[Intel-gfx] [PATCH 2/3] drm/i915: set ctx->initialized only after RCS

[Ben Widawsky]

The initialized flag is used to specify a context has been initialized
and it's context is safe to load, ie. the 3d state is setup properly.
With full PPGTT, we emit the address space loads during context switch
and this currently marks a context as initialized. With full PPGTT
patches, if a client first emits a batch to !RCS, then later, RCS, the
code will mistake the context as initialized and try to reload an
uninitialized context.

1. context 1 blit // context marked as initialized
2. context 1 render // loads random state

It is really easy to hit this with a planned upcoming patch which makes
default context reuse possible. See the mailing list thread in
references for more details about the issue.

Thanks to Chris for helping me track this down.

References: <1388266309-1178-1-git-send-email-benjamin.widawsky at intel.com>
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben at bwidawsk.net>
---
 drivers/gpu/drm/i915/i915_gem_context.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index 44dddc00..8c7a5b2 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -692,10 +692,11 @@ static int do_switch(struct intel_ring_buffer *ring,
 		i915_gem_context_unreference(from);
 	}
 
+	to->is_initialized = true;
+
 done:
 	i915_gem_context_reference(to);
 	ring->last_context = to;
-	to->is_initialized = true;
 	to->last_ring = ring;
 
 	return 0;
-- 
1.8.5.2