Root/Documentation/leds-class.txt

1
2LED handling under Linux
3========================
4
5If you're reading this and thinking about keyboard leds, these are
6handled by the input subsystem and the led class is *not* needed.
7
8In its simplest form, the LED class just allows control of LEDs from
9userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the
10LED is defined in max_brightness file. The brightness file will set the brightness
11of the LED (taking a value 0-max_brightness). Most LEDs don't have hardware
12brightness support so will just be turned on for non-zero brightness settings.
13
14The class also introduces the optional concept of an LED trigger. A trigger
15is a kernel based source of led events. Triggers can either be simple or
16complex. A simple trigger isn't configurable and is designed to slot into
17existing subsystems with minimal additional code. Examples are the ide-disk,
18nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
19optimises away.
20
21Complex triggers whilst available to all LEDs have LED specific
22parameters and work on a per LED basis. The timer trigger is an example.
23The timer trigger will periodically change the LED brightness between
24LED_OFF and the current brightness setting. The "on" and "off" time can
25be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds.
26You can change the brightness value of a LED independently of the timer
27trigger. However, if you set the brightness value to LED_OFF it will
28also disable the timer trigger.
29
30You can change triggers in a similar manner to the way an IO scheduler
31is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
32parameters can appear in /sys/class/leds/<device> once a given trigger is
33selected.
34
35
36Design Philosophy
37=================
38
39The underlying design philosophy is simplicity. LEDs are simple devices
40and the aim is to keep a small amount of code giving as much functionality
41as possible. Please keep this in mind when suggesting enhancements.
42
43
44LED Device Naming
45=================
46
47Is currently of the form:
48
49"devicename:colour:function"
50
51There have been calls for LED properties such as colour to be exported as
52individual led class attributes. As a solution which doesn't incur as much
53overhead, I suggest these become part of the device name. The naming scheme
54above leaves scope for further attributes should they be needed. If sections
55of the name don't apply, just leave that section blank.
56
57
58Hardware accelerated blink of LEDs
59==================================
60
61Some LEDs can be programmed to blink without any CPU interaction. To
62support this feature, a LED driver can optionally implement the
63blink_set() function (see <linux/leds.h>). To set an LED to blinking,
64however, it is better to use use the API function led_blink_set(),
65as it will check and implement software fallback if necessary.
66
67To turn off blinking again, use the API function led_brightness_set()
68as that will not just set the LED brightness but also stop any software
69timers that may have been required for blinking.
70
71The blink_set() function should choose a user friendly blinking value
72if it is called with *delay_on==0 && *delay_off==0 parameters. In this
73case the driver should give back the chosen value through delay_on and
74delay_off parameters to the leds subsystem.
75
76Setting the brightness to zero with brightness_set() callback function
77should completely turn off the LED and cancel the previously programmed
78hardware blinking function, if any.
79
80
81Known Issues
82============
83
84The LED Trigger core cannot be a module as the simple trigger functions
85would cause nightmare dependency issues. I see this as a minor issue
86compared to the benefits the simple trigger functionality brings. The
87rest of the LED subsystem can be modular.
88
89
90Future Development
91==================
92
93At the moment, a trigger can't be created specifically for a single LED.
94There are a number of cases where a trigger might only be mappable to a
95particular LED (ACPI?). The addition of triggers provided by the LED driver
96should cover this option and be possible to add without breaking the
97current interface.
98
99

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