drm/i915: Flush the switch-to-kernel-context harder for DROP_IDLE

To complete the idle worker, we must complete 2 passes of wait-for-idle.
At the end of the first pass, we queue a switch-to-kernel-context and
may only idle after waiting for its completion. Speed up the flush_work
by doing the wait explicitly, which then allows us to remove the
unbounded loop trying to complete the flush_work in the next patch.

References: 79ffac85 ("drm/i915: Invert the GEM wakeref hierarchy")
Testcase: igt/gem_ppgtt/flind-and-close-vma-leak
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/20190507121108.18377-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_debugfs.c

@@ -3901,14 +3901,26 @@ i915_drop_caches_set(void *data, u64 val)
 
 	/* No need to check and wait for gpu resets, only libdrm auto-restarts
 	 * on ioctls on -EAGAIN. */
-	if (val & (DROP_ACTIVE | DROP_RETIRE | DROP_RESET_SEQNO)) {
+	if (val & (DROP_ACTIVE | DROP_IDLE | DROP_RETIRE | DROP_RESET_SEQNO)) {
 		int ret;
 
 		ret = mutex_lock_interruptible(&i915->drm.struct_mutex);
 		if (ret)
 			return ret;
 
-		if (val & DROP_ACTIVE)
+		/*
+		 * To finish the flush of the idle_worker, we must complete
+		 * the switch-to-kernel-context, which requires a double
+		 * pass through wait_for_idle: first queues the switch,
+		 * second waits for the switch.
+		 */
+		if (ret == 0 && val & (DROP_IDLE | DROP_ACTIVE))
+			ret = i915_gem_wait_for_idle(i915,
+						     I915_WAIT_INTERRUPTIBLE |
+						     I915_WAIT_LOCKED,
+						     MAX_SCHEDULE_TIMEOUT);
+
+		if (ret == 0 && val & DROP_IDLE)
 			ret = i915_gem_wait_for_idle(i915,
 						     I915_WAIT_INTERRUPTIBLE |
 						     I915_WAIT_LOCKED,