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

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Date: Wed, 6 Mar 2024 07:05:31 -0800
From: Doug Anderson <dianders@...omium.org>
To: Thomas Zimmermann <tzimmermann@...e.de>
Cc: dri-devel@...ts.freedesktop.org, Rob Clark <robdclark@...omium.org>, 
	Javier Martinez Canillas <javierm@...hat.com>, Daniel Vetter <daniel@...ll.ch>, Dave Airlie <airlied@...hat.com>, 
	David Airlie <airlied@...il.com>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Maxime Ripard <mripard@...nel.org>, Maíra Canal <mcanal@...lia.com>, 
	Sean Paul <sean@...rly.run>, Ville Syrjälä <ville.syrjala@...ux.intel.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/udl: Add ARGB8888 as a format

Hi,

On Wed, Mar 6, 2024 at 4:07 AM Thomas Zimmermann <tzimmermann@...e.de> 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. ;-)

-Doug