[PATCH xserver] dri3: Fix DRI3.2 support for drivers other than modesetting-ddx.

[Adam Jackson]

On Tue, 2018-04-24 at 11:42 +0100, Daniel Stone wrote:
> Hi Mario,
> 
> On 24 April 2018 at 09:17, Mario Kleiner <mario.kleiner.de at gmail.com> wrote:
> > Both xf86-video-intel and xf86-video-nouveau cause OpenGL
> > clients to fail when used with DRI3 on server 1.20 with Mesa 18.1.
> > 
> > Reason is that the servers DRI3 version is now unconditionally
> > reported as DRI3 1.2 to 1.2 capable clients. This causes clients
> > using Mesa 18.1 to use the new DRI 3.2 requests DRI3GetSupportedModifiers,
> > DRI3PixmapFromBuffers, etc. Drivers other than modesetting-ddx
> > do not support the needed hooks like info->pixmap_from_fds or
> > info->get_formats, info->get_modifiers. Unfortunately we can't
> > simply report the servers DRI3 version as 1.0 in this case, as
> > the reported version can not be specific to a X-Screen, and
> > different screens may have drivers with different capabilities.

I think this implies we should not advertise 1.2 if any screen lacks
the 1.2 hooks.

> Thanks a lot for hunting this down! This fix does still make me
> uncomfortable though. I think a better fix would be, on the Mesa side,
> to never call any 1.2 codepaths for the server if we get no
> formats/modifiers back from the GetSupported calls, and to never call
> any 1.2 codepaths for the drawable if DRIImage's
> createImageWithModifiers does not exist or if it fails and we fall
> back to regular createImage. That way we make sure that both sides
> exercise the same behaviour: either both with explicit modifiers, or
> neither.

The problem with that strategy is it means 1.20 would be compatible
with zero already-released versions of Mesa.

- ajax