Backends aren't the right layer to take care of placeholder outputs, and
don't really have enough information to do it either. This also fixes a
crash, because the placeholder output currently gets created too late
Platform backends are provided as plugins. This is great for
extensibility, but the disadvantages of this design outweigh the
benefits.
The number of backends will be limited, it's safe to say that we will
have to maintain three backends for many years to come - kms/drm,
virtual, and wayland. The plugin system adds unnecessary complexity.
Startup logic is affected too. At the moment, platform backends provide
the session object, which is awkward as it starts adding dependencies
between backends. It will be nicer if the session is created depending
on the loaded session type.
In some cases, wayland code needs to talk to the backend directly, e.g.
for drm leasing, etc. With the plugin architecture it's hard to do that.
Not impossible though, we can approach it as in Qt 6, but it's still
harder than linking the code directly.
Of course, the main disadvantage of shipping backends in a lib is that
you will need to patch kwin if you need a custom platform, however such
cases will be rare.
Despite that disadvantage, I still think that it's a step in the right
direction where the goal is to have multi-purpose backends and other
reusable components of kwin.
The legacy X11 standalone platform is linked directly to kwin_x11
executable, while the remaining backends are linked to libkwin.
The main motivation behind this change is to make the drm backend
multi-purpose. That's it, to make it suitable for implementing all kinds
of compositors. At the moment, there's an artificial split between
"desktop" and "non-desktop" outputs, i.e. VR headsets, which stands in
the way of that and moving the remaining wayland code out of the drm
backend for better layering and architecture.
This allows us to get more information about the outputs like vendor
and model and for example provide them to effects which might find
the extra info useful.
It appears that importing gbm_bo's using eglCreateImageKHR() doesn't
work on nvidia.
Another issue is that Platform::createDmaBufTexture() uses
gbm_bo_create(). The nvidia driver doesn't like that, it's preferred to
use gbm_bo_create_with_modifiers().
In order to address those issues, this change refactors how gbm_bo
objects are imported and how gbm_bo's are allocated for dmabuf textures,
so the same code can be reused in EglGbmBackend::textureForOutput()
and Platform::createDmaBufTexture().
Instead of buffers being both drm framebuffers and gbm / dumb buffers, these
responsibilities are now split, which makes it possible to do zero copy
screen casting in the future.
Both framebuffers and gbm / dumb buffers also now always hold a valid underlying
resource, which simplifies code a bit.
In order to support layered rendering and tiled outputs KWin needs to be
able to split rendering of outputs into multiple surfaces. This commit
prepares the drm backend for that, by moving most of the code in EglGbmBackend
out to a EglGbmSurface class, which will later be used for overlay surfaces
and rendering to multiple connectors side by side.
In doing that, this commit also cleans up the code a bit, removes a lot of
now unnecessary multi-gpu stuff and potentially makes modesets a little
bit more efficient by re-using resources more often.
The ifdefs for have_gbm obfuscate the code unnecessarily - the drm backend
is not a great experience with qpainter, so in practice noone should ship
it without gbm anyways.
The proprietary NVidia driver now supports gbm, which vastly improves the
user experience. For older devices that will not get gbm support dropping
EglStreams will likely not have a big impact as it has several session breaking
issues anyways.
By removing the backend a lot of logic can be simplified, most notably multi-gpu.
This allows using base opengl backends in libkwin, which can be useful
later on for the purpose of moving the ownership of render backends from
the Scene class to the Compositor class.
This improves file organization in kwin by putting backends in a single
directory.
It also makes easier to discover kwin's low level components for new
contributors because the plugins directory may come as the last place to
look for. When one hears "plugin", the first thing that comes to mind is
regular plugins, not low level backends.
2021-11-02 09:02:41 +00:00
Renamed from src/plugins/platforms/drm/CMakeLists.txt (Browse further)
