Re: [PATCH] BDW swizzling

[Damien Lespiau <damien.lespiau@intel.com>]

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 honestly don't care what we do though, so long as the patches get
> tested both in simulation and silicon, and there is no measurable perf
> drop. I suppose my mild preference is to, "don't touch it if it ain't
> broke."
> 
> Sorry, but at the moment, I don't have time to test this for you. Maybe
> someone else can, or remind me in a couple of weeks.

Do you know if you have a configuration where we try to swizzle? If yes
and tests/gem_tiled_pread is passing that would give us a nice bit of
information. (which of course can be tried by the next person with time
to do so).

-- 
Damien