drm/i915: Wait for old resets before applying debugfs/i915_wedged

Since we use the debugfs to recover the device after modifying the
i915.reset parameter, we need to be sure that we apply the reset and not
piggy-back onto a concurrent one in order for the parameter to take
effect.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190208153708.20023-5-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_debugfs.c

@@ -3861,13 +3861,9 @@ i915_wedged_set(void *data, u64 val)
 {
 	struct drm_i915_private *i915 = data;
 
-	/*
-	 * There is no safeguard against this debugfs entry colliding
-	 * with the hangcheck calling same i915_handle_error() in
-	 * parallel, causing an explosion. For now we assume that the
-	 * test harness is responsible enough not to inject gpu hangs
-	 * while it is writing to 'i915_wedged'
-	 */
+	/* Flush any previous reset before applying for a new one */
+	wait_event(i915->gpu_error.reset_queue,
+		   !test_bit(I915_RESET_BACKOFF, &i915->gpu_error.flags));
 
 	i915_handle_error(i915, val, I915_ERROR_CAPTURE,
 			  "Manually set wedged engine mask = %llx", val);