[Intel-gfx] [PATCH] drm/i915: Force vblanks around evasion when i915.watermark_test is set.

[Daniel Vetter]

On Tue, Jun 14, 2016 at 08:27:57AM +0200, Maarten Lankhorst wrote:
> Op 13-06-16 om 16:45 schreef Daniel Vetter:
> > On Thu, Jun 09, 2016 at 01:36:50PM +0200, Maarten Lankhorst wrote:
> >> This will make it more likely that intermediary watermarks cause fifo
> >> underruns, which is useful when writing watermark specific tests,
> >> to make it more likely to trigger FIFO underruns in intermediary watermarks.
> > That's surprising ... with the vblank_wait added you're pretty much
> > guaranteed to never hit the evasion case. Note that the vblank evasion
> > code also just does a vblank wait if it detects a possible collision.
> >
> > Given that I'd have assumed that this simply slows down atomic flips,
> > which is probably not what we want for testing all. Does this really help?
> > Any ideas why?
> This is a sanity check on the intermediate watermarks. The vblank test will ensure
> that the old plane configuration doesn't cause underruns with watermarks, and that
> the intermediate watermarks will work correctly with the new plane configuration too.
> 
> I think the latter vblank_wait may be killed, but the former could be really useful,
> for example to find correct watervblank values in CHV.

Why can't we do the vblank wait in userspace? If we wait 1 vblank (or just
for the drm event of the previous commit), and then immediately issue the
next one, we will also run with intermediate watermarks for a full frame.
At least I still don't see a super-short race window for which we need to
have code in the kernel to properly provoke it ...
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch