Re: [RFC PATCH 3/4] dt-bindings: panel: Introduce dual-link LVDS panel

[Tomi Valkeinen]

On 09/01/2023 18:21, Aradhya Bhatia wrote:
> Hi Angelo,
>
> Thanks for taking a look at the patches!
>
> On 03-Jan-23 17:21, AngeloGioacchino Del Regno wrote:
> > Il 03/01/23 07:46, Aradhya Bhatia ha scritto:
> > > Dual-link LVDS interfaces have 2 links, with even pixels traveling on
> > > one link, and odd pixels on the other. These panels are also generic in
> > > nature, with no documented constraints, much like their single-link
> > > counterparts, "panel-lvds".
> > >
> > > Add a new compatible, "panel-dual-lvds", and a dt-binding document for
> > > these panels.
> > >
> > > Signed-off-by: Aradhya Bhatia <a-bhatia1@xxxxxx>
> > > ---
> > >   .../display/panel/panel-dual-lvds.yaml        | 157 ++++++++++++++++++
> > >   MAINTAINERS                                   |   1 +
> > >   2 files changed, 158 insertions(+)
> > >   create mode 100644 
> > > Documentation/devicetree/bindings/display/panel/panel-dual-lvds.yaml
> > >
> > > diff --git 
> > > a/Documentation/devicetree/bindings/display/panel/panel-dual-lvds.yaml b/Documentation/devicetree/bindings/display/panel/panel-dual-lvds.yaml
> > > new file mode 100644
> > > index 000000000000..88a7aa2410be
> > > --- /dev/null
> > > +++ 
> > > b/Documentation/devicetree/bindings/display/panel/panel-dual-lvds.yaml
> > > @@ -0,0 +1,157 @@
> > > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > > +%YAML 1.2
> > > +---
> > > +$id: http://devicetree.org/schemas/display/panel/panel-dual-lvds.yaml#
> > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > +
> > > +title: Generic Dual-Link LVDS Display Panel
> > > +
> > > +maintainers:
> > > +  - Aradhya Bhatia <a-bhatia1@xxxxxx>
> > > +  - Thierry Reding <thierry.reding@xxxxxxxxx>
> > > +
> > > +description: |
> > > +  A dual-LVDS interface is a dual-link connection with the even pixels
> > > +  traveling on one link, and the odd pixels traveling on the other.
> > > +
> > > +allOf:
> > > +  - $ref: panel-common.yaml#
> > > +  - $ref: /schemas/display/lvds.yaml/#
> > > +
> > > +properties:
> > > +  compatible:
> > > +    oneOf:
> > > +      - items:
> > > +          - enum:
> > > +              - lincolntech,lcd185-101ct
> > > +              - microtips,13-101hieb0hf0-s
> > > +          - const: panel-dual-lvds
> > > +      - const: panel-dual-lvds
> > > +
> > > +  ports:
> > > +    $ref: /schemas/graph.yaml#/properties/ports
> > > +
> > > +    properties:
> > > +      port@0:
> > > +        $ref: /schemas/graph.yaml#/$defs/port-base
> > > +        unevaluatedProperties: false
> > > +        description: The sink for first set of LVDS pixels.
> > > +
> > > +        properties:
> > > +          dual-lvds-odd-pixels:
> > > +            type: boolean
> > > +
> > > +          dual-lvds-even-pixels:
> > > +            type: boolean
> > > +
> > > +        oneOf:
> > > +          - required: [dual-lvds-odd-pixels]
> >
> > One question: why do we need a "panel-dual-lvds" compatible?
> > A Dual-LVDS panel is a LVDS panel using two ports, hence still a 
> > panel-lvds.
> >
> > If you're doing this to clearly distinguish, for human readability 
> > purposes,
> > single-link vs dual-link panels, I think that this would still be 
> > clear even
> > if we use panel-lvds alone because dual-link panels, as you wrote in this
> > binding, does *require* two ports, with "dual-lvds-{odd,even}-pixels" 
> > properties.
>
> Yes, while they are both LVDS based panels the extra LVDS sink in these
> panels, and the capability to decode and display the 2 sets of signals
> are enough hardware differences that warrant for an addition of a new
> compatible.
>
> > So... the devicetree node would look like this:
> >
> > panel {
> >      compatible = "vendor,panel", "panel-lvds";
> >      ....
> >      ports {
> >          port@0 {
> >              .....
> >              -> dual-lvds-odd-pixels <-
> >          }
> >
> >          port@1 {
> >              .....
> >              -> dual-lvds-even-pixels <-
> >          };
> >      };
> > };
> >
> > > +          - required: [dual-lvds-even-pixels]
> >
> > ...Though, if you expect dual-lvds panels to get other quirks in the 
> > future,
> > that's a whole different story and you may actually need the 
> > panel-dual-lvds
> > compatible.
>
> Yes, exactly. Even while being non-smart, there are going to be more
> quirks in future. And it would be better if they have their own
> compatible/binding, and are not getting appended in an ever-growing
> if-else ladder. :)

I can imagine a panel which you can use with a single LVDS link if the 
clock is high enough, or two LVDS links if the clock has to be lower. Is 
that a dual-lvds panel? =)

But probably that situation is no different than a panel that can work 
with DSI or DPI input.

Still, I'm agree with Angelo in that a new compatible string for dual 
link lvds feels a bit odd. That said, it's possible the panel-lvds 
bindings might get rather confusing. So I don't have a strong feeling here.

 Tomi