Re: Usage of _PAGE_PCD et al in i915 driver

[Jesse Barnes]

On Fri, 8 Aug 2014 15:14:15 +0200
Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:

> Adding relevant mailing lists.
> 
> On Fri, Aug 8, 2014 at 1:23 PM, Juergen Gross <jgross@xxxxxxxx> wrote:
> > I'm just about to create a patch for full PAT support in the Linux
> > kernel, including Xen. For this purpose I introduce a translation
> > between cache modes and pte bits.
> >
> > Scanning the kernel sources for usage of the cache mode bits in the
> > pte I discovered  drivers/gpu/drm/i915/i915_gem_gtt.h is using
> > _PAGE_PCD, _PAGE_PWT and _PAGE_PAT. I think those defines are used
> > to create ptes not for usage by the main processor, but for the
> > graphics processor. Is this true? In this case I'd suggest to define
> > i915-specific macros instead of using the x86 ones.
> 
> Yeah, those are gpu specific PAT tables, but the hw engineers
> specifically designed this to match, and we've tried to follow the cpu
> side to match it. Especially in the future that will be somewhat
> important, since we want to fully share the entire address space
> between cpu and gpu on the next platform. Jesse is working on that.

Right, we have an x86 compatible MMU in the GPU itself, so re-using the
defines makes sense.  I suppose with your work you'll move them and
make them a bit more opaque?  If so, we'll still want a way to get at
them directly, or access your mapping functions for generating PTE bits
for the GPU MMU.

Thanks,
-- 
Jesse Barnes, Intel Open Source Technology Center
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