Re: [PATCH] drm/i915: fix DP AUX errors due to false timeouts when using wait_event_timeout

[Imre Deak <imre.deak@intel.com>]

On Thu, 2013-05-02 at 10:17 +0100, Chris Wilson wrote:
> On Thu, May 02, 2013 at 12:00:03PM +0300, Imre Deak wrote:
> > Due to possible scheduling latencies wait_event_timeout doesn't
> > guarantee a non-zero return value, even if the condition becomes true
> > before the specified timeout expires. Thus we can incorrectly signal a
> > timeout and abort a DP AUX transaction.
> > 
> > If wait_event_timeout returns 0, it's guaranteed that at least the
> > specified timeout (minus one jiffies, see below) had passed, so we can
> > fix this by checking the condition explicitly in this case.
> > 
> > Also the timeout that wait_event_timeout() is guaranteed to wait if the
> > condition doesn't become true is one less jiffies than what is passed to
> > it as a parameter. This is because the absolute expiration time in
> > schedule_timeout() may be calculated at a moment close to the next
> > scheduling tick, when jiffies is incremented. So make sure we pass always
> > a jiffies value of 2 or greater. Here this makes a difference only for
> > HZ=100.
> > 
> > This fixes DP AUX errors I saw during booting on an ILK.
> > 
> > This should ideally be fixed in wait_event_timeout(), but that can take
> > a while. Until that's done use this fix as a band-aid.
> 
> As we have 3 such vulnerable callsite in our driver alone, perhaps we
> should push for your general fix.

Yes, I've sent a patch for wait_event_timeout() to start the discussion,
I'm not sure how long it will take..

--Imre