drm/i915: Cleanup context switching through do_switch()

When bug hunting, I found the interface to do_switch() overly
complicated and I believe festered the earlier bug. This aims to make
the code a little clearer.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_context.c

@@ -97,8 +97,7 @@
 
 static struct i915_hw_context *
 i915_gem_context_get(struct drm_i915_file_private *file_priv, u32 id);
-static int do_switch(struct drm_i915_gem_object *from_obj,
-		     struct i915_hw_context *to, u32 seqno);
+static int do_switch(struct i915_hw_context *to);
 
 static int get_context_size(struct drm_device *dev)
 {
@@ -220,19 +219,20 @@ static int create_default_context(struct drm_i915_private *dev_priv)
 	 */
 	dev_priv->ring[RCS].default_context = ctx;
 	ret = i915_gem_object_pin(ctx->obj, CONTEXT_ALIGN, false);
-	if (ret) {
-		do_destroy(ctx);
-		return ret;
-	}
+	if (ret)
+		goto err_destroy;
 
-	ret = do_switch(NULL, ctx, 0);
-	if (ret) {
-		i915_gem_object_unpin(ctx->obj);
-		do_destroy(ctx);
-	} else {
-		DRM_DEBUG_DRIVER("Default HW context loaded\n");
-	}
+	ret = do_switch(ctx);
+	if (ret)
+		goto err_unpin;
 
+	DRM_DEBUG_DRIVER("Default HW context loaded\n");
+	return 0;
+
+err_unpin:
+	i915_gem_object_unpin(ctx->obj);
+err_destroy:
+	do_destroy(ctx);
 	return ret;
 }
 
@@ -359,17 +359,18 @@ mi_set_context(struct intel_ring_buffer *ring,
 	return ret;
 }
 
-static int do_switch(struct drm_i915_gem_object *from_obj,
-		     struct i915_hw_context *to,
-		     u32 seqno)
+static int do_switch(struct i915_hw_context *to)
 {
-	struct intel_ring_buffer *ring = NULL;
+	struct intel_ring_buffer *ring = to->ring;
+	struct drm_i915_gem_object *from_obj = ring->last_context_obj;
 	u32 hw_flags = 0;
 	int ret;
 
-	BUG_ON(to == NULL);
 	BUG_ON(from_obj != NULL && from_obj->pin_count == 0);
 
+	if (from_obj == to->obj)
+		return 0;
+
 	ret = i915_gem_object_pin(to->obj, CONTEXT_ALIGN, false);
 	if (ret)
 		return ret;
@@ -393,7 +394,6 @@ static int do_switch(struct drm_i915_gem_object *from_obj,
 	else if (WARN_ON_ONCE(from_obj == to->obj)) /* not yet expected */
 		hw_flags |= MI_FORCE_RESTORE;
 
-	ring = to->ring;
 	ret = mi_set_context(ring, to, hw_flags);
 	if (ret) {
 		i915_gem_object_unpin(to->obj);
@@ -407,6 +407,7 @@ static int do_switch(struct drm_i915_gem_object *from_obj,
 	 * MI_SET_CONTEXT instead of when the next seqno has completed.
 	 */
 	if (from_obj != NULL) {
+		u32 seqno = i915_gem_next_request_seqno(ring);
 		from_obj->base.read_domains = I915_GEM_DOMAIN_INSTRUCTION;
 		i915_gem_object_move_to_active(from_obj, ring, seqno);
 		/* As long as MI_SET_CONTEXT is serializing, ie. it flushes the
@@ -417,7 +418,7 @@ static int do_switch(struct drm_i915_gem_object *from_obj,
 		 * swapped, but there is no way to do that yet.
 		 */
 		from_obj->dirty = 1;
-		BUG_ON(from_obj->ring != to->ring);
+		BUG_ON(from_obj->ring != ring);
 		i915_gem_object_unpin(from_obj);
 
 		drm_gem_object_unreference(&from_obj->base);
@@ -448,32 +449,26 @@ int i915_switch_context(struct intel_ring_buffer *ring,
 			int to_id)
 {
 	struct drm_i915_private *dev_priv = ring->dev->dev_private;
-	struct drm_i915_file_private *file_priv = NULL;
 	struct i915_hw_context *to;
-	struct drm_i915_gem_object *from_obj = ring->last_context_obj;
-	int ret;
 
 	if (dev_priv->hw_contexts_disabled)
 		return 0;
 
 	if (ring != &dev_priv->ring[RCS])
 		return 0;
 
-	if (file)
-		file_priv = file->driver_priv;
-
 	if (to_id == DEFAULT_CONTEXT_ID) {
 		to = ring->default_context;
 	} else {
-		to = i915_gem_context_get(file_priv, to_id);
+		if (file == NULL)
+			return -EINVAL;
+
+		to = i915_gem_context_get(file->driver_priv, to_id);
 		if (to == NULL)
 			return -ENOENT;
 	}
 
-	if (from_obj == to->obj)
-		return 0;
-
-	return do_switch(from_obj, to, i915_gem_next_request_seqno(to->ring));
+	return do_switch(to);
 }
 
 int i915_gem_context_create_ioctl(struct drm_device *dev, void *data,