[Intel-gfx] [PATCH] drm/i915: fixup i915_gem_object_get_page inline helper

[Daniel Vetter]

On Wed, Oct 10, 2012 at 10:01:47AM +0100, Chris Wilson wrote:
> On Tue, 09 Oct 2012 23:16:02 +0100, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > On Tue,  9 Oct 2012 22:50:48 +0200, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> > > The obj->pages to obj->pages->sgl rework introduced this helper, but
> > > it doesn't actually work for n >= SG_MAX_SINGLE_ALLOC.
> > > 
> > > For simplicity (and since right now I seem to be too stupid to see
> > > the bug), let's just grab the right page with a for_each_sg loop.
> > > 
> > > This is exercised by the improved hangman tests and the gem_exec_big
> > > test in i-g-t.
> > > 
> > > v2: Compared to v1, don't try to be clever since I seemingly only
> > > manage to prove that I'm not clever.
> > 
> > Only I expect that loop to show up on profiles even higher than the
> > sg_next() from pwrite. :|
> > 
> > I expect it to have a measureable impact upon relocation throughput,
> > so I should measure it...
> > -Chris
> > 
> > -- 
> > Chris Wilson, Intel Open Source Technology Centre
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/intel-gfx
> From: Chris Wilson <chris at chris-wilson.co.uk>
> Subject: Re: [PATCH] drm/i915: fixup i915_gem_object_get_page inline helper
> To: Daniel Vetter <daniel.vetter at ffwll.ch>, Intel Graphics Development <intel-gfx at lists.freedesktop.org>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> In-Reply-To: <1349815848-1824-1-git-send-email-daniel.vetter at ffwll.ch>
> References: <84c8a8$624sk4 at orsmga001.jf.intel.com> <1349815848-1824-1-git-send-email-daniel.vetter at ffwll.ch>
> 
> On Tue,  9 Oct 2012 22:50:48 +0200, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> > The obj->pages to obj->pages->sgl rework introduced this helper, but
> > it doesn't actually work for n >= SG_MAX_SINGLE_ALLOC.
> > 
> > For simplicity (and since right now I seem to be too stupid to see
> > the bug), let's just grab the right page with a for_each_sg loop.
> > 
> > This is exercised by the improved hangman tests and the gem_exec_big
> > test in i-g-t.
> > 
> > v2: Compared to v1, don't try to be clever since I seemingly only
> > manage to prove that I'm not clever.
> > 
> > Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> 
> Looks like my worries are baseless. It can always be attacked latter if
> need be. I'd still like to know what the mistake was...

I think it was two mistakes:
- One was the off-by-one fixed in v1.
- Second seemed to be the special case that if the table fits exactly,
  sg_alloc doesn't set a chain ptr with another sg table with just one
  entry.

But like the commit message says, I've failed to be clever enough
yesterday to make it work.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch