drm/i915: set ctx->initialized only after RCS

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, but isn't
2. context 1 render // loads random state from step 2

It is really easy to hit this with a planned upcoming patch which makes
default context reuse possible.

NOTE: This should only effect full PPGTT branches, ie. current
drm-intel-nightly.

Thanks to Chris for helping me track this down.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Simplify the failure scenario in the commit message according
to Chris' review a bit.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index c5975f6d12fb76bc25f175f86ca527de011dfdb8..112f8657db21a2610fc15999dd4e8ea477058cde 100644 (file)
--- 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;