[Intel-gfx] [PATCH] drm/i915: support low power watermarks on Ironlake

[Eric Anholt]

On Thu, 23 Dec 2010 12:30:29 +0000, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> On Thu, 23 Dec 2010 12:07:22 +0000, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > I tweaked the patch to pass the watermark structs through to the check
> > routine. The box survives s2ram, but I have now also experienced the
> > mysterious slow down after resuming.
> 
> As Eric first said the cause for the slow down is that writing through the
> GTT becomes very slow:
> 
> Before suspending:
> Time to pwrite 16k through the CPU: 		0.000004s
> Time to read 16k through a GTT map: 		0.000901s
> Time to write 16k through a GTT map: 		0.000023s
> Time to read 16k (again) through a GTT map: 	0.000900s
> Time to pwrite 16k through the GTT: 		0.000007s
> 
> After resume:
> Time to pwrite 16k through the CPU: 		0.000004s
> Time to read 16k through a GTT map: 		0.000900s
> Time to write 16k through a GTT map: 		0.000876s
> Time to read 16k (again) through a GTT map: 	0.000899s
> Time to pwrite 16k through the GTT: 		0.000431s
> 
> The implication being that our mapping goes from write-combining to
> uncached?
> -Chris

I found that the results were not just "slow like no write combining",
but timing-dependent, too.  Repeated runs of a timing app would get fast
sometimes, slow other times, and sometimes the switch from fast->slow or
slow->fast would happen in the middle of a run.
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