Re: Write GFX_FLSH_CNT after updating GGTT entries

Chris Wilson
2015-11-20T09:40:22Z

On Fri, Nov 20, 2015 at 09:23:12AM +0000, Tian, Kevin wrote:
> > From: Ville Syrjälä [mailto:ville.syrjala@linux.intel.com]
> > Sent: Thursday, November 19, 2015 6:35 PM
> >
> > 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.
> >
>
> Curious how much gain is observed by using WC vs. using UC on GTT
> entries?

Think back yonder when everything goes through the GGTT, and where
writing the PTEs was slower than allocating a bunch of pages and
applications would insist on submitting new objects every batch.

It was very easy to have workloads where UC GGTT updates were the
ratelimiting step. A WC update is ~8x faster, and sufficient to move the
bottleneck elsewhere.
-Chris

--
Chris Wilson, Intel Open Source Technology Centre