The first phase happens early, which discards pending page flips,
meaning the references held by those page flip closures are released.
The second phase happens late, after other units depending on the KMS
abstraction, have been cleaned up.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1822>
This adds a MetaKmsImplDevice backend using atomic drmMode* API in constrast to
non-atomic legacy drmMode* API used in MetaKmsImplDeviceSimple.
This has various behavioral differences worth noting, compared to the
simple backend:
* We can only commit once per CRTC per page flip.
This means that we can only update the cursor plane once. If a primary
plane composition missed a dead line, we cannot commit only a cursor
update that would be presented earlier.
* Partial success is not possible with the atomic backend.
Cursor planes may fail with the simple backend. This is not the case
with the atomic backend. This will instead later be handled using API
specific to the atomic backend, that will effectively translate into
TEST_ONLY commits.
For testing and debugging purposes, the environment variable
MUTTER_DEBUG_ENABLE_ATOMIC_KMS can be set to either 1 or 0 to
force-enable or force-disable atomic mode setting. Setting it to some
other value will cause mutter to abort().
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/548
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1488>
