[Mesa-dev] [PATCH v2 1/2] egl/wayland: Check queryImage return for wl_buffer

[Daniel Stone]

Hi Marek,

On 4 October 2017 at 12:38, Marek Olšák <maraeo at gmail.com> wrote:
> Yeah I don't have an answer to that. I think it's better to pageflip than
> blit. Can Wayland reallocate the buffer to make it displayable? If not, it
> may be better to create displayable buffers always.

Not sure what you mean by 'Wayland reallocate'? The buffers are
(DRI3-style) allocated by the client's EGLSurface, with no
intermediate server-allocated storage at any point There is no
explicit hint to the client whether it is being scanned out or not.
Even if there was, there is no indication of which device the buffer
might be scanned out on - a device the client might not have access to
(e.g. a display controller with no render nodes). I've mostly been
waiting for the allocator to come around before trying to deal with
this. But even with that, we'd still require the tiling information to
actually be exposed externally (i.e. through modifiers); currently the
compositor can't introspect the tiling flags at all, so it has no idea
that it shouldn't try to scan this particular buffer out.

Which is exactly the problem validateUsage() is supposed to solve. All
we can do is rely on gbm_bo_import failing (which is best, as it can
never be used for scanout), or at least the KMS commit fail (worse,
especially pre-atomic, but still beats displaying garbage?). We don't
need magic blits, or untiled views, or anything: just failing is fine
and we'll fall back to composition.

Cheers,
Daniel