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

[Ben Widawsky <ben@bwidawsk.net>]

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.

> 
> Also we should not introduce BUG_ON() trivially into user code paths
> (/dev/agp).

Remove 'em then?

> -Chris
>