Re: [PATCH 39/48] drm/i915: Do not allow buffers at offset 0

Daniel Vetter
2013-12-18T14:57:16Z

On Thu, Dec 12, 2013 at 10:59:03AM +0000, Chris Wilson wrote:
> On Fri, Dec 06, 2013 at 02:11:24PM -0800, Ben Widawsky wrote:
> > This is primarily a band aid for an unexplainable error in
> > gem_reloc_vs_gpu/forked-faulting-reloc-thrashing. Essentially as soon as
> > a relocated buffer (which had a non-zero presumed offset) moved to
> > offset 0, something goes bad. Since I have been unable to solve this,
> > and potentially this is a good thing to do anyway, since many things can
> > accidentally write to offset 0, why not?
>
> A better hack would have been to exclude the first page from the vm->mm,
> so that we didn't then introduce a change to ggtt.

Band aids need to come with much better explanations. Also, if we really
want a guard area it should be of sufficient size to avoid scribbling over
random gunk. With 4k screens that means about 32MB.

I'll smash a revert for this patch on top of the series.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch