1 ======================== 1 ========================
2 LED handling under Linux 2 LED handling under Linux
3 ======================== 3 ========================
4 4
5 In its simplest form, the LED class just allow 5 In its simplest form, the LED class just allows control of LEDs from
6 userspace. LEDs appear in /sys/class/leds/. Th 6 userspace. LEDs appear in /sys/class/leds/. The maximum brightness of the
7 LED is defined in max_brightness file. The bri 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). 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 f 9 brightness support so will just be turned on for non-zero brightness settings.
10 10
11 The class also introduces the optional concept 11 The class also introduces the optional concept of an LED trigger. A trigger
12 is a kernel based source of led events. Trigge 12 is a kernel based source of led events. Triggers can either be simple or
13 complex. A simple trigger isn't configurable a 13 complex. A simple trigger isn't configurable and is designed to slot into
14 existing subsystems with minimal additional co 14 existing subsystems with minimal additional code. Examples are the disk-activity,
15 nand-disk and sharpsl-charge triggers. With le 15 nand-disk and sharpsl-charge triggers. With led triggers disabled, the code
16 optimises away. 16 optimises away.
17 17
18 Complex triggers while available to all LEDs h 18 Complex triggers while available to all LEDs have LED specific
19 parameters and work on a per LED basis. The ti 19 parameters and work on a per LED basis. The timer trigger is an example.
20 The timer trigger will periodically change the 20 The timer trigger will periodically change the LED brightness between
21 LED_OFF and the current brightness setting. Th 21 LED_OFF and the current brightness setting. The "on" and "off" time can
22 be specified via /sys/class/leds/<device>/dela 22 be specified via /sys/class/leds/<device>/delay_{on,off} in milliseconds.
23 You can change the brightness value of a LED i 23 You can change the brightness value of a LED independently of the timer
24 trigger. However, if you set the brightness va 24 trigger. However, if you set the brightness value to LED_OFF it will
25 also disable the timer trigger. 25 also disable the timer trigger.
26 26
27 You can change triggers in a similar manner to 27 You can change triggers in a similar manner to the way an IO scheduler
28 is chosen (via /sys/class/leds/<device>/trigge 28 is chosen (via /sys/class/leds/<device>/trigger). Trigger specific
29 parameters can appear in /sys/class/leds/<devi 29 parameters can appear in /sys/class/leds/<device> once a given trigger is
30 selected. 30 selected.
31 31
32 32
33 Design Philosophy 33 Design Philosophy
34 ================= 34 =================
35 35
36 The underlying design philosophy is simplicity 36 The underlying design philosophy is simplicity. LEDs are simple devices
37 and the aim is to keep a small amount of code 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 su 38 as possible. Please keep this in mind when suggesting enhancements.
39 39
40 40
41 LED Device Naming 41 LED Device Naming
42 ================= 42 =================
43 43
44 Is currently of the form: 44 Is currently of the form:
45 45
46 "devicename:color:function" !! 46 "devicename:colour:function"
47 47
48 - devicename: !! 48 There have been calls for LED properties such as colour to be exported as
49 it should refer to a unique identifier <<
50 like e.g. phyN for network devices or <<
51 than to the hardware; the information <<
52 to which given device is hooked is ava <<
53 retrieved using get_led_device_info.sh <<
54 this section is expected mostly for LE <<
55 other devices. <<
56 <<
57 - color: <<
58 one of LED_COLOR_ID_* definitions from <<
59 include/dt-bindings/leds/common.h. <<
60 <<
61 - function: <<
62 one of LED_FUNCTION_* definitions from <<
63 include/dt-bindings/leds/common.h. <<
64 <<
65 If required color or function is missing, plea <<
66 to linux-leds@vger.kernel.org. <<
67 <<
68 It is possible that more than one LED with the <<
69 be required for given platform, differing only <<
70 In this case it is preferable to just concaten <<
71 name with required "-N" suffix in the driver. <<
72 function-enumerator property for that and then <<
73 automatically by the LED core upon LED class d <<
74 <<
75 LED subsystem has also a protection against na <<
76 when LED class device is created by a driver o <<
77 it doesn't provide unique devicename section. <<
78 suffix (e.g. "_1", "_2", "_3" etc.) is added t <<
79 device name. <<
80 <<
81 There might be still LED class drivers around <<
82 for devicename, but this approach is now depre <<
83 any added value. Product information can be fo <<
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 <<
102 meets the requirements pointed out here. It pe <<
103 devicename sections and gives hints on expecte <<
104 the validation fails for it. So far the script <<
105 of associations between LEDs and following typ <<
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 <<
113 individual led class attributes. As a solution 49 individual led class attributes. As a solution which doesn't incur as much
114 overhead, I suggest these become part of the d 50 overhead, I suggest these become part of the device name. The naming scheme
115 above leaves scope for further attributes shou 51 above leaves scope for further attributes should they be needed. If sections
116 of the name don't apply, just leave that secti 52 of the name don't apply, just leave that section blank.
117 53
118 54
119 Brightness setting API 55 Brightness setting API
120 ====================== 56 ======================
121 57
122 LED subsystem core exposes following API for s 58 LED subsystem core exposes following API for setting brightness:
123 59
124 - led_set_brightness: 60 - led_set_brightness:
125 it is guaranteed not to sleep, 61 it is guaranteed not to sleep, passing LED_OFF stops
126 blinking, 62 blinking,
127 63
128 - led_set_brightness_sync: 64 - led_set_brightness_sync:
129 for use cases when immediate e 65 for use cases when immediate effect is desired -
130 it can block the caller for th 66 it can block the caller for the time required for accessing
131 device registers and can sleep 67 device registers and can sleep, passing LED_OFF stops hardware
132 blinking, returns -EBUSY if so 68 blinking, returns -EBUSY if software blink fallback is enabled.
133 69
134 70
135 LED registration API 71 LED registration API
136 ==================== 72 ====================
137 73
138 A driver wanting to register a LED classdev fo 74 A driver wanting to register a LED classdev for use by other drivers /
139 userspace needs to allocate and fill a led_cla 75 userspace needs to allocate and fill a led_classdev struct and then call
140 `[devm_]led_classdev_register`. If the non dev 76 `[devm_]led_classdev_register`. If the non devm version is used the driver
141 must call led_classdev_unregister from its rem 77 must call led_classdev_unregister from its remove function before
142 free-ing the led_classdev struct. 78 free-ing the led_classdev struct.
143 79
144 If the driver can detect hardware initiated br 80 If the driver can detect hardware initiated brightness changes and thus
145 wants to have a brightness_hw_changed attribut 81 wants to have a brightness_hw_changed attribute then the LED_BRIGHT_HW_CHANGED
146 flag must be set in flags before registering. 82 flag must be set in flags before registering. Calling
147 led_classdev_notify_brightness_hw_changed on a 83 led_classdev_notify_brightness_hw_changed on a classdev not registered with
148 the LED_BRIGHT_HW_CHANGED flag is a bug and wi 84 the LED_BRIGHT_HW_CHANGED flag is a bug and will trigger a WARN_ON.
149 85
150 Hardware accelerated blink of LEDs 86 Hardware accelerated blink of LEDs
151 ================================== 87 ==================================
152 88
153 Some LEDs can be programmed to blink without a 89 Some LEDs can be programmed to blink without any CPU interaction. To
154 support this feature, a LED driver can optiona 90 support this feature, a LED driver can optionally implement the
155 blink_set() function (see <linux/leds.h>). To 91 blink_set() function (see <linux/leds.h>). To set an LED to blinking,
156 however, it is better to use the API function 92 however, it is better to use the API function led_blink_set(), as it
157 will check and implement software fallback if 93 will check and implement software fallback if necessary.
158 94
159 To turn off blinking, use the API function led 95 To turn off blinking, use the API function led_brightness_set()
160 with brightness value LED_OFF, which should st 96 with brightness value LED_OFF, which should stop any software
161 timers that may have been required for blinkin 97 timers that may have been required for blinking.
162 98
163 The blink_set() function should choose a user 99 The blink_set() function should choose a user friendly blinking value
164 if it is called with `*delay_on==0` && `*delay 100 if it is called with `*delay_on==0` && `*delay_off==0` parameters. In this
165 case the driver should give back the chosen va 101 case the driver should give back the chosen value through delay_on and
166 delay_off parameters to the leds subsystem. 102 delay_off parameters to the leds subsystem.
167 103
168 Setting the brightness to zero with brightness 104 Setting the brightness to zero with brightness_set() callback function
169 should completely turn off the LED and cancel 105 should completely turn off the LED and cancel the previously programmed
170 hardware blinking function, if any. 106 hardware blinking function, if any.
171 107
172 Hardware driven LEDs <<
173 ==================== <<
174 <<
175 Some LEDs can be programmed to be driven by ha <<
176 limited to blink but also to turn off or on au <<
177 To support this feature, a LED needs to implem <<
178 ops and needs to declare specific support for <<
179 <<
180 With hw control we refer to the LED driven by <<
181 <<
182 LED driver must define the following value to <<
183 <<
184 - hw_control_trigger: <<
185 unique trigger name supported b <<
186 mode. <<
187 <<
188 LED driver must implement the following API to <<
189 - hw_control_is_supported: <<
190 check if the flags passed by t <<
191 be parsed and activate hw cont <<
192 <<
193 Return 0 if the passed flags m <<
194 can be set with hw_control_set <<
195 <<
196 If the passed flags mask is no <<
197 must be returned, the LED trig <<
198 fallback in this case. <<
199 <<
200 Return a negative error in cas <<
201 device not ready or timeouts. <<
202 <<
203 - hw_control_set: <<
204 activate hw control. LED drive <<
205 flags passed from the supporte <<
206 a set of mode and setup the LE <<
207 following the requested modes. <<
208 <<
209 Set LED_OFF via the brightness <<
210 <<
211 Return 0 on success, a negativ <<
212 apply flags. <<
213 <<
214 - hw_control_get: <<
215 get active modes from a LED al <<
216 them and set in flags the curr <<
217 supported trigger. <<
218 <<
219 Return 0 on success, a negativ <<
220 parsing the initial mode. <<
221 Error from this function is NO <<
222 be in a not supported initial <<
223 trigger. <<
224 <<
225 - hw_control_get_device: <<
226 return the device associated w <<
227 hw control. A trigger might us <<
228 returned device from this func <<
229 device for the trigger as the <<
230 events and correctly enable hw <<
231 (example a netdev trigger conf <<
232 particular dev match the retur <<
233 to set hw control) <<
234 <<
235 Returns a pointer to a struct <<
236 is currently attached. <<
237 <<
238 LED driver can activate additional modes by de <<
239 impossibility of supporting each different mod <<
240 Examples are hardcoding the blink speed to a s <<
241 feature like bypassing blink if some requireme <<
242 <<
243 A trigger should first check if the hw control <<
244 driver and check if the trigger is supported t <<
245 use hw_control_is_supported to check if the fl <<
246 the end use hw_control_set to activate hw cont <<
247 <<
248 A trigger can use hw_control_get to check if a <<
249 and init their flags. <<
250 <<
251 When the LED is in hw control, no software bli <<
252 will effectively disable hw control. <<
253 108
254 Known Issues 109 Known Issues
255 ============ 110 ============
256 111
257 The LED Trigger core cannot be a module as the 112 The LED Trigger core cannot be a module as the simple trigger functions
258 would cause nightmare dependency issues. I see 113 would cause nightmare dependency issues. I see this as a minor issue
259 compared to the benefits the simple trigger fu 114 compared to the benefits the simple trigger functionality brings. The
260 rest of the LED subsystem can be modular. 115 rest of the LED subsystem can be modular.
>> 116
>> 117
>> 118 Future Development
>> 119 ==================
>> 120
>> 121 At the moment, a trigger can't be created specifically for a single LED.
>> 122 There are a number of cases where a trigger might only be mappable to a
>> 123 particular LED (ACPI?). The addition of triggers provided by the LED driver
>> 124 should cover this option and be possible to add without breaking the
>> 125 current interface.
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