Re: [PATCH] drm/i915: Drop the overzealous warning from i915_gem_set_cache_level

Ville Syrjälä
2013-08-13T12:38:00Z

On Tue, Aug 13, 2013 at 01:20:13PM +0100, Chris Wilson wrote:
> On Tue, Aug 13, 2013 at 03:12:59PM +0300, Ville Syrjälä wrote:
> > Thinking about this stuff a bit, I think I actually came up with a
> > scenario where we would currently fail to invalidate the CPU cache
> > between non-snooped GPU/GTT access and CPU access:
> >
> > 1. make bo non-snooped w/ pin_display=true (wd=0, rd|=gtt)
> > 2. set to CPU read domain (wd=0 rd|=cpu)
> > 3. set to GTT (or GPU) write domain (wd=gtt, rd=gtt) -> CPU cache is stale after this point
> > 4. make bo snooped -> pin_display=true still so we directly set (wd=cpu, rd=cpu)
> > 5. set to CPU domain -> CPU cache is still stale
>
> You will also find the scanout reads stale data as well.

Well, assuming you actually write something to the bo w/ the CPU. If
not, then it keeps scanning out the correct data.

> You've managed
> to shoot yourself in both feet. The kernel can't fix that, so should we
> care about the other foot?

Yeah, I suppose we shouldn't care too much about problems the user
created for himself.

--
Ville Syrjälä
Intel OTC