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Documentation / leds / leds-class.txt

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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

2	LED handling under Linux
3	========================
5	In its simplest form, the LED class just allows control of LEDs from
6	userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the
7	LED is defined in max_brightness file. The brightness file will set the brightness
8	of the LED (taking a value 0-max_brightness). Most LEDs don't have hardware
9	brightness support so will just be turned on for non-zero brightness settings.
11	The class also introduces the optional concept of an LED trigger. A trigger
12	is a kernel based source of led events. Triggers can either be simple or
13	complex. A simple trigger isn't configurable and is designed to slot into
14	existing subsystems with minimal additional code. Examples are the disk-activity,
15	nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
16	optimises away.
18	Complex triggers whilst available to all LEDs have LED specific
19	parameters and work on a per LED basis. The timer trigger is an example.
20	The timer trigger will periodically change the LED brightness between
21	LED_OFF and the current brightness setting. The "on" and "off" time can
22	be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds.
23	You can change the brightness value of a LED independently of the timer
24	trigger. However, if you set the brightness value to LED_OFF it will
25	also disable the timer trigger.
27	You can change triggers in a similar manner to the way an IO scheduler
28	is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
29	parameters can appear in /sys/class/leds/<device> once a given trigger is
30	selected.
33	Design Philosophy
34	=================
36	The underlying design philosophy is simplicity. LEDs are simple devices
37	and the aim is to keep a small amount of code giving as much functionality
38	as possible.  Please keep this in mind when suggesting enhancements.
41	LED Device Naming
42	=================
44	Is currently of the form:
46	"devicename:colour:function"
48	There have been calls for LED properties such as colour to be exported as
49	individual led class attributes. As a solution which doesn't incur as much
50	overhead, I suggest these become part of the device name. The naming scheme
51	above leaves scope for further attributes should they be needed. If sections
52	of the name don't apply, just leave that section blank.
55	Brightness setting API
56	======================
58	LED subsystem core exposes following API for setting brightness:
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.
68	LED registration API
69	====================
71	A driver wanting to register a LED classdev for use by other drivers /
72	userspace needs to allocate and fill a led_classdev struct and then call
73	[devm_]led_classdev_register. If the non devm version is used the driver
74	must call led_classdev_unregister from its remove function before
75	free-ing the led_classdev struct.
77	If the driver can detect hardware initiated brightness changes and thus
78	wants to have a brightness_hw_changed attribute then the LED_BRIGHT_HW_CHANGED
79	flag must be set in flags before registering. Calling
80	led_classdev_notify_brightness_hw_changed on a classdev not registered with
81	the LED_BRIGHT_HW_CHANGED flag is a bug and will trigger a WARN_ON.
83	Hardware accelerated blink of LEDs
84	==================================
86	Some LEDs can be programmed to blink without any CPU interaction. To
87	support this feature, a LED driver can optionally implement the
88	blink_set() function (see <linux/leds.h>). To set an LED to blinking,
89	however, it is better to use the API function led_blink_set(), as it
90	will check and implement software fallback if necessary.
92	To turn off blinking, use the API function led_brightness_set()
93	with brightness value LED_OFF, which should stop any software
94	timers that may have been required for blinking.
96	The blink_set() function should choose a user friendly blinking value
97	if it is called with *delay_on==0 && *delay_off==0 parameters. In this
98	case the driver should give back the chosen value through delay_on and
99	delay_off parameters to the leds subsystem.
101	Setting the brightness to zero with brightness_set() callback function
102	should completely turn off the LED and cancel the previously programmed
103	hardware blinking function, if any.
106	Known Issues
107	============
109	The LED Trigger core cannot be a module as the simple trigger functions
110	would cause nightmare dependency issues. I see this as a minor issue
111	compared to the benefits the simple trigger functionality brings. The
112	rest of the LED subsystem can be modular.
115	Future Development
116	==================
118	At the moment, a trigger can't be created specifically for a single LED.
119	There are a number of cases where a trigger might only be mappable to a
120	particular LED (ACPI?). The addition of triggers provided by the LED driver
121	should cover this option and be possible to add without breaking the
122	current interface.
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