[RFC PATCH] drm/panel: simple: panel-dpi: use bus-format to set bpc and bus_format

[Maxime Ripard]

On Mon, Mar 07, 2022 at 04:26:56PM +0100, Max Krummenacher wrote:
> On Wed, Mar 2, 2022 at 5:22 PM Marek Vasut <marex at denx.de> wrote:
> >
> > On 3/2/22 15:21, Maxime Ripard wrote:
> > > Hi,
> >
> > Hi,
> >
> > > Please try to avoid top posting
> Sorry.
> 
> > >
> > > On Wed, Feb 23, 2022 at 04:25:19PM +0100, Max Krummenacher wrote:
> > >> The goal here is to set the element bus_format in the struct
> > >> panel_desc. This is an enum with the possible values defined in
> > >> include/uapi/linux/media-bus-format.h.
> > >>
> > >> The enum values are not constructed in a way that you could calculate
> > >> the value from color channel width/shift/mapping/whatever. You rather
> > >> would have to check if the combination of color channel
> > >> width/shift/mapping/whatever maps to an existing value and otherwise
> > >> EINVAL out.
> > >>
> > >> I don't see the value in having yet another way of how this
> > >> information can be specified and then having to write a more
> > >> complicated parser which maps the dt data to bus_format.
> > >
> > > Generally speaking, sending an RFC without explicitly stating what you
> > > want a comment on isn't very efficient.
> >
> > Isn't that what RFC stands for -- Request For Comment ?
> 
> I hoped that the link to the original discussion was enough.
> 
> panel-simple used to have a finite number of hardcoded panels selected
> by their compatible.
> The following patchsets added a compatible 'panel-dpi' which should
> allow to specify the panel in the device tree with timing etc.
>   https://patchwork.kernel.org/project/dri-devel/patch/20200216181513.28109-6-sam@ravnborg.org/
> In the same release cycle part of it got reverted:
>   https://patchwork.kernel.org/project/dri-devel/patch/20200314153047.2486-3-sam@ravnborg.org/
> With this it is no longer possible to set bus_format.
>
> The explanation what makes the use of a property "data-mapping" not a
> suitable way in that revert
> is a bit vague.

Indeed, but I can only guess. BGR666 in itself doesn't mean much for
example. Chances are the DPI interface will use a 24 bit bus, so where
is the padding?

I think that's what Sam and Laurent were talking about: there wasn't
enough information encoded in that property to properly describe the
format, hence the revert.

> The RFC revert of the revert
>   https://patchwork.kernel.org/project/dri-devel/patch/20220201110717.3585-1-cniedermaier@dh-electronics.com/
> tried to get feedback what would be a way forward. This RFC tries the
> same by giving a possible solution should
> the property name and/or the a bit short strings of the original be
> the reason why it is not suitable.
> 
> So the requested comments would be about what was not good enough with
> 'data-mapping' and what would be a way forward.
> 
> Especially since in my limited view it is not clear why in panel-lvds
> 'data-mapping' is used to state how the bits representing colour are
> mapped to the 21 or 28 possible bit position in the LVDS lanes vs.
> here where we want to say how the bits representing colour are mapped
> to the 16/18/24 lines of the parallel interface would need a different
> binding pattern.

There's only a few data format in LVDS, so it's ok. A DPI interface is
much more free-form, so you need to be a bit more accurate than what is
done for LVDS.

Maxime
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