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

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

On Fri, 18 Mar 2022 at 17:16, Maxime Ripard <maxime@cerno.tech> wrote:
>
> On Fri, Mar 18, 2022 at 05:05:11PM +0000, Dave Stevenson wrote:
> > 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.
>
> Yeah, that's kind of my point actually :)

Ah, OK :)

> Just having a rgb666 string won't allow to differentiate between
> MEDIA_BUS_FMT_RGB666_1X18 and MEDIA_BUS_FMT_RGB666_1X24_CPADHI: both are
> RGB666 formats. Or we could say that it's MEDIA_BUS_FMT_RGB666_1X18 and
> then when we'll need MEDIA_BUS_FMT_RGB666_1X24_CPADHI we'll add a new
> string but that usually leads to inconsistent or weird names, so this
> isn't ideal.
>
> > 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).
>
> I think having an integer value is indeed better: it doesn't change much
> in the device tree if we're using a header, it makes the driver simpler
> since we don't have to parse a string, and we can easily extend it or
> rename the define, it won't change the ABI.
>
> I'm not sure using the raw media bus format value is ideal though, since
> that value could then be used by any OS, and it would effectively force
> the mbus stuff down their throat.

I'll agree that the media bus format isn't the nicest, but I was
looking for a quick fix that could be configured from an overlay.

If using defines, then possibly go for a partial bitmask?
3 bits for RGB order can be defined across the board. An encoding of
the bus width. And then the packing within that bus width would have
to be a lookup table, with no padding, padhi, and padlo being defined
as 0, 1, and 2 respectively. >=3 are extensions per bus width.
MEDIA_BUS_FMT_RGB666_1X24_CPADHI might then be described as ORDER_RGB
| BUS_24 | PAD_HI.
And MEDIA_BUS_FMT_BGR666 as ORDER_BGR | BUS_18 | NO_PAD.

Hmm, a bit more thought needed for RGB565, as a bus width of 16
wouldn't guarantee that.

  Dave

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