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

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

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@ti.com>
>>> ---
>>>   .../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@ti.com>
>>> +  - Thierry Reding <thierry.reding@gmail.com>
>>> +
>>> +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