Re: [PATCH] drm/udl: Add ARGB8888 as a format

[Thomas Zimmermann]

Hi

Am 06.03.24 um 16:05 schrieb Doug Anderson:
> Hi,
>
> On Wed, Mar 6, 2024 at 4:07 AM Thomas Zimmermann <tzimmermann@xxxxxxx> wrote:
> > Hi,
> >
> > sorry that I did not see the patch before.
> >
> > Am 27.02.24 um 23:19 schrieb Douglas Anderson:
> > > Even though the UDL driver converts to RGB565 internally (see
> > > pixel32_to_be16() in udl_transfer.c), it advertises XRGB8888 for
> > > compatibility. Let's add ARGB8888 to that list.
> > We had a heated discussion about the emulation of color formats. It was
> > decided that XRGB8888 is the only format to support; and that's only
> > because legacy userspace sometimes expects it. Adding other formats to
> > the list should not be done easily.
> Sorry! I wasn't aware of the previous discussion and nobody had
> brought it up till now. As discussed on #dri-devel IRC, I've posted a
> revert:
>
> https://lore.kernel.org/r/20240306063721.1.I4a32475190334e1fa4eef4700ecd2787a43c94b5@changeid
>
>
> > > This makes UDL devices work on ChromeOS again after commit
> > > c91acda3a380 ("drm/gem: Check for valid formats"). Prior to that
> > > commit things were "working" because we'd silently treat the ARGB8888
> > > that ChromeOS wanted as XRGB8888.
> > This problem has been caused by userspace. Why can it not be fixed there?
> I guess the one argument I could make is that the kernel isn't
> supposed to break userspace. Before the extra format validation patch,
> AKA commit c91acda3a380 ("drm/gem: Check for valid formats"),
> userspace worked. Now it doesn't.
>
> That being said, one can certainly argue that userspace was working in
> the past simply due to relying on a bug. ...and in such a case fixing
> the bug in userspace is preferred.
>
> I don't personally know _how_ to fix userspace but it feels like it
> should be possible.
>
>
> > And udl is just one driver. Any other driver without ARGB8888, such as
> > simpledrm or ofdrm, would be affected. Do these work?
> It's the ChromeOS compositor. I can totally believe that those drivers
> don't work. In this case, though, those drivers aren't needed by a USB
> peripheral that someone might plug in. ;-)

If you can fix to problem in the compositor, that would be the correct 
solution. If it's really not possible, we'd have to figure out something 
else.

We supported various combinations of source and destination formats in 
the kernel. But that results in state explosion at some point. And we 
wanted to move conversion code to userspace where possible. So only 
XRGB8888 was allowed, as it's sometimes necessary to support legacy 
userspace. (Ironically, one could make that argument for ARGB and the 
ChromeOS compositor. :) We even have code that works around incorrect 
ARGB reported by hardware or firmware, so that userspace absolutely does 
not see ARGB for the primary planes of simpledrm/ofdrm. [1]

Best regards
Thomas

[1] 
https://elixir.bootlin.com/linux/v6.7/source/drivers/gpu/drm/drm_format_helper.c#L1158

> -Doug

--
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Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
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