About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / leds / leds-class.txt

Custom Search

Based on kernel version 4.9. Page generated on 2016-12-21 14:35 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	Hardware accelerated blink of LEDs
69	==================================
71	Some LEDs can be programmed to blink without any CPU interaction. To
72	support this feature, a LED driver can optionally implement the
73	blink_set() function (see <linux/leds.h>). To set an LED to blinking,
74	however, it is better to use the API function led_blink_set(), as it
75	will check and implement software fallback if necessary.
77	To turn off blinking, use the API function led_brightness_set()
78	with brightness value LED_OFF, which should stop any software
79	timers that may have been required for blinking.
81	The blink_set() function should choose a user friendly blinking value
82	if it is called with *delay_on==0 && *delay_off==0 parameters. In this
83	case the driver should give back the chosen value through delay_on and
84	delay_off parameters to the leds subsystem.
86	Setting the brightness to zero with brightness_set() callback function
87	should completely turn off the LED and cancel the previously programmed
88	hardware blinking function, if any.
91	Known Issues
92	============
94	The LED Trigger core cannot be a module as the simple trigger functions
95	would cause nightmare dependency issues. I see this as a minor issue
96	compared to the benefits the simple trigger functionality brings. The
97	rest of the LED subsystem can be modular.
100	Future Development
101	==================
103	At the moment, a trigger can't be created specifically for a single LED.
104	There are a number of cases where a trigger might only be mappable to a
105	particular LED (ACPI?). The addition of triggers provided by the LED driver
106	should cover this option and be possible to add without breaking the
107	current interface.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.