You can change triggers in a similar manner to the way an IO scheduler
28is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
29parameters can appear in /sys/class/leds/<device> once a given trigger is
30selected.
31
32
33Design Philosophy
34=================
35
36The underlying design philosophy is simplicity. LEDs are simple devices
37and the aim is to keep a small amount of code giving as much functionality
38as possible. Please keep this in mind when suggesting enhancements.
39
40
41LED Device Naming
42=================
43
44Is currently of the form:
45
Richard Purdie6c152be2007-10-31 15:00:07 +010046"devicename:colour:function"
Richard Purdie75c1d312006-03-31 02:31:03 -080047
48There have been calls for LED properties such as colour to be exported as
49individual led class attributes. As a solution which doesn't incur as much
50overhead, I suggest these become part of the device name. The naming scheme
Richard Purdie6c152be2007-10-31 15:00:07 +010051above leaves scope for further attributes should they be needed. If sections
52of the name don't apply, just leave that section blank.
Richard Purdie75c1d312006-03-31 02:31:03 -080053
54
Jacek Anaszewski648da8f2015-10-07 11:10:44 +020055Brightness setting API
56======================
57
58LED subsystem core exposes following API for setting brightness:
59
60 - led_set_brightness : it is guaranteed not to sleep, passing LED_OFF stops
61 blinking,
62 - led_set_brightness_sync : for use cases when immediate effect is desired -
63 it can block the caller for the time required for accessing
64 device registers and can sleep, passing LED_OFF stops hardware
65 blinking, returns -EBUSY if software blink fallback is enabled.
66
67
Márton Németh4c791412007-10-31 15:07:12 +010068Hardware accelerated blink of LEDs
69==================================
70
71Some LEDs can be programmed to blink without any CPU interaction. To
72support this feature, a LED driver can optionally implement the
Johannes Berg5ada28b2010-11-11 14:05:21 -080073blink_set() function (see <linux/leds.h>). To set an LED to blinking,
Bryan Wuee318922011-11-04 11:22:29 -070074however, it is better to use the API function led_blink_set(), as it
75will check and implement software fallback if necessary.
Márton Németh4c791412007-10-31 15:07:12 +010076
Johannes Berg5ada28b2010-11-11 14:05:21 -080077To turn off blinking again, use the API function led_brightness_set()
78as that will not just set the LED brightness but also stop any software
79timers that may have been required for blinking.
80
81The blink_set() function should choose a user friendly blinking value
82if it is called with *delay_on==0 && *delay_off==0 parameters. In this
83case the driver should give back the chosen value through delay_on and
84delay_off parameters to the leds subsystem.
Márton Németh4c791412007-10-31 15:07:12 +010085
Németh Márton0013b232008-03-09 20:54:37 +000086Setting the brightness to zero with brightness_set() callback function
87should completely turn off the LED and cancel the previously programmed
88hardware blinking function, if any.
Márton Németh4c791412007-10-31 15:07:12 +010089
90
Richard Purdie75c1d312006-03-31 02:31:03 -080091Known Issues
92============
93
94The LED Trigger core cannot be a module as the simple trigger functions
95would cause nightmare dependency issues. I see this as a minor issue
96compared to the benefits the simple trigger functionality brings. The
97rest of the LED subsystem can be modular.
98
Richard Purdie75c1d312006-03-31 02:31:03 -080099
100Future Development
101==================
102
103At the moment, a trigger can't be created specifically for a single LED.
104There are a number of cases where a trigger might only be mappable to a
105particular LED (ACPI?). The addition of triggers provided by the LED driver
106should cover this option and be possible to add without breaking the
107current interface.