On Thu, Jun 12, 2014 at 07:16:48PM +0300, Ville Syrjälä wrote:
> On Thu, Jun 12, 2014 at 08:25:52AM -0700, Ben Widawsky wrote:
> > On GEN8 the PDPs are saved and restored with context, which means we
> > must set them after the context switch has occurred. If we do not do
> > this, we end up saving the new PDPs for the old context.
> > 
> > Example of a problem
> > LRI PDPs 1
> > MI_SET_CONTEXT bar
> > LRI_PDPs 2
> > MI_SET_CONTEXT foo // save PDPs 2 to bar's context
> >               //  load foos PDPs
> > LRI PDPs 1
> > MI_SET_CONTEXT bar // save PDPs 1 to foo's context
> > 
> > It's all wacky. This should allow full PPGTT on Broadwell to work.
> 
> Hmm. I had this impression too but now I'm not sure. The PDPs are listed
> as being in the execlist context. Do we save/restore that in ring buffer
> mode too? IIRC on ivb/hsw it got skipped.
> 
> And if the PDPs are really part of the context we shouldn't need to
> load them at all I think, except when we skip the restore (unitialized
> or default context). Or am I missing something here?

Yes, I believe you are correct. However, I don't see any harm with the
existing solution either, and I was hesitant to change stuff around,
since at the time I just needed to make it work for the 64b series. But
I think what you're saying is just adding an "&& !to->is_initialized" to
the USES_FULL_PPGTT check, right? I can try that.

> 
> And what about ivb/hsw? Doesn't this reordering risk the context restore
> accessing the wrong PPGTT. That's assuming the context restore can
> already chase some pointers.

This is not meant for IVB/HSW. Empirically we've found it doesn't
have the same behavior, as you said above. So that's definite an issue
which requires a patch rev.

> 
> > 
> > Signed-off-by: Ben Widawsky <b...@bwidawsk.net>
> > ---
> >  drivers/gpu/drm/i915/i915_gem_context.c | 24 ++++++++++++++++++------
> >  1 file changed, 18 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_gem_context.c 
> > b/drivers/gpu/drm/i915/i915_gem_context.c
> > index 3ffe308..b2434e0 100644
> > --- a/drivers/gpu/drm/i915/i915_gem_context.c
> > +++ b/drivers/gpu/drm/i915/i915_gem_context.c
> > @@ -623,13 +623,12 @@ static int do_switch(struct intel_engine_cs *ring,
> >      */
> >     from = ring->last_context;
> >  
> > -   if (USES_FULL_PPGTT(ring->dev)) {
> > -           ret = ppgtt->switch_mm(ppgtt, ring, false);
> > -           if (ret)
> > -                   goto unpin_out;
> > -   }
> > -
> >     if (ring != &dev_priv->ring[RCS]) {
> > +           if (USES_FULL_PPGTT(ring->dev)) {
> > +                   ret = ppgtt->switch_mm(ppgtt, ring, false);
> > +                   if (ret)
> > +                           goto unpin_out;
> > +           }
> >             if (from)
> >                     i915_gem_context_unreference(from);
> >             goto done;
> > @@ -660,6 +659,19 @@ static int do_switch(struct intel_engine_cs *ring,
> >     if (ret)
> >             goto unpin_out;
> >  
> > +   if (USES_FULL_PPGTT(ring->dev)) {
> > +           ret = ppgtt->switch_mm(ppgtt, ring, false);
> > +           /* The hardware context switch is emitted, but we haven't
> > +            * actually changed the state - so it's probably safe to bail
> > +            * here. Still, let the user know something dangerous has
> > +            * happened.
> > +            */
> > +           if (ret) {
> > +                   DRM_ERROR("Failed to change address space on context 
> > switch\n");
> > +                   goto unpin_out;
> > +           }
> > +   }
> > +
> >     for (i = 0; i < MAX_L3_SLICES; i++) {
> >             if (!(to->remap_slice & (1<<i)))
> >                     continue;
> > -- 
> > 2.0.0
> > 
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx@lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 
> -- 
> Ville Syrjälä
> Intel OTC

-- 
Ben Widawsky, Intel Open Source Technology Center
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