Subject: [PATCH RFC net-next] dt-bindings: ethernet-controller: document signal multiplexer

From: Marek Behún <[email-masked]>
Date: Thu, 1 Jul 2021 02:53:47 +0200

There are devices where the MAC signals from the ethernet controller are
not directly connected to an ethernet PHY or a SFP cage, but to a
multiplexer, so that the device can switch between the endpoints.

For example on Turris Omnia the WAN controller is connected to a SerDes
switch, which multiplexes the SerDes lanes between SFP cage and ethernet
PHY, depending on whether a SFP module is present (MOD_DEF0 GPIO from
the SFP cage).

Document how to describe such a situation for an ethernet controller in
the device tree bindings.

Example usage could then look like:
  &eth2 {
    status = "okay";
    phys = <&comphy5 2>;
    buffer-manager = <&bm>;
    bm,pool-long = <2>;
    bm,pool-short = <3>;

    signal-multiplexer {
      compatible = "gpio-signal-multiplexer";
      gpios = <&pcawan 4 GPIO_ACTIVE_LOW>;

      endpoint@0 {
        phy-mode = "sgmii";
	phy-handle = <&phy1>;
      };

      endpoint@1 {
        sfp = <&sfp>;
	phy-mode = "sgmii";
	managed = "in-band-status";
      };
    };
  };

Signed-off-by: Marek Behún <[email-masked]>
---
I wonder if this is the proper way to do this.

We already have framework for multiplexers in Linux, in drivers/mux.
But as I understand it, that framework is meant to be used when the
multiplexer state is to be set by kernel, while here it is possible
that the multiplexer state can be (and on Turris Omnia is) set by
the user plugging a SFP module into the SFP cage.

We theoretically could add a method for getting mux state into the mux
framework and state notification support. But using the mux framework
to solve this case in phylink would be rather complicated, especially
since mux framework is abstract, and if the multiplexer state is
determined by the MOD_DEF0 GPIO, which is also used by SFP code, the
implementation would get rather complicate in phylink...

I wonder whether driver implementation complexity should play a role
when proposing device tree bindings :-)

Some thoughts?
---
 .../bindings/net/ethernet-controller.yaml     | 60 +++++++++++++++++++
 1 file changed, 60 insertions(+)

diff --git a/Documentation/devicetree/bindings/net/ethernet-controller.yaml b/Documentation/devicetree/bindings/net/ethernet-controller.yaml
index b0933a8c295a..a7770edaec2b 100644
--- a/Documentation/devicetree/bindings/net/ethernet-controller.yaml
+++ b/Documentation/devicetree/bindings/net/ethernet-controller.yaml
@@ -226,6 +226,66 @@ properties:
           required:
             - speed
 
+  signal-multiplexer:
+    type: object
+    description:
+      Specifies that the signal pins (for example SerDes lanes) are connected
+      to a multiplexer from which they can be multiplexed to several different
+      endpoints, depending on the multiplexer configuration. (For example SerDes
+      lanes can be switched between an ethernet PHY and a SFP cage.)
+
+    properties:
+      compatible:
+        const: gpio-signal-multiplexer
+
+      gpios:
+        maxItems: 1
+        description:
+          GPIO to determine which endpoint the multiplexer is switched to.
+
+    patternProperties:
+      "^endpoint@[01]$":
+        type: object
+        description:
+          Specifies a multiplexer endpoint settings. Each endpoint can have
+          different settings. (For example in the case when multiplexing between
+          an ethernet PHY and a SFP cage, the SFP cage endpoint should specify
+          SFP phandle, while the PHY endpoint should specify PHY handle.)
+
+        properties:
+          reg:
+            enum: [ 0, 1 ]
+
+          phy-connection-type:
+            $ref: #/properties/phy-connection-type
+
+          phy-mode:
+            $ref: #/properties/phy-mode
+
+          phy-handle:
+            $ref: #/properties/phy-handle
+
+          phy:
+            $ref: #/properties/phy
+
+          phy-device:
+            $ref: #/properties/phy-device
+
+          sfp:
+            $ref: #/properties/sfp
+
+          managed:
+            $ref: #/properties/managed
+
+          fixed-link:
+            $ref: #/properties/fixed-link
+
+        required:
+          - reg
+
+    required:
+      - gpios
+
 additionalProperties: true
 
 ...
-- 
2.31.1