Re: [PATCH] drm/i915: Ironlake GPU with VT-d fix

[Chris Wilson <chris@chris-wilson.co.uk>]

On Fri, 23 Sep 2011 08:38:49 -0700, Ben Widawsky <ben@bwidawsk.net> wrote:
> On Fri, 23 Sep 2011 08:37:10 +0100
> Chris Wilson <chris@chris-wilson.co.uk> wrote:
> 
> > On Thu, 22 Sep 2011 22:09:39 -0700, Ben Widawsky <ben@bwidawsk.net>
> > wrote:
> > > On Thu, 22 Sep 2011 21:50:49 -0700
> > > Keith Packard <keithp@keithp.com> wrote:
> > > 
> > > > On Thu, 22 Sep 2011 17:11:52 -0700, Ben Widawsky
> > > > <ben@bwidawsk.net> wrote:
> > > > 
> > > > > It requires an additional IOMMU patch.
> > > > 
> > > > Can we collect those two patches into one sequence?
> > > > 
> > > 
> > > Yes. Although not sure who would do the pull request to Linus.
> > > 
> > > > > +	if ((gpu_devid == PCI_DEVICE_ID_INTEL_IRONLAKE_M_HB ||
> > > > > +	     gpu_devid == PCI_DEVICE_ID_INTEL_IRONLAKE_M_IG) &&
> > > > > +	     intel_private.base.needs_dmar)
> > > > > +		intel_private.base.do_idle_maps = 1;
> > > > > +
> > > > 
> > > > I'd like to make this conditional on whether IOMMU is actually in
> > > > use; needs_dmar is based solely on whether the DMA_API is
> > > > compiled into the kernel and the GTT gen is > 2.
> > > 
> > > David did mention a way for me to detect it. I wasn't sure if there
> > > was a deadline to get this patch out so I submitted it now. I will
> > > work on the detection portion of it while the rest gets review.
> > > It's something like, check if the bus address == the physical
> > > address and if not, assume the IOMMU is in use.
> > 
> > And for starters, we can check no_iommu. But really we want
> > intel_iommu.c to report whether or not it enabled VTd on the gfx.
> >  
> > > > 
> > > > > -	if (lists_empty)
> > > > > +	if (lists_empty && !!dev_priv->mm.gtt->do_idle_maps)
> > > > >  		return 0;
> > > > 
> > > > Is it necessary to change the semantic of this function in cases
> > > > which aren't related to GTT remapping? Seems like you're imposing
> > > > a fairly high cost on operations which don't actually need it.
> > > > 
> > > 
> > > Not sure I follow. On that specific hunk it's apparently needed. I
> > > tried the patch without this and we got some data back which
> > > suggested it didn't work. It would be nice if we could reproduce
> > > this locally...
> > 
> > Keith is right, there should be no reason to touch that function,
> > and especially not for the normal code paths.
> 
> Crap... This is a bug in the patch.

I'm still can't explain the necessity of forcing the i915_ring_idle()
here. If those two lists are empty, then each of the ring write/active
lists should also be empty and it will be a no-op. Hence bafflement and
time to be slightly afraid.

> > 
> > Also we should not introduce BUG_ON() trivially into user code paths
> > (/dev/agp).
> 
> Remove 'em then?

Not quite, in this case I think returning -ENODEV is the sensible
approach.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre