drm/i915/execlists: Wait for ELSP submission on restart

After a reset, we will ensure that there is at least one request
submitted to HW to ensure that a context is loaded for powersaving.
Let's wait for this submission via a tasklet to complete before we drop
our forcewake, ensuring the system is ready for rc6 before we let it
possibly sleep.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180522101937.7738-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>

drivers/gpu/drm/i915/i915_gem.h

@@ -82,4 +82,10 @@ static inline void __tasklet_disable_sync_once(struct tasklet_struct *t)
 		tasklet_unlock_wait(t);
 }
 
+static inline void __tasklet_enable_sync_once(struct tasklet_struct *t)
+{
+	if (atomic_dec_return(&t->count) == 0)
+		tasklet_kill(t);
+}
+
 #endif /* __I915_GEM_H__ */

drivers/gpu/drm/i915/intel_lrc.c

@@ -2008,7 +2008,16 @@ static void execlists_reset(struct intel_engine_cs *engine,
 
 static void execlists_reset_finish(struct intel_engine_cs *engine)
 {
-	tasklet_enable(&engine->execlists.tasklet);
+	/*
+	 * Flush the tasklet while we still have the forcewake to be sure
+	 * that it is not allowed to sleep before we restart and reload a
+	 * context.
+	 *
+	 * As before (with execlists_reset_prepare) we rely on the caller
+	 * serialising multiple attempts to reset so that we know that we
+	 * are the only one manipulating tasklet state.
+	 */
+	__tasklet_enable_sync_once(&engine->execlists.tasklet);
 
 	GEM_TRACE("%s\n", engine->name);
 }