1========================
2LED handling under Linux
3========================
4
5In its simplest form, the LED class just allows control of LEDs from
6userspace. 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.
10
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 disk-activity,
15nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
16optimises away.
17
18Complex triggers while available to all LEDs have LED specific
19parameters and work on a per LED basis. The timer trigger is an example.
20The 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.
26
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
46	"devicename:colour:function"
47
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
51above leaves scope for further attributes should they be needed. If sections
52of the name don't apply, just leave that section blank.
53
54
55Brightness setting API
56======================
57
58LED subsystem core exposes following API for setting brightness:
59
60    - led_set_brightness:
61		it is guaranteed not to sleep, passing LED_OFF stops
62		blinking,
63
64    - led_set_brightness_sync:
65		for use cases when immediate effect is desired -
66		it can block the caller for the time required for accessing
67		device registers and can sleep, passing LED_OFF stops hardware
68		blinking, returns -EBUSY if software blink fallback is enabled.
69
70
71LED registration API
72====================
73
74A driver wanting to register a LED classdev for use by other drivers /
75userspace needs to allocate and fill a led_classdev struct and then call
76`[devm_]led_classdev_register`. If the non devm version is used the driver
77must call led_classdev_unregister from its remove function before
78free-ing the led_classdev struct.
79
80If the driver can detect hardware initiated brightness changes and thus
81wants to have a brightness_hw_changed attribute then the LED_BRIGHT_HW_CHANGED
82flag must be set in flags before registering. Calling
83led_classdev_notify_brightness_hw_changed on a classdev not registered with
84the LED_BRIGHT_HW_CHANGED flag is a bug and will trigger a WARN_ON.
85
86Hardware accelerated blink of LEDs
87==================================
88
89Some LEDs can be programmed to blink without any CPU interaction. To
90support this feature, a LED driver can optionally implement the
91blink_set() function (see <linux/leds.h>). To set an LED to blinking,
92however, it is better to use the API function led_blink_set(), as it
93will check and implement software fallback if necessary.
94
95To turn off blinking, use the API function led_brightness_set()
96with brightness value LED_OFF, which should stop any software
97timers that may have been required for blinking.
98
99The blink_set() function should choose a user friendly blinking value
100if it is called with `*delay_on==0` && `*delay_off==0` parameters. In this
101case the driver should give back the chosen value through delay_on and
102delay_off parameters to the leds subsystem.
103
104Setting the brightness to zero with brightness_set() callback function
105should completely turn off the LED and cancel the previously programmed
106hardware blinking function, if any.
107
108
109Known Issues
110============
111
112The LED Trigger core cannot be a module as the simple trigger functions
113would cause nightmare dependency issues. I see this as a minor issue
114compared to the benefits the simple trigger functionality brings. The
115rest of the LED subsystem can be modular.
116
117
118Future Development
119==================
120
121At the moment, a trigger can't be created specifically for a single LED.
122There are a number of cases where a trigger might only be mappable to a
123particular LED (ACPI?). The addition of triggers provided by the LED driver
124should cover this option and be possible to add without breaking the
125current interface.
126