drm/i915/gt: Do not restore invalid RS state

Only restore valid resource streamer state from the context image, i.e.
avoid restoring if we know the image is invalid.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/446
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191229183153.3719869-4-chris@chris-wilson.co.uk
Cc: stable@vger.kernel.org

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

@@ -1408,14 +1408,6 @@ static inline int mi_set_context(struct i915_request *rq, u32 flags)
 	int len;
 	u32 *cs;
 
-	flags |= MI_MM_SPACE_GTT;
-	if (IS_HASWELL(i915))
-		/* These flags are for resource streamer on HSW+ */
-		flags |= HSW_MI_RS_SAVE_STATE_EN | HSW_MI_RS_RESTORE_STATE_EN;
-	else
-		/* We need to save the extended state for powersaving modes */
-		flags |= MI_SAVE_EXT_STATE_EN | MI_RESTORE_EXT_STATE_EN;
-
 	len = 4;
 	if (IS_GEN(i915, 7))
 		len += 2 + (num_engines ? 4 * num_engines + 6 : 0);
@@ -1607,15 +1599,21 @@ static int switch_context(struct i915_request *rq)
 		return ret;
 
 	if (ce->state) {
-		u32 hw_flags;
+		u32 flags;
 
 		GEM_BUG_ON(rq->engine->id != RCS0);
 
-		hw_flags = 0;
-		if (!test_bit(CONTEXT_VALID_BIT, &ce->flags))
-			hw_flags = MI_RESTORE_INHIBIT;
+		/* For resource streamer on HSW+ and power context elsewhere */
+		BUILD_BUG_ON(HSW_MI_RS_SAVE_STATE_EN != MI_SAVE_EXT_STATE_EN);
+		BUILD_BUG_ON(HSW_MI_RS_RESTORE_STATE_EN != MI_RESTORE_EXT_STATE_EN);
+
+		flags = MI_SAVE_EXT_STATE_EN | MI_MM_SPACE_GTT;
+		if (test_bit(CONTEXT_VALID_BIT, &ce->flags))
+			flags |= MI_RESTORE_EXT_STATE_EN;
+		else
+			flags |= MI_RESTORE_INHIBIT;
 
-		ret = mi_set_context(rq, hw_flags);
+		ret = mi_set_context(rq, flags);
 		if (ret)
 			return ret;
 	}