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

[Chris Wilson]

On Sat, Dec 28, 2013 at 01:31:49PM -0800, Ben Widawsky wrote:
> 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 initialized
+context marked as initialised

> 2. context 2 <X operation> // saves context 1 random state
> 3. context 1 render // loads random state from step 2

Note that step 2 is not required since the tracking is per-ring.
 
> 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.

Though we did postulate that mesa could hit this in a pathological
scenario with a freshly created context.
 
> Thanks to Chris for helping me track this down.
> 
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Signed-off-by: Ben Widawsky <ben at bwidawsk.net>
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre