linux-kernel - Re: [PATCH 3/5] dt-bindings: display: simple-bridge: Document DPI color encoder

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Message-ID: <20250310-hopeful-helpful-quoll-e0dee8@houat>
Date: Mon, 10 Mar 2025 10:44:24 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Liu Ying <victor.liu@....com>
Cc: Rob Herring <robh@...nel.org>, 
	Alexander Stein <alexander.stein@...tq-group.com>, dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, krzk+dt@...nel.org, conor+dt@...nel.org, andrzej.hajda@...el.com, 
	neil.armstrong@...aro.org, rfoss@...nel.org, Laurent.pinchart@...asonboard.com, 
	jonas@...boo.se, jernej.skrabec@...il.com, maarten.lankhorst@...ux.intel.com, 
	tzimmermann@...e.de, airlied@...il.com, simona@...ll.ch
Subject: Re: [PATCH 3/5] dt-bindings: display: simple-bridge: Document DPI
 color encoder

On Fri, Mar 07, 2025 at 11:10:00AM +0800, Liu Ying wrote:
> On 03/06/2025, Maxime Ripard wrote:
> > On Thu, Mar 06, 2025 at 03:02:41PM +0800, Liu Ying wrote:
> >> On 03/06/2025, Rob Herring wrote:
> >>> On Wed, Mar 05, 2025 at 10:35:26AM +0100, Alexander Stein wrote:
> >>>> Hi,
> >>>>
> >>>> Am Dienstag, 4. März 2025, 16:23:20 CET schrieb Rob Herring:
> >>>>> On Tue, Mar 04, 2025 at 06:15:28PM +0800, Liu Ying wrote:
> >>>>>> A DPI color encoder, as a simple display bridge, converts input DPI color
> >>>>>> coding to output DPI color coding, like Adafruit Kippah DPI hat[1] which
> >>>>>> converts input 18-bit pixel data to 24-bit pixel data(with 2 low padding
> >>>>>> bits in every color component though). Document the DPI color encoder.
> >>>>>
> >>>>> Why do we need a node for this? Isn't this just wired how it is wired 
> >>>>> and there's nothing for s/w to see or do? I suppose if you are trying to 
> >>>>> resolve the mode with 24-bit on one end and 18-bit on the other end, you 
> >>>>> need to allow that and not require an exact match. You still might need 
> >>>>> to figure out which pins the 18-bit data comes out on, but you have that 
> >>>>> problem with an 18-bit panel too. IOW, how is this any different if you 
> >>>>> have an 18-bit panel versus 24-bit panel?
> >>>>
> >>>> Especially panel-simple.c has a fixed configuration for each display, such as:
> >>>>> .bus_format = MEDIA_BUS_FMT_RGB666_1X18
> >>>>
> >>>> How would you allow or even know it should be addressed as
> >>>> MEDIA_BUS_FMT_RGB888_1X24 instead? I see different ways:
> >>>> 1. Create a new display setting/compatible
> >>>> 2. Add an overwrite property to the displays
> >>>> 3. Use a (transparent) bridge (this series)
> >>>>
> >>>> Number 1 is IMHO out of question. 
> >>>
> >>> Agreed.
> >>>
> >>>> I personally don't like number 2 as this
> >>>> feels like adding quirks to displays, which they don't have.
> >>>
> >>> This is what I would do except apply it to the controller side. We know 
> >>> the panel side already. This is a board variation, so a property makes 
> >>> sense. I don't think you need any more than knowing what's on each end. 
> >>
> >> With option 2, no matter putting a property in source side or sink side,
> >> impacted display drivers and DT bindings need to be changed, once a board
> >> manipulates the DPI color coding.  This adds burdens and introduces new
> >> versions of those DT bindings.  Is this what we want?
> > 
> > There's an option 4: make it a property of the OF graph endpoints. In
> > essence, it's similar to properties that are already there like
> > lane-mapping, and it wouldn't affect the panel drivers, or create an
> > intermediate bridge.
> 
> I don't see lane-mapping anywhere. Do you mean data-mapping instead?
> data-mapping is not defined in dtschema. Only lvds-codec.yaml defines
> data-mapping in endpoint.

I meant as a general concept. The properties are data-lanes and
clock-lanes in
Documentation/devicetree/bindings/media/video-interfaces.yaml

> With option 4, I guess you meant display sink drivers, i.e., panel and
> bridge drivers, wouldn't be affected. Then, display source drivers, i.e.,
> display controller and bridge drivers, would be affected. This adds
> burdens for driver developers/maintainers(though almost no effort from
> DT's PoV), doesn't it?

Not necessarily, panels have a phandle to the parent endpoint too so
they can do that walk and configure their format if it's any easier.

> Moreover, why it has to be the display sink drivers which are not affected?
> DT writers might choose to set the format at the sink endpoint, e.g., setting
> RGB666 at the sink endpoint of a RGB888 DPI panel or bridge.

Why wouldn't you run the panel at the highest bpc possible?

Maxime

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