On Fri, Nov 20, 2015 at 09:23:12AM +0000, Tian, Kevin wrote: > > From: Ville Syrjälä [mailto:ville.syrjala@xxxxxxxxxxxxxxx] > > 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 _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
