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

[Daniel Vetter]

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