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

[Dave Stevenson <dave.stevenson@raspberrypi.com>]

Hi Maxime

On Fri, 18 Mar 2022 at 16:35, Maxime Ripard <maxime@cerno.tech> wrote:
>
> 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@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.

MEDIA_BUS_FMT_RGB666_1X18 defines an 18bit bus, therefore there is no
padding. "bgr666" was selecting that media bus code (I won't ask about
the rgb/bgr swap).

If there is padding on a 24 bit bus, then you'd use (for example)
MEDIA_BUS_FMT_RGB666_1X24_CPADHI to denote that the top 2 bits of each
colour are the padding. Define and use a PADLO variant if the padding
is the low bits.
The string matching would need to be extended to have some string to
select those codes ("lvds666" is a weird choice from the original
patch).

Taking those media bus codes and handling them appropriately is
already done in vc4_dpi [1], and the vendor tree has gained
BGR666_1X18 and BGR666_1X24_CPADHI [2] as they aren't defined in
mainline.

Now this does potentially balloon out the number of MEDIA_BUS_FMT_xxx
defines needed, but that's the downside of having defines for all
formats.

(I will admit to having a similar change in the Pi vendor tree that
allows the media bus code to be selected explicitly by hex value).

  Dave

[1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/vc4/vc4_dpi.c#L154
[2] https://github.com/raspberrypi/linux/blob/rpi-5.15.y/include/uapi/linux/media-bus-format.h#L49

> > 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