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Documentation / leds / leds-class.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

1	
2	LED handling under Linux
3	========================
4	
5	If you're reading this and thinking about keyboard leds, these are
6	handled by the input subsystem and the led class is *not* needed.
7	
8	In its simplest form, the LED class just allows control of LEDs from
9	userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the
10	LED is defined in max_brightness file. The brightness file will set the brightness
11	of the LED (taking a value 0-max_brightness). Most LEDs don't have hardware
12	brightness support so will just be turned on for non-zero brightness settings.
13	
14	The class also introduces the optional concept of an LED trigger. A trigger
15	is a kernel based source of led events. Triggers can either be simple or
16	complex. A simple trigger isn't configurable and is designed to slot into
17	existing subsystems with minimal additional code. Examples are the ide-disk,
18	nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
19	optimises away.
20	
21	Complex triggers whilst available to all LEDs have LED specific
22	parameters and work on a per LED basis. The timer trigger is an example.
23	The timer trigger will periodically change the LED brightness between
24	LED_OFF and the current brightness setting. The "on" and "off" time can
25	be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds.
26	You can change the brightness value of a LED independently of the timer
27	trigger. However, if you set the brightness value to LED_OFF it will
28	also disable the timer trigger.
29	
30	You can change triggers in a similar manner to the way an IO scheduler
31	is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
32	parameters can appear in /sys/class/leds/<device> once a given trigger is
33	selected.
34	
35	
36	Design Philosophy
37	=================
38	
39	The underlying design philosophy is simplicity. LEDs are simple devices
40	and the aim is to keep a small amount of code giving as much functionality
41	as possible.  Please keep this in mind when suggesting enhancements.
42	
43	
44	LED Device Naming
45	=================
46	
47	Is currently of the form:
48	
49	"devicename:colour:function"
50	
51	There have been calls for LED properties such as colour to be exported as
52	individual led class attributes. As a solution which doesn't incur as much
53	overhead, I suggest these become part of the device name. The naming scheme
54	above leaves scope for further attributes should they be needed. If sections
55	of the name don't apply, just leave that section blank.
56	
57	
58	Hardware accelerated blink of LEDs
59	==================================
60	
61	Some LEDs can be programmed to blink without any CPU interaction. To
62	support this feature, a LED driver can optionally implement the
63	blink_set() function (see <linux/leds.h>). To set an LED to blinking,
64	however, it is better to use the API function led_blink_set(), as it
65	will check and implement software fallback if necessary.
66	
67	To turn off blinking again, use the API function led_brightness_set()
68	as that will not just set the LED brightness but also stop any software
69	timers that may have been required for blinking.
70	
71	The blink_set() function should choose a user friendly blinking value
72	if it is called with *delay_on==0 && *delay_off==0 parameters. In this
73	case the driver should give back the chosen value through delay_on and
74	delay_off parameters to the leds subsystem.
75	
76	Setting the brightness to zero with brightness_set() callback function
77	should completely turn off the LED and cancel the previously programmed
78	hardware blinking function, if any.
79	
80	
81	Known Issues
82	============
83	
84	The LED Trigger core cannot be a module as the simple trigger functions
85	would cause nightmare dependency issues. I see this as a minor issue
86	compared to the benefits the simple trigger functionality brings. The
87	rest of the LED subsystem can be modular.
88	
89	
90	Future Development
91	==================
92	
93	At the moment, a trigger can't be created specifically for a single LED.
94	There are a number of cases where a trigger might only be mappable to a
95	particular LED (ACPI?). The addition of triggers provided by the LED driver
96	should cover this option and be possible to add without breaking the
97	current interface.
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