dt-bindings: nvmem: fixed-cell: add compatible for MAC cells

A lot of home routers have NVMEM fixed cells containing MAC address that
need some further processing. In ~99% cases MAC needs to be:
1. Optionally parsed from ASCII format
2. Increased by a vendor-picked value

There was already an attempt to design a binding for that at NVMEM
device level in the past. It wasn't accepted though as it didn't really
fit NVMEM device layer.

The introduction of NVMEM fixed-cells layout seems to be an opportunity
to provide a relevant binding in a clean way.

This commit adds a *generic* compatible string: "mac-base". As always it
needs to be carefully reviewed.

OpenWrt project currently supports ~300 home routers that have NVMEM
cell with binary-stored base MAC.T hose devices are manufactured by
multiple vendors. There are TP-Link devices (76 of them), Netgear (19),
D-Link (11), OpenMesh (9), EnGenius (8), GL.iNet (8), ZTE (7),
Xiaomi (5), Ubiquiti (6) and more. Those devices don't share an
architecture or SoC.

Another 200 devices have base MAC stored in an ASCII format (not all
those devices have been converted to DT though).

It would be impractical to provide unique "compatible" strings for NVMEM
layouts of all those devices. It seems like a valid case for allowing a
generic binding instead. Even if this binding will not be sufficient for
some further devices it seems to be useful enough as it is.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20230823132744.350618-2-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/nvmem/layouts/fixed-cell.yaml

@@ -11,6 +11,15 @@ maintainers:
   - Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
 
 properties:
+  compatible:
+    oneOf:
+      - const: mac-base
+        description: >
+          Cell with base MAC address to be used for calculating extra relative
+          addresses.
+          It can be stored in a plain binary format (cell length 6) or as an
+          ASCII text like "00:11:22:33:44:55" (cell length 17).
+
   reg:
     maxItems: 1
 
@@ -25,6 +34,23 @@ properties:
         description:
           Size in bit within the address range specified by reg.
 
+allOf:
+  - if:
+      required: [ compatible ]
+    then:
+      if:
+        properties:
+          compatible:
+            contains:
+              const: mac-base
+      then:
+        properties:
+          "#nvmem-cell-cells":
+            description: The first argument is a MAC address offset.
+            const: 1
+        required:
+          - "#nvmem-cell-cells"
+
 required:
   - reg
 

Documentation/devicetree/bindings/nvmem/layouts/fixed-layout.yaml

@@ -44,6 +44,18 @@ examples:
         #address-cells = <1>;
         #size-cells = <1>;
 
+        mac@100 {
+            compatible = "mac-base";
+            reg = <0x100 0x6>;
+            #nvmem-cell-cells = <1>;
+        };
+
+        mac@110 {
+            compatible = "mac-base";
+            reg = <0x110 0x11>;
+            #nvmem-cell-cells = <1>;
+        };
+
         calibration@4000 {
             reg = <0x4000 0x100>;
         };

Documentation/devicetree/bindings/nvmem/nvmem.yaml

@@ -49,7 +49,10 @@ properties:
 patternProperties:
   "@[0-9a-f]+(,[0-7])?$":
     type: object
-    $ref: layouts/fixed-cell.yaml
+    allOf:
+      - $ref: layouts/fixed-cell.yaml
+      - properties:
+          compatible: false
     deprecated: true
 
 additionalProperties: true