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+.. SPDX-License-Identifier: GPL-2.0
+.. include:: <isonum.txt>
+
+========================================
+Describing and referring to LEDs in ACPI
+========================================
+
+Individual LEDs are described by hierarchical data extension [5] 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. [leds] 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 [video-interfaces], 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 [devicetree], in
+ACPI references are limited to device nodes only [acpi]. For this reason using
+the same mechanism on ACPI is not possible. A mechanism to refer to non-device
+ACPI nodes is documented in [data-node-ref].
+
+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.
+
+Example
+=======
+
+An ASL example of a camera sensor device and a LED driver device for two LEDs is
+show below. 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)
+
+References
+==========
+
+[acpi] Advanced Configuration and Power Interface Specification.
+    https://uefi.org/specifications/ACPI/6.4/, referenced 2021-11-30.
+
+[data-node-ref] Documentation/firmware-guide/acpi/dsd/data-node-references.rst
+
+[devicetree] Devicetree. https://www.devicetree.org, referenced 2019-02-21.
+
+[dsd-guide] DSD Guide.
+    https://github.com/UEFI/DSD-Guide/blob/main/dsd-guide.adoc, referenced
+    2021-11-30.
+
+[leds] Documentation/devicetree/bindings/leds/common.yaml
+
+[video-interfaces] Documentation/devicetree/bindings/media/video-interfaces.yaml