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

[Ben Widawsky <ben@bwidawsk.net>]

On Tue, Jan 07, 2014 at 10:47:18AM +0000, Chris Wilson wrote:
> On Tue, Jan 07, 2014 at 08:10:33AM +0100, Daniel Vetter 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
> > > 2. context 2 <X operation> // saves context 1 random state
> > > 3. 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>
> > 
> > Do we have a testcase for this or a bug report?
> 
> Odd, since QA should have hit this...
> -Chris

I was only able to hit it on one machine. The test case was 'startx.' I
think getting it to fail is somewhat dependent upon how the backing
pages for the context are initialized (at least, that is my theory).

-- 
Ben Widawsky, Intel Open Source Technology Center