[Intel-gfx] [PATCH] drm/i915: Defer modeset cleanup to a secondary task

[Chris Wilson]

Quoting Chris Wilson (2018-06-23 11:39:51)
> If we avoid cleaning up the old state immediately in
> intel_atomic_commit_tail() and defer it to a second task, we can avoid
> taking heavily contended locks when the caller is ready to procede.
> Subsequent modesets will wait for the cleanup operation (either directly
> via the ordered modeset wq or indirectly through the atomic helperr)
> which keeps the number of inflight cleanup tasks in check.
> 
> As an example, during reset an immediate modeset is performed to disable
> the displays before the HW is reset, which must avoid struct_mutex to
> avoid recursion. Moving the cleanup to a separate task, defers acquiring
> the struct_mutex to after the GPU is running again, allowing it to
> complete. Even in a few patches time (optimist!) when we no longer
> require struct_mutex to unpin the framebuffers, it will still be good
> practice to minimise the number of contention points along reset. The
> mutex dependency still exists (as one modeset flushes the other), but in
> the short term it resolves the deadlock for simple reset cases.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>

This silences
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101600
which is one of the *four* remaining failures in BAT.

We're almost green, so let's board up this window as well.
-Chris