1 ========================
2 LED handling under Linux
3 ========================
4
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.
10
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.
17
18 Complex triggers while 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.
26
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.
31
32
33 Design Philosophy
34 =================
35
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.
39
40
41 LED Device Naming
42 =================
43
44 Is 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
65 If required color or function is missing, please submit a patch
66 to linux-leds@vger.kernel.org.
67
68 It is possible that more than one LED with the same color and function will
69 be required for given platform, differing only with an ordinal number.
70 In this case it is preferable to just concatenate the predefined LED_FUNCTION_*
71 name with required "-N" suffix in the driver. fwnode based drivers can use
72 function-enumerator property for that and then the concatenation will be handled
73 automatically by the LED core upon LED class device registration.
74
75 LED subsystem has also a protection against name clash, that may occur
76 when LED class device is created by a driver of hot-pluggable device and
77 it doesn't provide unique devicename section. In this case numerical
78 suffix (e.g. "_1", "_2", "_3" etc.) is added to the requested LED class
79 device name.
80
81 There might be still LED class drivers around using vendor or product name
82 for devicename, but this approach is now deprecated as it doesn't convey
83 any added value. Product information can be found in other places in sysfs
84 (see tools/leds/get_led_device_info.sh).
85
86 Examples 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
101 get_led_device_info.sh script can be used for verifying if the LED name
102 meets the requirements pointed out here. It performs validation of the LED class
103 devicename sections and gives hints on expected value for a section in case
104 the validation fails for it. So far the script supports validation
105 of associations between LEDs and following types of devices:
106
107 - input devices
108 - ieee80211 compliant USB devices
109
110 The script is open to extensions.
111
112 There have been calls for LED properties such as color to be exported as
113 individual led class attributes. As a solution which doesn't incur as much
114 overhead, I suggest these become part of the device name. The naming scheme
115 above leaves scope for further attributes should they be needed. If sections
116 of the name don't apply, just leave that section blank.
117
118
119 Brightness setting API
120 ======================
121
122 LED 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
135 LED registration API
136 ====================
137
138 A driver wanting to register a LED classdev for use by other drivers /
139 userspace 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
141 must call led_classdev_unregister from its remove function before
142 free-ing the led_classdev struct.
143
144 If the driver can detect hardware initiated brightness changes and thus
145 wants to have a brightness_hw_changed attribute then the LED_BRIGHT_HW_CHANGED
146 flag must be set in flags before registering. Calling
147 led_classdev_notify_brightness_hw_changed on a classdev not registered with
148 the LED_BRIGHT_HW_CHANGED flag is a bug and will trigger a WARN_ON.
149
150 Hardware accelerated blink of LEDs
151 ==================================
152
153 Some LEDs can be programmed to blink without any CPU interaction. To
154 support this feature, a LED driver can optionally implement the
155 blink_set() function (see <linux/leds.h>). To set an LED to blinking,
156 however, it is better to use the API function led_blink_set(), as it
157 will check and implement software fallback if necessary.
158
159 To turn off blinking, use the API function led_brightness_set()
160 with brightness value LED_OFF, which should stop any software
161 timers that may have been required for blinking.
162
163 The blink_set() function should choose a user friendly blinking value
164 if it is called with `*delay_on==0` && `*delay_off==0` parameters. In this
165 case the driver should give back the chosen value through delay_on and
166 delay_off parameters to the leds subsystem.
167
168 Setting the brightness to zero with brightness_set() callback function
169 should completely turn off the LED and cancel the previously programmed
170 hardware blinking function, if any.
171
172 Hardware driven LEDs
173 ====================
174
175 Some LEDs can be programmed to be driven by hardware. This is not
176 limited to blink but also to turn off or on autonomously.
177 To support this feature, a LED needs to implement various additional
178 ops and needs to declare specific support for the supported triggers.
179
180 With hw control we refer to the LED driven by hardware.
181
182 LED 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
188 LED 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
238 LED driver can activate additional modes by default to workaround the
239 impossibility of supporting each different mode on the supported trigger.
240 Examples are hardcoding the blink speed to a set interval, enable special
241 feature like bypassing blink if some requirements are not met.
242
243 A trigger should first check if the hw control API are supported by the LED
244 driver and check if the trigger is supported to verify if hw control is possible,
245 use hw_control_is_supported to check if the flags are supported and only at
246 the end use hw_control_set to activate hw control.
247
248 A trigger can use hw_control_get to check if a LED is already in hw control
249 and init their flags.
250
251 When the LED is in hw control, no software blink is possible and doing so
252 will effectively disable hw control.
253
254 Known Issues
255 ============
256
257 The LED Trigger core cannot be a module as the simple trigger functions
258 would cause nightmare dependency issues. I see this as a minor issue
259 compared to the benefits the simple trigger functionality brings. The
260 rest of the LED subsystem can be modular.
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