[PATCH v4 5/5] drm/i915: Solve the GPU reset vs. modeset deadlocks with an rw_semaphore

[Chris Wilson]

Quoting ville.syrjala at linux.intel.com (2017-06-29 15:36:42)
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> Introduce an rw_semaphore to protect the display commits. All normal
> commits use down_read() and hence can proceed in parallel, but GPU reset
> will use down_write() making sure no other commits are in progress when
> we have to pull the plug on the display engine on pre-g4x platforms.
> There are no modeset/gem locks taken inside __intel_atomic_commit_tail()
> itself, and we wait for all dependencies before the down_read(), and
> thus there is no chance of deadlocks with this scheme.

This matches what I thought should be done (I didn't think of using
rwsem just a mutex, nice touch). The point I got stuck on was what
should be done after the reset? I expected another modeset to return the
state back or otherwise the inflight would get confused?
 
> During reset we should be recommiting the state that was committed last.
> But for now we'll settle for recommiting the last state for each object.

Ah, I guess that explains the above. What is the complication with
restoring the current state as opposed to the next state?

But I have to leave debating the merits of atomic internals to others.  :|
-Chris