1 .. SPDX-License-Identifier: GPL-2.0
2 .. include:: <isonum.txt>
3
4 ========================================
5 Describing and referring to LEDs in ACPI
6 ========================================
7
8 Individual LEDs are described by hierarchical data extension [5] nodes under the
9 device node, the LED driver chip. The "reg" property in the LED specific nodes
10 tells the numerical ID of each individual LED output to which the LEDs are
11 connected. [leds] The hierarchical data nodes are named "led@X", where X is the
12 number of the LED output.
13
14 Referring to LEDs in Device tree is documented in [video-interfaces], in
15 "flash-leds" property documentation. In short, LEDs are directly referred to by
16 using phandles.
17
18 While Device tree allows referring to any node in the tree [devicetree], in
19 ACPI references are limited to device nodes only [acpi]. For this reason using
20 the same mechanism on ACPI is not possible. A mechanism to refer to non-device
21 ACPI nodes is documented in [data-node-ref].
22
23 ACPI allows (as does DT) using integer arguments after the reference. A
24 combination of the LED driver device reference and an integer argument,
25 referring to the "reg" property of the relevant LED, is used to identify
26 individual LEDs. The value of the "reg" property is a contract between the
27 firmware and software, it uniquely identifies the LED driver outputs.
28
29 Under the LED driver device, The first hierarchical data extension package list
30 entry shall contain the string "led@" followed by the number of the LED,
31 followed by the referred object name. That object shall be named "LED" followed
32 by the number of the LED.
33
34 Example
35 =======
36
37 An ASL example of a camera sensor device and a LED driver device for two LEDs is
38 show below. Objects not relevant for LEDs or the references to them have been
39 omitted. ::
40
41 Device (LED)
42 {
43 Name (_DSD, Package () {
44 ToUUID("dbb8e3e6-5886-4ba6-8795-1319f52a966b"),
45 Package () {
46 Package () { "led@0", LED0 },
47 Package () { "led@1", LED1 },
48 }
49 })
50 Name (LED0, Package () {
51 ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
52 Package () {
53 Package () { "reg", 0 },
54 Package () { "flash-max-microamp", 1000000 },
55 Package () { "flash-timeout-us", 200000 },
56 Package () { "led-max-microamp", 100000 },
57 Package () { "label", "white:flash" },
58 }
59 })
60 Name (LED1, Package () {
61 ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
62 Package () {
63 Package () { "reg", 1 },
64 Package () { "led-max-microamp", 10000 },
65 Package () { "label", "red:indicator" },
66 }
67 })
68 }
69
70 Device (SEN)
71 {
72 Name (_DSD, Package () {
73 ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
74 Package () {
75 Package () {
76 "flash-leds",
77 Package () { ^LED, "led@0", ^LED, "led@1" },
78 }
79 }
80 })
81 }
82
83 where
84 ::
85
86 LED LED driver device
87 LED0 First LED
88 LED1 Second LED
89 SEN Camera sensor device (or another device the LED is related to)
90
91 References
92 ==========
93
94 [acpi] Advanced Configuration and Power Interface Specification.
95 https://uefi.org/specifications/ACPI/6.4/, referenced 2021-11-30.
96
97 [data-node-ref] Documentation/firmware-guide/acpi/dsd/data-node-references.rst
98
99 [devicetree] Devicetree. https://www.devicetree.org, referenced 2019-02-21.
100
101 [dsd-guide] DSD Guide.
102 https://github.com/UEFI/DSD-Guide/blob/main/dsd-guide.adoc, referenced
103 2021-11-30.
104
105 [leds] Documentation/devicetree/bindings/leds/common.yaml
106
107 [video-interfaces] Documentation/devicetree/bindings/media/video-interfaces.yaml
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