Describing and referring to LEDs in ACPI

Individual LEDs are described by hierarchical data extension [6] nodes under the
device node, the LED driver chip. The "reg" property in the LED specific nodes
tells the numerical ID of each individual LED output to which the LEDs are
connected. [3] The hierarchical data nodes are named "led@X", where X is the
number of the LED output.

Referring to LEDs in Device tree is documented in [4], in "flash-leds" property
documentation. In short, LEDs are directly referred to by using phandles.

While Device tree allows referring to any node in the tree[1], in ACPI
references are limited to device nodes only [2]. For this reason using the same
mechanism on ACPI is not possible. A mechanism to refer to non-device ACPI nodes
is documented in [7].

ACPI allows (as does DT) using integer arguments after the reference. A
combination of the LED driver device reference and an integer argument,
referring to the "reg" property of the relevant LED, is used to identify
individual LEDs. The value of the "reg" property is a contract between the
firmware and software, it uniquely identifies the LED driver outputs.

Under the LED driver device, The first hierarchical data extension package list
entry shall contain the string "led@" followed by the number of the LED,
followed by the referred object name. That object shall be named "LED" followed
by the number of the LED.

An ASL example of a camera sensor device and a LED driver device for two LEDs.
Objects not relevant for LEDs or the references to them have been omitted.

	Device (LED)
	{
		Name (_DSD, Package () {
			ToUUID("dbb8e3e6-5886-4ba6-8795-1319f52a966b"),
			Package () {
				Package () { "led@0", LED0 },
				Package () { "led@1", LED1 },
			}
		})
		Name (LED0, Package () {
			ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
			Package () {
				Package () { "reg", 0 },
				Package () { "flash-max-microamp", 1000000 },
				Package () { "flash-timeout-us", 200000 },
				Package () { "led-max-microamp", 100000 },
				Package () { "label", "white:flash" },
			}
		})
		Name (LED1, Package () {
			ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
			Package () {
				Package () { "reg", 1 },
				Package () { "led-max-microamp", 10000 },
				Package () { "label", "red:indicator" },
			}
		})
	}

	Device (SEN)
	{
		Name (_DSD, Package () {
			ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
			Package () {
				Package () {
					"flash-leds",
					Package () { ^LED, "led@0", ^LED, "led@1" },
				}
			}
		})
	}

where

	LED	LED driver device
	LED0	First LED
	LED1	Second LED
	SEN	Camera sensor device (or another device the LED is
		related to)

[1] Device tree. <URL:http://www.devicetree.org>, referenced 2019-02-21.

[2] Advanced Configuration and Power Interface Specification.
    <URL:https://uefi.org/sites/default/files/resources/ACPI_6_3_final_Jan30.pdf>,
    referenced 2019-02-21.

[3] Documentation/devicetree/bindings/leds/common.txt

[4] Documentation/devicetree/bindings/media/video-interfaces.txt

[5] Device Properties UUID For _DSD.
    <URL:http://www.uefi.org/sites/default/files/resources/_DSD-device-properties-UUID.pdf>,
    referenced 2019-02-21.

[6] Hierarchical Data Extension UUID For _DSD.
    <URL:http://www.uefi.org/sites/default/files/resources/_DSD-hierarchical-data-extension-UUID-v1.1.pdf>,
    referenced 2019-02-21.

[7] Documentation/firmware-guide/acpi/dsd/data-node-references.rst