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? > > Also, can you do the same fix for gmbus_wait_idle in intel_i2c.c please? 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
