(NOTE: This is the minimal backportable fix, a full fix is being
developed at https://patchwork.freedesktop.org/patch/388048/)

The flags passed to the wait_entry.func are passed onwards to
try_to_wake_up(), which has a very particular interpretation for its
wake_flags. In particular, beyond the published WF_SYNC, it has a few
internal flags as well. Since we passed the fence->error down the chain
via the flags argument, these ended up in the default_wake_function
confusing the kernel/sched.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2110
Fixes: ef4688497512 ("drm/i915: Propagate fence errors")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: <stable@vger.kernel.org> # v5.4+
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200728152144.1100-1-chris@chris-wilson.co.uk
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
[Joonas: Rebased and reordered into drm-intel-gt-next branch]
[Joonas: Added a note and link about more complete fix]
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
(cherry picked from commit f4b3c395540aa3d4f5a6275c5bdd83ab89034806)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>