[Intel-gfx] Write GFX_FLSH_CNT after updating GGTT entries

[Ville Syrjälä]

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