[Intel-gfx] [PATCH 1/3] drm/i915/ppgtt: Load address space after mi_set_context

[Ben Widawsky]

On GEN8 the PDPs are saved and restored with context, which means we
must set them after the context switch has occurred. If we do not do
this, we end up saving the new PDPs for the old context.

The next patch will go one step further and actually only load the PDPs
once.

Example of a problem
LRI PDPs 1
MI_SET_CONTEXT bar
LRI_PDPs 2
MI_SET_CONTEXT foo // save PDPs 2 to bar's context
		  //  load foos PDPs
LRI PDPs 1
MI_SET_CONTEXT bar // save PDPs 1 to foo's context

It's all wacky. This should allow full PPGTT on Broadwell to work.

Signed-off-by: Ben Widawsky <ben at bwidawsk.net>
---
 drivers/gpu/drm/i915/i915_gem_context.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index 8fe7d72..0e8fd1e 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -667,13 +667,12 @@ static int do_switch(struct intel_ring_buffer *ring,
 	 */
 	from = ring->last_context;
 
-	if (USES_FULL_PPGTT(ring->dev)) {
-		ret = ppgtt->switch_mm(ppgtt, ring, false);
-		if (ret)
-			goto unpin_out;
-	}
-
 	if (ring != &dev_priv->ring[RCS]) {
+		if (USES_FULL_PPGTT(ring->dev)) {
+			ret = ppgtt->switch_mm(ppgtt, ring, false);
+			if (ret)
+				goto unpin_out;
+		}
 		if (from)
 			i915_gem_context_unreference(from);
 		goto done;
@@ -711,6 +710,19 @@ static int do_switch(struct intel_ring_buffer *ring,
 	if (ret)
 		goto unpin_out;
 
+	if (USES_FULL_PPGTT(ring->dev)) {
+		ret = ppgtt->switch_mm(ppgtt, ring, false);
+		/* The hardware context switch is emitted, but we haven't
+		 * actually changed the state - so it's probably safe to bail
+		 * here. Still, let the user know something dangerous has
+		 * happened.
+		 */
+		if (ret) {
+			DRM_ERROR("Failed to change address space on context switch\n");
+			goto unpin_out;
+		}
+	}
+
 	for (i = 0; i < MAX_L3_SLICES; i++) {
 		if (!(to->remap_slice & (1<<i)))
 			continue;
-- 
1.9.1