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

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Message-ID: <20250306-kangaroo-of-pastoral-typhoon-8aefb2@houat>
Date: Thu, 6 Mar 2025 12:35:49 +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 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.

Maxime

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