[Intel-gfx] [PATCH 4/5] drm/i915/contexts: Add forced switches

[Ben Widawsky]

A force parameter for switch currently only has one use, the first time
we load the default context.

Slightly hand-wavy explanation: We want to get the default context
actually loaded so that the GPU has some real state to load (instead of
garbage) after a reset or resume. The reason it's hand-wavy is because
the context we load is still somewhat random. Certain things may be
initialized, and certain other things may not. However it should
definitely be better than he random state that HW comes up in (and
better than all 0's).

Therefore, the benefit to adding a parameter instead of trying to
determine when to force is that we can strictly limit when such switches
occur.

References: https://bugs.freedesktop.org/show_bug.cgi?id=52429
Tested-by: Guang A Yang <guang.a.yang at intel.com>
Signed-off-by: Ben Widawsky <ben at bwidawsk.net>
---
 drivers/gpu/drm/i915/i915_gem_context.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index 3e884dc..3db84e2 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -97,7 +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 i915_hw_context *to);
+static int do_switch(struct i915_hw_context *to, bool force);
 
 static int get_context_size(struct drm_device *dev)
 {
@@ -225,7 +225,7 @@ static int create_default_context(struct drm_i915_private *dev_priv)
 	if (ret)
 		goto err_destroy;
 
-	ret = do_switch(ctx);
+	ret = do_switch(ctx, false);
 	if (ret)
 		goto err_unpin;
 
@@ -256,7 +256,7 @@ void i915_gem_context_init(struct drm_device *dev)
 	 * context so our HW CCID matches what we think it should be in software.
 	 */
 	if (dev_priv->ring[RCS].default_context) {
-		int ret = do_switch(dev_priv->ring[RCS].default_context);
+		int ret = do_switch(dev_priv->ring[RCS].default_context, false);
 		if (ret) {
 			DRM_ERROR("HW contexts were broken after resume\n");
 			i915_gem_context_fini(dev);
@@ -373,7 +373,7 @@ mi_set_context(struct intel_ring_buffer *ring,
 	return ret;
 }
 
-static int do_switch(struct i915_hw_context *to)
+static int do_switch(struct i915_hw_context *to, bool force)
 {
 	struct intel_ring_buffer *ring = to->ring;
 	struct drm_i915_gem_object *from_obj = ring->last_context_obj;
@@ -382,7 +382,7 @@ static int do_switch(struct i915_hw_context *to)
 
 	BUG_ON(from_obj != NULL && from_obj->pin_count == 0);
 
-	if (from_obj == to->obj)
+	if (!force && (from_obj == to->obj))
 		return 0;
 
 	ret = i915_gem_object_pin(to->obj, CONTEXT_ALIGN, false);
@@ -403,10 +403,10 @@ static int do_switch(struct i915_hw_context *to)
 	if (!to->obj->has_global_gtt_mapping)
 		i915_gem_gtt_bind_object(to->obj, to->obj->cache_level);
 
-	if (!to->is_initialized || is_default_context(to))
-		hw_flags |= MI_RESTORE_INHIBIT;
-	else if (WARN_ON_ONCE(from_obj == to->obj)) /* not yet expected */
+	if (force) {
 		hw_flags |= MI_FORCE_RESTORE;
+	} else if (!to->is_initialized || is_default_context(to))
+		hw_flags |= MI_RESTORE_INHIBIT;
 
 	ret = mi_set_context(ring, to, hw_flags);
 	if (ret) {
@@ -482,7 +482,7 @@ int i915_switch_context(struct intel_ring_buffer *ring,
 			return -ENOENT;
 	}
 
-	return do_switch(to);
+	return do_switch(to, false);
 }
 
 int i915_gem_context_create_ioctl(struct drm_device *dev, void *data,
-- 
1.7.11.4