Make DRM plane-assignment algorithm tolerant of more than one fullscreen opaque surface?

[Matt Hoosier]

Any interest (this is probably mostly a question for Daniel)?

On Thu, Aug 10, 2017 at 1:41 PM, Matt Hoosier <matt.hoosier at gmail.com>
wrote:

> Currently the DRM backend ends up picking the bottom-most view which meets
> all the checks for eligibility, for direct scanout usage. If more than one
> such view exists, we get a visual result counter to expectations -- it
> should be the highest-sorted such view that the user sees.
>
> The big loop in drm_assign_planes() that assigns views to planes iterates
> top-down through the weston_layer's, and the particular way that the
> results are progressively stored in variables means that if more than one
> pass through the loop encounters a view which is fullscreen and opaque then
> the last (bottom-most) of these passes is the one whose results are
> preserved upon exit from the loop.
>
> This normally isn't a problem because mostly only the desktop shell has a
> notion of fullscreen surfaces. desktop-shell takes care (whether
> intentionally or not I can't tell) that when running full-screen only the
> logical topmost view is actively left in a visible weston_layer.
>
> Is it informally expected that the shell must not allow more than one
> fullscreen view at a time? Would there be interest in a patch to
> drm_assign_planes() that adds a bit more awareness to the plane assignment
> so that only the topmost fullscreen opaque view is picked for scanout?
>
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