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

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Jan 09, 2014 at 03:17:25PM -0800, Ben Widawsky wrote:
> 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).

Well I've hoped we'd understand the fallout a bit better, but meh. Patch
merged with the failure scenario in the commit message simplified as noted
by Chris.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch