When GraphicsBuffer::dropped() is emitted, the buffer can be
unreferenced. If that's the case, the GraphicsBuffer will be deleted
twice: first in GraphicsBuffer::unref(), the second time in drop().
In order to address the issue, this change gets rid of the
GraphicsBuffer::dropped() signal, so it's always guaranteed that the
buffer stays alive until the reference count is checked.
The GraphicsBuffer::dropped() signal is used to remove the mapping
between wl_resource and ShmClientBuffer when the corresponding
wl_shm_buffer object is destroyed. On the other hand, we could perform
such cleanup when calling drop() too. This code can be further improved
by reimplementing wl-shm, which we need to do at some point in the
future.
The main motivation is to avoid scattering graphics buffer things around
kwin.
DmaBufParams struct has been moved to the OutputBackend, but with the
introduction of buffer allocators, we need to port screencasting code to
the new abstractions some time in the future.
A client can specify the following flags when creating a linux dmabuf
client buffer:
- y_invert
- interlaced
- bottom_first
Only the y_invert flag is supported by kwin. The interlaced and the
bottom_first flags are ignored. On the other hand, most clients don't
specify the dmabuf flags. For example, neither EGL nor Vulkan WSIs
use the y_invert flag.
The y_invert flag is undesired because it also blocks optimizations such
as direct scanout because DRM assumes that the origin is in the top left
corner.
Therefore, this change drops the support for linux dmabuf flags. From
the protocol perspective, this is fine. It can be viewed as buffer
import failing with the specified flags.
LinuxDmaBufV1ClientBuffer contains properties (formats, and flags) that
are not available in the base GraphicsBuffer type and there's no reason
to move it there.
In order to get rid of those properties (and eventually hide the
LinuxDmaBufV1ClientBuffer type from the public api), this change adds a
DmaBufAttributes getter in the GraphicsBuffer.
Currently, there exists the separation between the buffers provided by
the clients and the buffers created by the compositor.
In hindsight, this separation is not great because it leads to
specialized code paths in the output backend. For example, we have a
separate code path for direct scanout and presenting composited frame.
But you could view the latter case as "direct scanout of a compositor
buffer".
The main idea behind the buffer type is to provide a base buffer type
for client buffers and composited frame buffers (not drm fbs) that we
could pass around, import as textures, etc.
