Re: [PATCH] dt-bindings: media: Convert video-mux to DT schema

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Rob,

On Fri, Mar 12, 2021 at 08:33:06AM -0700, Rob Herring wrote:
> On Fri, Mar 12, 2021 at 3:06 AM Laurent Pinchart wrote:
> > On Fri, Mar 12, 2021 at 09:29:04AM +0200, Sakari Ailus wrote:
> > > On Fri, Mar 12, 2021 at 03:25:11AM +0200, Laurent Pinchart wrote:
> > > > On Thu, Mar 11, 2021 at 04:40:42PM -0700, Rob Herring wrote:
> > > > > Now that we have the graph schema, convert the video-mux binding to DT
> > > > > schema.
> > > > >
> > > > > Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
> > > > > Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> > > > > Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
> > > > > Cc: linux-media@vger.kernel.org
> > > > > Signed-off-by: Rob Herring <robh@kernel.org>
> > > > > ---
> > > > >  .../devicetree/bindings/media/video-mux.txt   | 60 ------------
> > > > >  .../devicetree/bindings/media/video-mux.yaml  | 93 +++++++++++++++++++
> > > > >  2 files changed, 93 insertions(+), 60 deletions(-)
> > > > >  delete mode 100644 Documentation/devicetree/bindings/media/video-mux.txt
> > > > >  create mode 100644 Documentation/devicetree/bindings/media/video-mux.yaml
> > > > >
> > > > > diff --git a/Documentation/devicetree/bindings/media/video-mux.txt b/Documentation/devicetree/bindings/media/video-mux.txt
> > > > > deleted file mode 100644
> > > > > index 63b9dc913e45..000000000000
> > > > > --- a/Documentation/devicetree/bindings/media/video-mux.txt
> > > > > +++ /dev/null
> > > > > @@ -1,60 +0,0 @@
> > > > > -Video Multiplexer
> > > > > -=================
> > > > > -
> > > > > -Video multiplexers allow to select between multiple input ports. Video received
> > > > > -on the active input port is passed through to the output port. Muxes described
> > > > > -by this binding are controlled by a multiplexer controller that is described by
> > > > > -the bindings in Documentation/devicetree/bindings/mux/mux-controller.txt
> > > > > -
> > > > > -Required properties:
> > > > > -- compatible : should be "video-mux"
> > > > > -- mux-controls : mux controller node to use for operating the mux
> > > > > -- #address-cells: should be <1>
> > > > > -- #size-cells: should be <0>
> > > > > -- port@*: at least three port nodes containing endpoints connecting to the
> > > > > -  source and sink devices according to of_graph bindings. The last port is
> > > > > -  the output port, all others are inputs.
> > > > > -
> > > > > -Optionally, #address-cells, #size-cells, and port nodes can be grouped under a
> > > > > -ports node as described in Documentation/devicetree/bindings/graph.txt.
> > > > > -
> > > > > -Example:
> > > > > -
> > > > > - mux: mux-controller {
> > > > > -         compatible = "gpio-mux";
> > > > > -         #mux-control-cells = <0>;
> > > > > -
> > > > > -         mux-gpios = <&gpio1 15 GPIO_ACTIVE_HIGH>;
> > > > > - };
> > > > > -
> > > > > - video-mux {
> > > > > -         compatible = "video-mux";
> > > > > -         mux-controls = <&mux>;
> > > > > -         #address-cells = <1>;
> > > > > -         #size-cells = <0>;
> > > > > -
> > > > > -         port@0 {
> > > > > -                 reg = <0>;
> > > > > -
> > > > > -                 mux_in0: endpoint {
> > > > > -                         remote-endpoint = <&video_source0_out>;
> > > > > -                 };
> > > > > -         };
> > > > > -
> > > > > -         port@1 {
> > > > > -                 reg = <1>;
> > > > > -
> > > > > -                 mux_in1: endpoint {
> > > > > -                         remote-endpoint = <&video_source1_out>;
> > > > > -                 };
> > > > > -         };
> > > > > -
> > > > > -         port@2 {
> > > > > -                 reg = <2>;
> > > > > -
> > > > > -                 mux_out: endpoint {
> > > > > -                         remote-endpoint = <&capture_interface_in>;
> > > > > -                 };
> > > > > -         };
> > > > > - };
> > > > > -};
> > > > > diff --git a/Documentation/devicetree/bindings/media/video-mux.yaml b/Documentation/devicetree/bindings/media/video-mux.yaml
> > > > > new file mode 100644
> > > > > index 000000000000..780fbbd46a38
> > > > > --- /dev/null
> > > > > +++ b/Documentation/devicetree/bindings/media/video-mux.yaml
> > > > > @@ -0,0 +1,93 @@
> > > > > +# SPDX-License-Identifier: GPL-2.0
> > > > > +%YAML 1.2
> > > > > +---
> > > > > +$id: http://devicetree.org/schemas/media/video-mux.yaml#
> > > > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > > > +
> > > > > +title: Video Multiplexer
> > > > > +
> > > > > +maintainers:
> > > > > +  - Sakari Ailus <sakari.ailus@linux.intel.com>
> > > > > +  - Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> > > > > +
> > > > > +description:
> > > > > +  Video multiplexers allow to select between multiple input ports. Video
> > > > > +  received on the active input port is passed through to the output port. Muxes
> > > > > +  described by this binding are controlled by a multiplexer controller.
> > > > > +
> > > > > +properties:
> > > > > +  compatible:
> > > > > +    const: video-mux
> > > > > +
> > > > > +  mux-controls:
> > > > > +    maxItems: 1
> > > > > +
> > > > > +  '#address-cells':
> > > > > +    const: 1
> > > > > +
> > > > > +  '#size-cells':
> > > > > +    const: 0
> > > > > +
> > > > > +  ports:
> > > > > +    $ref: /schemas/graph.yaml#/properties/ports
> > > > > +
> > > > > +    patternProperties:
> > > > > +      '^port@':
> > > > > +        $ref: /schemas/graph.yaml#/properties/port
> > > >
> > > > Should we require at least port@0, port@1 and port@2 ?
> > > >
> > > > > +
> > > > > +patternProperties:
> > > > > +  '^port@':
> > > > > +    $ref: /schemas/graph.yaml#/properties/port
> > > > > +    description:
> > > > > +      At least three port nodes containing endpoints connecting to the source
> > > > > +      and sink devices according to of_graph bindings. The last port is the
> > > > > +      output port, all others are inputs.
> > > > > +
> > > > > +required:
> > > > > +  - compatible
> > > > > +  - mux-controls
> > > >
> > > > Should a constraint be added to ensure that either a ports node or
> > > > port@0, port@1 and port@2 nodes exists ?
> > >
> > > It's not meaningful to have this device without such nodes. But a mux with
> > > more ports could be connected in a way that leaves one or both of ports 1
> > > and 2 unconnected. It's still not a likely configuration but a possible
> > > one.
> >
> > Those ports wouldn't be connected, but they could still exist in DT.
> 
> Could or must? I think we could have both cases. If there's never a
> connection on a board no point in having the node there. If there's a
> possible connection (via a connector and overlay), then we may want
> the node.

I tend to treat ports as mandatory and endpoints as optional, as the
former describe a intrinsic property of the device and the latter a
connection in a specific system.

Looking at the video-mux driver, it first finds the number of the last
port, and then creates N V4L2 subdev pads, regardless of whether a port
exists for it. There's thus an implicit assumption that ports will be
numbered 0 to N-1 and will all exist. If a port doesn't exist in DT, the
driver should still operate correctly as far as I can tell, but it will
in any case create N pads.

Now, that's a driver implementation, but given that the bindings are for
a generic mux device, the driver matters here. I'd be fine making ports
optional and modifying the driver, or making them mandatory in DT, but I
think it's important to at least synchronize the bindings and the
driver.

-- 
Regards,

Laurent Pinchart