drm/i915/gt: Treat a busy timeline as 'active' while waiting

If we cannot claim the timeline->mutex while preparing for a wait on it,
we have to skip the timeline. In doing so, treat it as active so that
under a intel_gt_wait_for_idle() loop, we repeat the wait after
scheduling away.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191006165002.30312-4-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_gt_requests.c

@@ -34,8 +34,10 @@ long intel_gt_retire_requests_timeout(struct intel_gt *gt, long timeout)
 
 	spin_lock_irqsave(&timelines->lock, flags);
 	list_for_each_entry_safe(tl, tn, &timelines->active_list, link) {
-		if (!mutex_trylock(&tl->mutex))
+		if (!mutex_trylock(&tl->mutex)) {
+			active_count++; /* report busy to caller, try again? */
 			continue;
+		}
 
 		intel_timeline_get(tl);
 		GEM_BUG_ON(!tl->active_count);