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

[Chris Wilson]

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

-- 
Chris Wilson, Intel Open Source Technology Centre