[PATCH v2 2/6] dt-bindings: pinctrl: Add generic pinctrl for board-level mux chips

[Frank Li]

Add a generic pinctrl binding for board-level pinmux chips that are
controlled through the multiplexer subsystem.

On some boards, especially development boards, external mux chips are used
to switch SoC signals between different peripherals (e.g. MMC and UART).
The mux select lines are often driven by a GPIO expander over I2C,
as illustrated below:

	┌──────┐      ┌─────┐
	│ SOC  │      │     │    ┌───────┐
	│      │      │     │───►│ MMC   │
	│      │      │ MUX │    └───────┘
	│      ├─────►│     │    ┌───────┐
	│      │      │     │───►│ UART  │
	│      │      └─────┘    └───────┘
	│      │         ▲
	│      │    ┌────┴──────────────┐
	│ I2C  ├───►│ GPIO Expander     │
	└──────┘    └───────────────────┘

Traditionally, gpio-hog is used to configure the onboard mux at boot.
However, the GPIO expander may probe later than consumer devices such as
MMC. As a result, the MUX might not be configured when the peripheral
driver probes, leading to initialization failures or data transfer errors.

Introduce a generic pinctrl binding that models the board-level MUX as a
pin control provider and builds proper device links between the MUX, its
GPIO controller, and peripheral devices. This ensures correct probe
ordering and reliable mux configuration.

The implementation leverages the standard multiplexer subsystem, which
provides broad support for onboard mux controllers and avoids the need for
per-driver custom MUX handling.

Allow pinctrl-* pattern as node name because this pinctrl device have not
reg property.

Signed-off-by: Frank Li <Frank.Li@xxxxxxx>
---
change in v2:
 - change descriptions for device, not for driver
 - add missed additionalProperties: false
---
 .../bindings/pinctrl/pinctrl-multiplexer.yaml      | 57 ++++++++++++++++++++++
 .../devicetree/bindings/pinctrl/pinctrl.yaml       |  2 +-
 2 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/Documentation/devicetree/bindings/pinctrl/pinctrl-multiplexer.yaml b/Documentation/devicetree/bindings/pinctrl/pinctrl-multiplexer.yaml
new file mode 100644
index 0000000000000000000000000000000000000000..2b0385ed879b70b24ca9c39b098c3840d08d7482
--- /dev/null
+++ b/Documentation/devicetree/bindings/pinctrl/pinctrl-multiplexer.yaml
@@ -0,0 +1,57 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/pinctrl/pinctrl-multiplexer.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Generic pinctrl device for on-board MUX Chips
+
+maintainers:
+  - Frank Li <Frank.Li@xxxxxxx>
+
+description:
+  Generic pinctrl device for on-board MUX Chips, which switch SoC signals
+  between different peripherals (e.g. MMC and UART).
+
+  The MUX select lines are often driven by a I2C GPIO expander.
+
+properties:
+  compatible:
+    const: pinctrl-multiplexer
+
+patternProperties:
+  '-grp$':
+    type: object
+    additionalProperties: false
+    properties:
+      mux-states:
+        maxItems: 1
+
+    required:
+      - mux-states
+
+required:
+  - compatible
+
+allOf:
+  - $ref: pinctrl.yaml#
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    pinctrl-mux {
+        compatible = "pinctrl-multiplexer";
+
+        uart-grp {
+            mux-states = <&mux 0>;
+        };
+
+        spi-grp {
+            mux-states = <&mux 1>;
+        };
+
+        i2c-grp {
+            mux-states = <&mux 2>;
+        };
+    };
diff --git a/Documentation/devicetree/bindings/pinctrl/pinctrl.yaml b/Documentation/devicetree/bindings/pinctrl/pinctrl.yaml
index 290438826c507ec6725f486d18cf686aa7c35e67..20176bf3074757de30f208e69b968a6bd6125273 100644
--- a/Documentation/devicetree/bindings/pinctrl/pinctrl.yaml
+++ b/Documentation/devicetree/bindings/pinctrl/pinctrl.yaml
@@ -27,7 +27,7 @@ description: |
 
 properties:
   $nodename:
-    pattern: "^(pinctrl|pinmux)(@[0-9a-f]+)?$"
+    pattern: "^(pinctrl|pinmux)(@[0-9a-f]+|-[a-z0-9]+)?$"
 
   "#pinctrl-cells":
     description: >

-- 
2.43.0