Re: [PATCH 2/2] drm/i915: Handle inaccurate time conversion issues

Daniel Vetter
2014-11-28T13:30:22Z

On Fri, Nov 28, 2014 at 12:08 PM, Chris Wilson <chris@chris-wilson.co.uk> wrote:
>> - Using jiffies. Way to inaccurate, which means way too much drift
>> with signals plus automatic ioctl restarting in userspace. In
>> hindsight we should have used an absolute timeout, but hey we need
>> something for v3 of the i915 gem wait interfaces ;-)
>
> I am still trying to get my head around an ioctl that only works in
> jiffie intervals where the jiffie is too inaccurate.

The problem is the rounding down behaviour of jiffies - if you wake up
sufficiently often compared to HZ that adds up surprisingly fast.
Adding a flag for absolute timeouts might work better, but then we'd
still need to make sure that with the back&forth conversion we don't
end up rounding all the time.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch