Re: [PATCH] agp/intel, drm/i915: Use a write-combining map for updating PTEs

[Chris Wilson <chris@chris-wilson.co.uk>]

On Sun, 12 Aug 2012 17:01:08 +0100, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> On Sun, 12 Aug 2012 17:47:46 +0200, Daniel Vetter <daniel@ffwll.ch> wrote:
> > On Sun, Aug 12, 2012 at 12:04:39PM +0100, Chris Wilson wrote:
> > > In order to be able to ioremap_wc the GTT space, we need to remove the
> > > conflicting pci_iomap from drm/i915, so we limit the register map in
> > > drm/i915 to the suitable range for each generation. The benefit of doing
> > > this is an order of magnitude reduction in time spent rewriting the GTT
> > > entries when inserting and removing objects. For example, this halves the
> > > CPU time spent in X when pushing pixels for chromium through a userptr
> > > (chromium has a bug where it likes to recreate its ShmPixmap on every
> > > draw).
> > > 
> > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > 
> > How well does this work with ums?
> > 
> > I guess if it blows up, we could ioremap uncached, but when kms
> > initializes drop that uc mapping and try to remap wc. But I fear that ums
> > will map the entire bar and hence we can't just unconditionally map the
> > gatt wc.
> 
> It will work equisitely with ums. It will fail to do as it wishes and
> fallback to VESA and everybody will be much happier...

So having rediscovered the hard truth that i915.modeset=1 and
xf86-video-2.6.0 results in nasty hangs, setting the GTT table to WC has
no effect upon the ancient UMS module - it shows the retro background
and appears to function. We struck lucky. \o/
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre