Smooth gdm -> GNOME3 session transition broken with 1.20 modesetting driver

[Daniel Stone]

Hi Hans,

On Wed, 5 Sep 2018 at 19:23, Hans de Goede <hdegoede at redhat.com> wrote:
> Under Fedora 29 (xserver-1.20) the transition from GDM to
> the GNOME3 session is no longer smooth, it seems that the
> screen is cleared to black when the Xserver starts instead
> of inheriting the framebuffer contents from GDM as before.
>
> Changing the DDX driver from modesetting to intel fixes this,
> I think this may be caused by the new atomic support in the
> modesetting driver.

It's caused by support for modifiers: this allows Mesa to use a
multi-planar framebuffer (auxiliary compression plane), which the new
X server can't then read back from because drmModeGetFB only supports
a single plane. I've written up the patches to do that[0], but last
time I tried to respin the current mainline was broken and didn't
actually boot. I've actually just picked that up this week and am
planning to send a new series out tomorrow.

[0]: https://lists.freedesktop.org/archives/xorg-devel/2018-March/056363.html

Cheers,
Daniel