udelay minimum delay guarantee and maximum supported delay?

From: Tvrtko Ursulin
Date: Fri Mar 09 2012 - 10:20:41 EST



Hi all,

I was debugging some weird driver behaviour under 3.3.0-rc6+ (amd64) and
eventually I got to discovering udelay's driver is issuing are occasionally
short. That results in random hardware behaviour, but that is beside the
point.

Driver in question wants to delay for 500us at a time, which is not a terribly
nice thing to do, but putting that aside and talking more in general I would
have three questions:

1. Are 500us udelays supposed to work? (I know they are not recommended and
I'll fix that.)
2. Should udelay guarantee it won't delay by less than the time asked?
3. Is ktime_get() considered accurate enough to measure how long udelay
actually delayed? (Empirical evidence suggests it is, because hardware
weirdness correlates perfectly with occurences of these short udelays.)

If answers to all are yes then we might have a bug here.

Because I am seeing udelay(500) (_occasionally_) being short, and that by
delaying for some duration between 0us (yep) and 491us.

As far as I can see this box is using TSC delay and CPU (Intel(R) Core(TM)
i5-2400S CPU @ 2.50GH) exposes the constant_tsc flag:

[ 1.717050] Refined TSC clocksource calibration: 2494.334 MHz.
[ 1.717054] Switching to clocksource tsc

Am I missing something and what are your opinions?

Thanks,

Tvrtko
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