Where possible, try to export the buffer rendered by the primary GPU as a
dmabuf and import it to the secondary GPU and turn it into a DRM FB for
scanout. If this works, we get a zero-copy path to secondary GPU outputs.
This is especially useful on virtual drivers like EVDI (used for DisplayLink
devices) which are not picky at all about what kind of FBs they can handle.
The zero-copy path is prioritised after the secondary GPU copy path, which
should avoid regressions for existing working systems. Attempting zero-copy
would have the risk of being less performant than doing the copy on the
secondary GPU. This does not affect the DisplayLink use case, because there is
no GPU in a DisplayLink device.
The zero-copy path is prioritised before the primary GPU and CPU copy paths. It
will be tried on the first frame of an output and the copy path is executed
too. If zero-copy fails, the result from the copy path will take over on that
frame. Furthermore, zero-copy will not be attemped again on that output. If
zero-copy succeeds, the copy path is de-initialized.
Zero-copy is assumed to be always preferable over the primary GPU and CPU copy
paths. Whether this is universally true remains to be seen.
This patch has one unhandled failure mode: if zero-copy path first succeeds and
then fails later, there is no fallback and the output is left frozen or black.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/810
