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

[Ben Widawsky <ben@bwidawsk.net>]

On Sun, Dec 29, 2013 at 09:59:26AM +0000, Chris Wilson wrote:
> 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.
>  

This is missing an extremely important caveat which I was
incorrectly correlating earlier in our discussion. Yes, step 2 is not
required.  The step which is required is the page allocated must be
non-zero when allocated, and the contents of the page must be capable of
hanging the GPU when used as a context object.

Otherwise, the uninitialized context would always be all 0, which if I
understand the HW correctly, is "safe"

As long as you don't disagree, I'll fix the commit message with that
info.

> > 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@chris-wilson.co.uk>
> > Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
> -Chris
> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre

-- 
Ben Widawsky, Intel Open Source Technology Center