linux-kernel - Re: udelay minimum delay guarantee and maximum supported delay?

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Message-Id: <201203091521.50916.tvrtko.ursulin@onelan.co.uk>
Date:	Fri, 9 Mar 2012 15:21:50 +0000
From:	Tvrtko Ursulin <tvrtko.ursulin@...lan.co.uk>
To:	x86@...nel.org
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: udelay minimum delay guarantee and maximum supported delay?


[Adding some people to the CC list, courtesy of get_maintainer.pl -f 
arch/x86/lib/delay.c, if you consider that a good file choice for this.]

On Friday 09 Mar 2012 14:37:50 Tvrtko Ursulin wrote:
> 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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