Re: Write GFX_FLSH_CNT after updating GGTT entries

Ville Syrjälä
2015-11-19T13:26:10Z

On Thu, Nov 19, 2015 at 09:04:11PM +0800, Zhi Wang wrote:
> Hi Ville:
>
> Thanks for the answer! :) Learned a lot.
>
> I think the following scenario should be typical for a general PCI
> devices(perhaps a dedicated video card). How do other PCI devices handle
> this kinds of WC MMIO writes without GFX_FLSH_CNT? Only support UC mapping?

The standard rule is that you can't use WC, except for prefetchable BARs.

>
> Thanks,
> Zhi.
>
> On 11/19/15 18:35, Ville Syrjälä wrote:
> > On Thu, Nov 19, 2015 at 06:20:23PM +0800, Zhi Wang wrote:
> >> Hi Gurus:
> >> I'm curious about the register GFX_FLSH_CNT(0x101008) in
> >> i915_gem_gtt.c. Does these register exist in recently generations? After
> >> digging into b-spec, it looks only BXT and CHV has this register. Does
> >> the desktop platform also have this register which needs to be written
> >> after updating GGTT MMIOs?
> >>
> >> BTW: Looks windows driver haven't used this MMIO... So whose behavior is
> >> the right behavior?
> >
> > As I understand it that register flushes the CPU GTT TLBs, and we need
> > to do it because of the WC mapping we have for the GTT PTEs. If we used
> > UC mapping we wouldn't need it since there's supposedly an automagic
> > TLB flush that happens on PTE writes.
> >
> > BSpec is bad at finding some registers via bxml. Using dtsearch and
> > looking for both 0x<offset> and <offset>h is the method I use to track
> > such things down.
> >

--
Ville Syrjälä
Intel OTC