The type of render device used for a specific GPU affects the mode
setting backend that can be used, more specifically, when the render
device is an EGLStream based one, atomic mode setting isn't possible, as
page flipping is done via EGL, not via atomic mode setting commits.
Preparing the render devices before KMS devices means can make a more
informed decision whether to deny-list atomic mode setting for when
a certain GPU uses a EGLStream based render device instance.
This also means we need to translate mode setting devices to render node
devices when creating the render device itself, as doing it later when
creating the mode setting device is already too late.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2578>
This changes the way the KMS backends load; if we're headless, we always
use the dummy one and fail otherwise; in other cases, we first try the
atomic backend, and if that fails, fall back on the simple one.
The aim for this is to have the impl device open and close the device
when needed, using the device pool directly.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1828>
Currently our only entry point for DRM devices is MetaKms*, but in order
to run without being DRM master, we cannot use /dev/dri/card*, nor can
we be either of the existing MetaKmsImplDevice implementation (legacy
KMS, and atomic KMS), as they both depend on being DRM master.
Thus to handle running without being DRM master (i.e. headless), add a
"dummy" MetaKmsImplDevice implementation, that doesn't do any mode
setting at all, and that switches to operate on the render node, instead
of the card node itself.
This means we still use the same GBM code paths as the regular native
backend paths, except we never make use of any CRTC backed onscreen
framebuffers.
Eventually, this "dummy" MetaKmsImplDevice will be replaced separating
"KMS" device objects from "render" device objects, but that will require
more significant changes. It will, however, be necessary for e.g. going
from being headless, only having access to a render node, to turning
into a real session, with a seat, being DRM master, and having access to
a card node.
This is currently not hooked up, but will be in a later commit.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1698>
