[Intel-gfx] [PATCH 3/4] drm/i915: opt-out CPU and WC mmaps from FBC

[chris at chris-wilson.co.uk]

On Tue, Mar 29, 2016 at 01:55:02PM +0200, Daniel Vetter wrote:
> *Blonk* or whatever the sound for suddenly realization is. Totally forgot
> that we're reuseding set_domain(GTT) for wc mmaps because this it's a nice
> trick.
> 
> Otoh, is that trick the reason why wc mmaps aren't coherent enough? One
> possible difference is that this won't do the magic chipset flush in
> intel-gtt.c that we have on gen3-5. Let's pretend wbinv doesn't exist on
> gen2 ;-) But that's just an aside really ...

No, writes through the *GTT* are not coherent.

igt/gem_mmap_wc/coherency -> PASS
igt/gem_mmap_gtt/coherency -> FAIL on byt/bsw

Also see patch on list to insert a magic delay that fixes the issue
exposed in the kernel (which causes various hangs and EINVALs).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre