Add device tree binding documentation for the Synopsys DesignWare
XPCS integrated on the Rockchip RK3568 SoC.

The XPCS is accessed over the APB3 bus and internally connected to
a Naneng Combo SerDes PHY.  It supports 1000BASE-X, SGMII, and
QSGMII modes, with four MII ports.

The four MII ports are described as ethernet-pcs-mii@N child nodes,
consumed by the Rockchip XPCS glue driver later in this series.

phys and phy-names are required because dtbs_check only validates
required properties for enabled nodes. The SerDes link is a board-level
design choice (combphy1 on some boards, combphy2 on others), so these
properties must be provided by the board device tree, not the SoC dtsi.

The CRU reset lines (SRST_XPCS*) are intentionally not described: no
in-tree user requests them, and bring-up relies on the PD_PIPE power
domain, the SerDes PHY and the in-IP soft reset. They can be added
later as optional without breaking ABI.

Signed-off-by: Coia Prant <coiaprant@gmail.com>
---
 .../net/pcs/rockchip,rk3568-xpcs.yaml         | 110 ++++++++++++++++++
 1 file changed, 110 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/net/pcs/rockchip,rk3568-xpcs.yaml

diff --git a/Documentation/devicetree/bindings/net/pcs/rockchip,rk3568-xpcs.yaml b/Documentation/devicetree/bindings/net/pcs/rockchip,rk3568-xpcs.yaml
new file mode 100644
index 0000000000000..703fcff0e3f70
--- /dev/null
+++ b/Documentation/devicetree/bindings/net/pcs/rockchip,rk3568-xpcs.yaml
@@ -0,0 +1,110 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/net/pcs/rockchip,rk3568-xpcs.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Rockchip RK3568 Synopsys DesignWare Ethernet PCS
+
+maintainers:
+  - Coia Prant <coiaprant@gmail.com>
+
+description: |
+  Rockchip RK3568 SoC integrates a Synopsys DesignWare Ethernet Physical
+  Coding Sublayer (XPCS).
+  The PCS provides an interface between the Media Access Control (MAC)
+  and the Physical Medium Attachment (PMA) sublayer through a Media
+  Independent Interface (GMII).
+
+  The XPCS is accessed over the APB3 bus and internally connected to a
+  Naneng Combo SerDes PHY.
+  It supports 1000BASE-X, SGMII and QSGMII modes.
+
+  The block contains four MII ports that can be individually enabled and
+  routed to one of the Ethernet GMAC controllers via the pcs-handle
+  property in the MAC device tree node.
+
+properties:
+  compatible:
+    const: rockchip,rk3568-xpcs
+
+  reg:
+    maxItems: 1
+
+  "#address-cells":
+    const: 1
+
+  "#size-cells":
+    const: 0
+
+  clocks:
+    items:
+      - description: APB3 bus interface clock (clk_csr_i), required for register access
+      - description: EEE clock (clk_eee_i), required for Energy Efficient Ethernet operation
+
+  clock-names:
+    items:
+      - const: csr
+      - const: eee
+
+  phys:
+    maxItems: 1
+
+  phy-names:
+    const: serdes
+
+  power-domains:
+    maxItems: 1
+
+patternProperties:
+  "^ethernet-pcs-mii@[0-3]$":
+    type: object
+    description:
+      One of the four MII ports of the XPCS. The port is linked to an
+      Ethernet MAC controller via the pcs-handle property in the MAC's
+      device tree node.
+
+    properties:
+      reg:
+        description: MII port number.
+        enum: [0, 1, 2, 3]
+
+    required:
+      - reg
+
+    additionalProperties: false
+
+required:
+  - compatible
+  - reg
+  - "#address-cells"
+  - "#size-cells"
+  - clocks
+  - clock-names
+  - phys
+  - phy-names
+  - power-domains
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/rk3568-cru.h>
+    #include <dt-bindings/power/rk3568-power.h>
+    #include <dt-bindings/phy/phy.h>
+
+    ethernet-pcs@fda00000 {
+      compatible = "rockchip,rk3568-xpcs";
+      reg = <0xfda00000 0x200000>;
+      #address-cells = <1>;
+      #size-cells = <0>;
+      clocks = <&cru PCLK_XPCS>, <&cru CLK_XPCS_EEE>;
+      clock-names = "csr", "eee";
+      phys = <&combphy2 PHY_TYPE_SGMII>;
+      phy-names = "serdes";
+      power-domains = <&power RK3568_PD_PIPE>;
+
+      ethernet-pcs-mii@0 {
+        reg = <0>;
+      };
+    };
-- 
2.47.3