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:color:function"
47
48- devicename:
49        it should refer to a unique identifier created by the kernel,
50        like e.g. phyN for network devices or inputN for input devices, rather
51        than to the hardware; the information related to the product and the bus
52        to which given device is hooked is available in sysfs and can be
53        retrieved using get_led_device_info.sh script from tools/leds; generally
54        this section is expected mostly for LEDs that are somehow associated with
55        other devices.
56
57- color:
58        one of LED_COLOR_ID_* definitions from the header
59        include/dt-bindings/leds/common.h.
60
61- function:
62        one of LED_FUNCTION_* definitions from the header
63        include/dt-bindings/leds/common.h.
64
65If required color or function is missing, please submit a patch
66to linux-leds@vger.kernel.org.
67
68It is possible that more than one LED with the same color and function will
69be required for given platform, differing only with an ordinal number.
70In this case it is preferable to just concatenate the predefined LED_FUNCTION_*
71name with required "-N" suffix in the driver. fwnode based drivers can use
72function-enumerator property for that and then the concatenation will be handled
73automatically by the LED core upon LED class device registration.
74
75LED subsystem has also a protection against name clash, that may occur
76when LED class device is created by a driver of hot-pluggable device and
77it doesn't provide unique devicename section. In this case numerical
78suffix (e.g. "_1", "_2", "_3" etc.) is added to the requested LED class
79device name.
80
81There might be still LED class drivers around using vendor or product name
82for devicename, but this approach is now deprecated as it doesn't convey
83any added value. Product information can be found in other places in sysfs
84(see tools/leds/get_led_device_info.sh).
85
86Examples of proper LED names:
87
88  - "red:disk"
89  - "white:flash"
90  - "red:indicator"
91  - "phy1:green:wlan"
92  - "phy3::wlan"
93  - ":kbd_backlight"
94  - "input5::kbd_backlight"
95  - "input3::numlock"
96  - "input3::scrolllock"
97  - "input3::capslock"
98  - "mmc1::status"
99  - "white:status"
100
101get_led_device_info.sh script can be used for verifying if the LED name
102meets the requirements pointed out here. It performs validation of the LED class
103devicename sections and gives hints on expected value for a section in case
104the validation fails for it. So far the script supports validation
105of associations between LEDs and following types of devices:
106
107        - input devices
108        - ieee80211 compliant USB devices
109
110The script is open to extensions.
111
112There have been calls for LED properties such as color to be exported as
113individual led class attributes. As a solution which doesn't incur as much
114overhead, I suggest these become part of the device name. The naming scheme
115above leaves scope for further attributes should they be needed. If sections
116of the name don't apply, just leave that section blank.
117
118
119Brightness setting API
120======================
121
122LED subsystem core exposes following API for setting brightness:
123
124    - led_set_brightness:
125		it is guaranteed not to sleep, passing LED_OFF stops
126		blinking,
127
128    - led_set_brightness_sync:
129		for use cases when immediate effect is desired -
130		it can block the caller for the time required for accessing
131		device registers and can sleep, passing LED_OFF stops hardware
132		blinking, returns -EBUSY if software blink fallback is enabled.
133
134
135LED registration API
136====================
137
138A driver wanting to register a LED classdev for use by other drivers /
139userspace needs to allocate and fill a led_classdev struct and then call
140`[devm_]led_classdev_register`. If the non devm version is used the driver
141must call led_classdev_unregister from its remove function before
142free-ing the led_classdev struct.
143
144If the driver can detect hardware initiated brightness changes and thus
145wants to have a brightness_hw_changed attribute then the LED_BRIGHT_HW_CHANGED
146flag must be set in flags before registering. Calling
147led_classdev_notify_brightness_hw_changed on a classdev not registered with
148the LED_BRIGHT_HW_CHANGED flag is a bug and will trigger a WARN_ON.
149
150Hardware accelerated blink of LEDs
151==================================
152
153Some LEDs can be programmed to blink without any CPU interaction. To
154support this feature, a LED driver can optionally implement the
155blink_set() function (see <linux/leds.h>). To set an LED to blinking,
156however, it is better to use the API function led_blink_set(), as it
157will check and implement software fallback if necessary.
158
159To turn off blinking, use the API function led_brightness_set()
160with brightness value LED_OFF, which should stop any software
161timers that may have been required for blinking.
162
163The blink_set() function should choose a user friendly blinking value
164if it is called with `*delay_on==0` && `*delay_off==0` parameters. In this
165case the driver should give back the chosen value through delay_on and
166delay_off parameters to the leds subsystem.
167
168Setting the brightness to zero with brightness_set() callback function
169should completely turn off the LED and cancel the previously programmed
170hardware blinking function, if any.
171
172
173Known Issues
174============
175
176The LED Trigger core cannot be a module as the simple trigger functions
177would cause nightmare dependency issues. I see this as a minor issue
178compared to the benefits the simple trigger functionality brings. The
179rest of the LED subsystem can be modular.
180
181
182Future Development
183==================
184
185At the moment, a trigger can't be created specifically for a single LED.
186There are a number of cases where a trigger might only be mappable to a
187particular LED (ACPI?). The addition of triggers provided by the LED driver
188should cover this option and be possible to add without breaking the
189current interface.
190