Re: [RFC net-next 2/2] dt-bindings: ethernet-phy: define `unsupported-mac-connection-types` property

On Mon, 22 Mar 2021 19:56:05 +0000
Russell King - ARM Linux admin [off-list ref] wrote:

On Mon, Mar 22, 2021 at 08:49:59PM +0100, Marek Behún wrote:

quoted

diff --git a/Documentation/devicetree/bindings/net/ethernet-phy.yaml b/Documentation/devicetree/bindings/net/ethernet-phy.yaml
index 2766fe45bb98..4c5b8fabbec3 100644
--- a/Documentation/devicetree/bindings/net/ethernet-phy.yaml
+++ b/Documentation/devicetree/bindings/net/ethernet-phy.yaml

@@ -136,6 +136,20 @@ properties:
       used. The absence of this property indicates the muxers
       should be configured so that the external PHY is used.
 
+  unsupported-mac-connection-types:
+    $ref: "ethernet-controller.yaml#/$defs/phy-connection-type-array"
+    description:
+      The PHY device may support different interface types for
+      connecting the Ethernet MAC device to the PHY device (i.e.
+      rgmii, sgmii, xaui, ...), but not all of these interface
+      types must necessarily be supported for a specific board
+      (either not all of them are wired, or there is a known bug
+      for a specific mode).
+      This property specifies a list of interface modes are not
+      supported on the board.  

I think this needs to be clearer. "This property specifies a list
of interface modes supported by the PHY hardware but are not
supported on the board."

I would also suggest having a think about a PHY that supports some
interface types that we don't have support in the kernel for, but
which also are not part of the board. Should these be listed
somehow as well? If not, how do we deal with the kernel later gaining
support for those interface modes, potentially the PHY driver as well,
and then having a load of boards not listing this?

My feeling is that listing negative properties presents something of
a problem, and we ought to stick with boards specifying what they
support, rather than what they don't.

That is a good point. And if this alternative `supported-modes` property
is missing, we can just assume that all modes are supported, in order
to be backward compatible.