Documentation: ACPI: Use all-string data node references

Document that references to data nodes shall use string-only references
instead of a device reference and a succession of the first package
entries of hierarchical data node references.

Fixes: 9880702 ("ACPI: property: Support using strings in reference properties")
Cc: 6.8+ <stable@vger.kernel.org> # 6.8+
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Link: https://patch.msgid.link/20250409084738.3657079-1-sakari.ailus@linux.intel.com
[ rjw: Clarifying edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: Sakari Ailus

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

@@ -12,11 +12,14 @@ ACPI in general allows referring to device objects in the tree only.
 Hierarchical data extension nodes may not be referred to directly, hence this
 document defines a scheme to implement such references.
 
-A reference consist of the device object name followed by one or more
-hierarchical data extension [dsd-guide] keys. Specifically, the hierarchical
-data extension node which is referred to by the key shall lie directly under
-the parent object i.e. either the device object or another hierarchical data
-extension node.
+A reference to a _DSD hierarchical data node is a string consisting of a
+device object reference followed by a dot (".") and a relative path to a data
+node object. Do not use non-string references as this will produce a copy of
+the hierarchical data node, not a reference!
+
+The hierarchical data extension node which is referred to shall be located
+directly under its parent object i.e. either the device object or another
+hierarchical data extension node [dsd-guide].
 
 The keys in the hierarchical data nodes shall consist of the name of the node,
 "@" character and the number of the node in hexadecimal notation (without pre-
@@ -33,11 +36,9 @@ extension key.
 Example
 =======
 
-In the ASL snippet below, the "reference" _DSD property contains a
-device object reference to DEV0 and under that device object, a
-hierarchical data extension key "node@1" referring to the NOD1 object
-and lastly, a hierarchical data extension key "anothernode" referring to
-the ANOD object which is also the final target node of the reference.
+In the ASL snippet below, the "reference" _DSD property contains a string
+reference to a hierarchical data extension node ANOD under DEV0 under the parent
+of DEV1. ANOD is also the final target node of the reference.
 ::
 
 	Device (DEV0)
@@ -76,10 +77,7 @@ the ANOD object which is also the final target node of the reference.
 	    Name (_DSD, Package () {
 		ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
 		Package () {
-		    Package () {
-			"reference", Package () {
-			    ^DEV0, "node@1", "anothernode"
-			}
+		    Package () { "reference", "^DEV0.ANOD" }
 		    },
 		}
 	    })

Documentation/firmware-guide/acpi/dsd/graph.rst

@@ -66,12 +66,9 @@ of that port shall be zero. Similarly, if a port may only have a single
 endpoint, the number of that endpoint shall be zero.
 
 The endpoint reference uses property extension with "remote-endpoint" property
-name followed by a reference in the same package. Such references consist of
-the remote device reference, the first package entry of the port data extension
-reference under the device and finally the first package entry of the endpoint
-data extension reference under the port. Individual references thus appear as::
+name followed by a string reference in the same package. [data-node-ref]::
 
-    Package() { device, "port@X", "endpoint@Y" }
+    "device.datanode"
 
 In the above example, "X" is the number of the port and "Y" is the number of
 the endpoint.
@@ -109,7 +106,7 @@ A simple example of this is show below::
 		ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
 		Package () {
 		    Package () { "reg", 0 },
-		    Package () { "remote-endpoint", Package() { \_SB.PCI0.ISP, "port@4", "endpoint@0" } },
+		    Package () { "remote-endpoint", "\\_SB.PCI0.ISP.EP40" },
 		}
 	    })
 	}
@@ -141,7 +138,7 @@ A simple example of this is show below::
 		ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
 		Package () {
 		    Package () { "reg", 0 },
-		    Package () { "remote-endpoint", Package () { \_SB.PCI0.I2C2.CAM0, "port@0", "endpoint@0" } },
+		    Package () { "remote-endpoint", "\\_SB.PCI0.I2C2.CAM0.EP00" },
 		}
 	    })
 	}

Documentation/firmware-guide/acpi/dsd/leds.rst

@@ -15,11 +15,6 @@ 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
@@ -74,7 +69,7 @@ omitted. ::
 			Package () {
 				Package () {
 					"flash-leds",
-					Package () { ^LED, "led@0", ^LED, "led@1" },
+					Package () { "^LED.LED0", "^LED.LED1" },
 				}
 			}
 		})