Even though FENCE_SUBMIT is only documented to wait until the request in
the in-fence starts instead of waiting until it completes, it has a bit
more magic than that.  If FENCE_SUBMIT is used to submit something to a
balanced engine, we would wait to assign engines until the primary
request was ready to start and then attempt to assign it to a different
engine than the primary.  There is an IGT test (the bonded-slice subtest
of gem_exec_balancer) which exercises this by submitting a primary batch
to a specific VCS and then using FENCE_SUBMIT to submit a secondary
which can run on any VCS and have i915 figure out which VCS to run it on
such that they can run in parallel.

However, this functionality has never been used in the real world.  The
media driver (the only user of FENCE_SUBMIT) always picks exactly two
physical engines to bond and never asks us to pick which to use.

v2 (Daniel Vetter):
 - Mention the exact IGT test this breaks
Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-11-jason@jlekstrand.net