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

[Max Krummenacher <max.krummenacher@active.ch>]

Am Freitag, den 18.03.2022, 17:53 +0000 schrieb Dave Stevenson:
> 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.

I agree that the strings used to set "data-mapping" weren't self explaining.
However, as there was a
clear 1:1 relation to the bus_format value the meaning
wasn't ambiguous at all.

> > > 
> > > 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.

We're on the same page that the strings that were used aren't self
explaining and do not follow a pattern which would make it easy to
extend. However that is something I addressed in my RFC proposal, not?

> > 
> > > 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.

Fine with me.

> > 
> > 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 disagree here, this forces us to use code to map the device tree enum
to the kernel enum for Linux, i.e. adds complexity and maintenance work
if additional bus_formats are needed.
Assuming there is another OS which uses the device tree it would not
make a difference, that OS would still need to map the device tree enum
to the corresponding representation in their kernel.
I would copy the definitions of media-bus-format.h into a header in
include/dt-bindings similarly as it is done for
include/dt-bindings/display/sdtv-standards.h for TV standards.

> 
> 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.

I disagree here, I don't see value in that structuring. It won't
help us mapping it to the corresponding bus_format enum and it
might be incomplete for bus_formats added in the future. 
E.g. besides your RGB565 example consider a Tegra 30 which for RGB666
outputs them on [23:8] and for RGB888 the 6 most significant bits are
kept in [23:8], the 2 least significant ones in [7:0]. That wouldn't
fit in this structured enum you propose, however one could easily add
a new consecutive number to the enum.

Max

> 
>   Dave


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