Re: [PATCH] drm/i915: Enable full PPGTT on gen7

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Sep 09, 2014 at 02:07:24PM +0100, Chris Wilson wrote:
> On Tue, Sep 09, 2014 at 12:41:34PM +0000, Thierry, Michel wrote:
> > 
> > 
> > On Tue, Sep 9, 2014 at 1:34 PM, Ville Syrjälä
> > <ville.syrjala@linux.intel.com> wrote: 
> > > On Tue, Sep 09, 2014 at 12:57:11PM +0100, Chris Wilson wrote:
> > > > On Fri, Sep 05, 2014 at 02:13:16PM +0100, Michel Thierry wrote:
> > > > > Use full PPGTT as the default option in gen7.
> > > > > Note that aliasing PPGTT is the default option for gen8 (see
> > HAS_PPGTT).
> > > > >
> > > > > This may well come back to bite me later.
> > > >
> > > > Indeed. So something I spotted was that bspec mentions that the per-ring
> > > > PDE registers (RING_PP_DIR_DCLV and RING_PP_DIR_BASE) are stored in
> > > the
> > > > logical context and so the registers are restored along with the
> > > > context. If this is correct what happens when we switch logical contexts
> > > > on RCS whilst we have active work on BCS etc? Does this mean that we
> > > > have to serialise context switches across rings, or is my reading of the
> > > > bspec false?
> > > 
> > > How does rcs PP_DIR_* affect bcs? Also IIRC that stuff is part of
> > > the execlist context which isn't saved/restored unless execlists
> > > are actually enabled. IIRC when I tried it, snb did reserve the
> > > space for that stuff in the context image but didn't save/restore
> > > it, but ivb+ didn't even reserve the space.
> > > 
> > Yes, my understanding is that these registers are per engine, and bcs
> > couldn't be affected by rcs.  
> 
> They are per-engine, but are they stored in the logical context (which
> is what bspec says afaict) and so reloaded with the wrong values when
> RCS executes MI_SET_CONTEXT? That is the question.

I don't see any !RCS PP_DIR_* registers listed in the execlist context.
Also BSpec says that RCS PP_DIR_* registers are stored in the power context
rather than the execlist context in ring buffer mode. BSpec is a bit thin
on what happens to these registers on other engines eg. during rc6. I guess
they must have their own power contexts.

-- 
Ville Syrjälä
Intel OTC