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
172Hardware driven LEDs
173====================
174
175Some LEDs can be programmed to be driven by hardware. This is not
176limited to blink but also to turn off or on autonomously.
177To support this feature, a LED needs to implement various additional
178ops and needs to declare specific support for the supported triggers.
179
180With hw control we refer to the LED driven by hardware.
181
182LED driver must define the following value to support hw control:
183
184    - hw_control_trigger:
185               unique trigger name supported by the LED in hw control
186               mode.
187
188LED driver must implement the following API to support hw control:
189    - hw_control_is_supported:
190                check if the flags passed by the supported trigger can
191                be parsed and activate hw control on the LED.
192
193                Return 0 if the passed flags mask is supported and
194                can be set with hw_control_set().
195
196                If the passed flags mask is not supported -EOPNOTSUPP
197                must be returned, the LED trigger will use software
198                fallback in this case.
199
200                Return a negative error in case of any other error like
201                device not ready or timeouts.
202
203     - hw_control_set:
204                activate hw control. LED driver will use the provided
205                flags passed from the supported trigger, parse them to
206                a set of mode and setup the LED to be driven by hardware
207                following the requested modes.
208
209                Set LED_OFF via the brightness_set to deactivate hw control.
210
211                Return 0 on success, a negative error number on failing to
212                apply flags.
213
214    - hw_control_get:
215                get active modes from a LED already in hw control, parse
216                them and set in flags the current active flags for the
217                supported trigger.
218
219                Return 0 on success, a negative error number on failing
220                parsing the initial mode.
221                Error from this function is NOT FATAL as the device may
222                be in a not supported initial state by the attached LED
223                trigger.
224
225    - hw_control_get_device:
226                return the device associated with the LED driver in
227                hw control. A trigger might use this to match the
228                returned device from this function with a configured
229                device for the trigger as the source for blinking
230                events and correctly enable hw control.
231                (example a netdev trigger configured to blink for a
232                particular dev match the returned dev from get_device
233                to set hw control)
234
235                Returns a pointer to a struct device or NULL if nothing
236                is currently attached.
237
238LED driver can activate additional modes by default to workaround the
239impossibility of supporting each different mode on the supported trigger.
240Examples are hardcoding the blink speed to a set interval, enable special
241feature like bypassing blink if some requirements are not met.
242
243A trigger should first check if the hw control API are supported by the LED
244driver and check if the trigger is supported to verify if hw control is possible,
245use hw_control_is_supported to check if the flags are supported and only at
246the end use hw_control_set to activate hw control.
247
248A trigger can use hw_control_get to check if a LED is already in hw control
249and init their flags.
250
251When the LED is in hw control, no software blink is possible and doing so
252will effectively disable hw control.
253
254Known Issues
255============
256
257The LED Trigger core cannot be a module as the simple trigger functions
258would cause nightmare dependency issues. I see this as a minor issue
259compared to the benefits the simple trigger functionality brings. The
260rest of the LED subsystem can be modular.
261