On Thu, 2013-05-02 at 11:33 +0200, Daniel Vetter wrote: > On Thu, May 2, 2013 at 11:31 AM, Daniel Vetter <daniel at ffwll.ch> wrote: > > On Thu, May 2, 2013 at 11:17 AM, Chris Wilson <chris at chris-wilson.co.uk> 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. > > > > Yeah, I think this should be fixed in general, doesn't really make > > much sense given that the jiffies value is relative this way. Also, > > the actual timeout we need to wait is just 400 us, so I think we're > > pretty safe here. Can you please resumbit just the recheck part of > > your patch? Well, without the +1 to msecs_to_jiffies(10) part, I still can trigger the problem on HZ=100. Since w/o that we pass 1 jiffies and wait less than 400us.. > > > > Also, can you do the same fix for gmbus_wait_idle in intel_i2c.c please? Yes, can do this. --Imre > > Oops, just now I've seen your generic patch. Yeah, I like that much better ;-) > -Daniel > -- > Daniel Vetter > Software Engineer, Intel Corporation > +41 (0) 79 365 57 48 - http://blog.ffwll.ch
