[PATCH v2 1/7] dt-bindings: Add support for export-symbols node

[Herve Codina <herve.codina@bootlin.com>]

An export-symbols node allows to export symbols for symbols resolution
performed when applying a device tree overlay.

When a device tree overlay is applied on a node having an export-symbols
node, symbols listed in the export-symbols node are used to resolve
undefined symbols referenced from the overlay.

This allows:
  - Referencing symbols from an device tree overlay without the need to
    know the full base board. Only the connector definition is needed.

  - Using the exact same overlay on several connectors available on a given
    board.

For instance, the following description is supported with the
export-symbols node:
 - Base device tree board A:
    ...
    foo_connector: connector1 {
        export-symbols {
           connector = <&foo_connector>;
        };
    };

    bar_connector: connector2 {
        export-symbols {
           connector = <&bar_connector>;
        };
    };
    ...

 - Base device tree board B:
    ...
    front_connector: addon-connector {
        export-symbols {
           connector = <&front_connector>;
        };
    };
    ...

 - Overlay describing an addon board the can be connected on connectors:
    ...
    node {
        ...
        connector = <&connector>;
        ...
    };
    ...

Thanks to the export-symbols node, the overlay can be applied on
connector1 or connector2 available on board A but also on
addon-connector available on board B.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Tested-by: Ayush Singh <ayush@beagleboard.org>
---
 .../devicetree/bindings/export-symbols.yaml   | 43 +++++++++++++++++++
 1 file changed, 43 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/export-symbols.yaml

diff --git a/Documentation/devicetree/bindings/export-symbols.yaml b/Documentation/devicetree/bindings/export-symbols.yaml
new file mode 100644
index 000000000000..0e404eff8937
--- /dev/null
+++ b/Documentation/devicetree/bindings/export-symbols.yaml
@@ -0,0 +1,43 @@
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/export-symbols.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Export symbols
+
+maintainers:
+  - Herve Codina <herve.codina@bootlin.com>
+
+description: |
+  An export-symbols node allows to export symbols for symbols resolution
+  performed when applying a device tree overlay.
+
+  When a device tree overlay is applied on a node having an export-symbols
+  node, symbols listed in the export-symbols node are used to resolve undefined
+  symbols referenced from the overlay.
+
+properties:
+  $nodename:
+    const: export-symbols
+
+patternProperties:
+  "^[a-zA-Z_]?[a-zA-Z0-9_]*$":
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      A symbol exported in the form <symbol_name>=<phandle>.
+
+additionalProperties: false
+
+examples:
+  - |
+    /*
+     * Allows 'connector' symbol used in a device-tree overlay to be resolved to
+     * connector0 when the device-tree overlay is applied on connector0 node.
+     */
+    connector0: connector0 {
+      export-symbols {
+        connector = <&connector0>;
+      };
+    };
+...
-- 
2.49.0