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

[Chris Wilson]

Quoting Chris Wilson (2018-06-25 11:16:58)
> 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.

Acked-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>

Maarten said he would ack it if I could dig up the bugzilla link for the
CI fail. This is not the long term solution, just a convenient hack that
adds a schedule point after the modeset (which in imo is beneficial).
-Chris