[Intel-gfx] [PATCH] drm/i915: Disable stolen memory when DMAR is active

[Daniel Vetter]

On Wed, Mar 05, 2014 at 03:32:54PM +0000, Chris Wilson wrote:
> On Wed, Mar 05, 2014 at 04:17:17PM +0100, Daniel Vetter wrote:
> > On Fri, Feb 28, 2014 at 12:14:02PM +0000, Chris Wilson wrote:
> > > We have reports of heavy screen corruption if we try to use the stolen
> > > memory reserved by the BIOS whilst the DMA-Remapper is active. This
> > > quirk may be only specific to a few machines or BIOSes, but first lets
> > > apply the big hammer and always disable use of stolen memory when DMAR
> > > is active.
> > > 
> > > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=68535
> > > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > > Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> > > ---
> > >  drivers/gpu/drm/i915/i915_gem_stolen.c | 14 ++++++++++++++
> > >  1 file changed, 14 insertions(+)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/i915_gem_stolen.c b/drivers/gpu/drm/i915/i915_gem_stolen.c
> > > index 984ada1b0084..13a849afe479 100644
> > > --- a/drivers/gpu/drm/i915/i915_gem_stolen.c
> > > +++ b/drivers/gpu/drm/i915/i915_gem_stolen.c
> > > @@ -325,11 +325,25 @@ void i915_gem_cleanup_stolen(struct drm_device *dev)
> > >  	drm_mm_takedown(&dev_priv->mm.stolen);
> > >  }
> > >  
> > > +static bool need_dmar_wa(struct drm_device *dev)
> > > +{
> > > +#ifdef CONFIG_INTEL_IOMMU
> > > +	if (INTEL_INFO(dev)->gen >= 6 && intel_iommu_gfx_mapped)
> > 
> > Why the gen check in here? Without that we could just check
> > intel_iommu_gfx_mapped and shovel the #ifdef into stolen init. No need to
> > spread it out like that imo.
> 
> It's almost as if it was cut'n'paste from another VTd w/a. I'm worried
> that this is too big a hammer. I would have expected more complaints if
> VTd was this broken since 3.9. Otoh, it is a severe bug.

We've had piles of bugs in our stolen detection, combined with very few
people having vt-d capable machines (thanks to intel's differentation) and
probably even fewer having it enabled I'm not so much surprised.

On top of that lots of noise on the interwebz that dmar is trouble for
intel gfx on linux, so people probably just disable it and carry on ...
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch