Re: [PATCH v3 1/4] drm: bridge: dw_hdmi: Add flag to indicate output port is optional

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

On Tue, Dec 31, 2024 at 10:10:51PM +0100, Marek Vasut wrote:
> On 12/31/24 9:31 PM, Laurent Pinchart wrote:
> > Hi Marek,
> 
> Hi,
> 
> > Thank you for the patch.
> > 
> > On Tue, Dec 31, 2024 at 08:28:48PM +0100, Marek Vasut wrote:
> >> Add a flag meant purely to work around broken i.MX8MP DTs which enable
> >> HDMI but do not contain the HDMI connector node. This flag allows such
> >> DTs to work by creating the connector in the HDMI bridge driver. Do not
> >> use this flag, do not proliferate this flag, please fix your DTs.
> > 
> > What's the rationale for this, what prevents fixing DT instead of using
> > this flag ? Adding such a flag will most likely open the door to
> > proliferation.
> 
> See the V2 series discussion, there are a few in-tree DTs which do not 
> have the HDMI connector node. The rationale is there might be more and 
> they might come from vendors, so this flag is necessary to work around 
> those DTs.
>
> > If you can't fix the DT on particular boards, patching it could be an
> > option. We had a similar problem on Renesas boards, which we fixed with
> > a DT overlay, see commit 81c0e3dd82927064 ("drm: rcar-du: Fix legacy DT
> > to create LVDS encoder nodes"). This made the workaround self-contained,
> > and allowed dropping it several kernel versions later (in commit
> > 841281fe52a769fe, "drm: rcar-du: Drop LVDS device tree backward
> > compatibility").
>
> Frankly, I would much rather fix the few in-tree DTs and mandate the 
> HDMI connector node in DT, which would keep the code simple, rather than 
> maintain a backward compatibility workaround for problem which might not 
> even exist.

The in-tree device tree sources should be converted as part of the
series, I don't see a point trying to maintain backward compatibility
for in-tree DT sources.

For out-of-tree sources it depends on how likely the problem is. There's
no regression if nobody is affected. I personally like restricting
backward compatibility to the strict minimum, to ensure that all new DTs
will use proper bindings. Making the backward compatibility code
self-contained helps there, and we could also print a loud warning
(WARN_ON() seems appropriate) and set a date for the removal of the
workaround.

-- 
Regards,

Laurent Pinchart