[Intel-gfx] [PATCH] BDW swizzling

[Chris Wilson]

On Thu, Apr 10, 2014 at 03:50:35PM -0700, Ben Widawsky wrote:
> On Thu, Apr 10, 2014 at 06:51:50PM +0100, Damien Lespiau wrote:
> > On Thu, Apr 10, 2014 at 10:32:46AM -0700, Ben Widawsky wrote:
> > > On Thu, Apr 10, 2014 at 05:24:07PM +0100, Damien Lespiau wrote:
> > > > While cruising through the specs, I noticed a note about swizzling changes on
> > > > BDW. My understanding is that we don't need to either initialize swizzling on
> > > > the GPU side nor swizzle the address ourselves on the CPU side.
> > > > 
> > > > That could be totally wrong though, and I unfortunately don't have a machine to
> > > > test this theory on.
> > > 
> > > I fought with this too. My resolution was we can either set all the
> > > swizzling bits, or set none. There is no motivation to do either, and
> > > the spec simply is telling us what they do for windows. That was well
> > > over a year ago, so it all can be different now.
> > 
> > My (limited) understanding is telling me that if we don't return
> > I915_BIT_6_SWIZZLE_NONE, user space is going to swizzle the address and
> > the controller is going to swizzle it again, even from the CPU, so we
> > end up at the wrong place.
> 
> I thought this type of swizzling was usually determined by our code
> i915_gem_detect_bit_6_swizzle(). I am also unfamiliar with whether or
> not userspace controls it, and whether or not it should. I never really
> looked at that aspect of the code. Maybe Chris can answer this part.

Userspace only queries swizzling.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre