linux-kernel - Re: [PATCH 0/2] Add test to validate udelay

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Date:	Tue, 6 May 2014 21:19:43 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	David Riley <davidriley@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] Add test to validate udelay

John,

On Tue, May 6, 2014 at 5:25 PM, John Stultz <john.stultz@...aro.org> wrote:
> On 05/06/2014 05:12 PM, David Riley wrote:
>> This change adds a module and a script that makes use of it to
>> validate that udelay delays for at least as long as requested
>> (as compared to ktime).
>
> Interesting.
>
> So fundamentally, udelay is a good bit fuzzier accuracy wise then
> ktime_get(), as it may be backed by relatively coarsely calibrated delay
> loops, or very rough tsc freq estimates.
>
> ktime_get on the other hand is as fine grained as we can be, and is ntp
> corrected, so that a second can really be a second.
>
> So your comparing the fast and loose interface so we can delay a bit
> before hitting some hardware again with a fairly precise interface.
> Thus  I'd not be surprised if your test failed on various hardware. I'd
> really only trust udelay to be roughly accurate, so you might want to
> consider adding some degree of acceptable error to the test.

My understanding is that udelay should be >= the true delay.
Specifically it tends to be used when talking to hardware.  We used it
to ensure a minimum delay between SPI transactions when talking to a
slow embedded controller.  I think the regulator code uses udelay() to
wait for voltage to ramp up, for instance.  Waiting too long isn't
terrible, but too short is bad.

That being said, I think if udelay was within 1% we're probably OK.  I
believe I have seen systems where udelay is marginally shorter than it
ought to be and it didn't upset me too much.

> Really, I'm curious about the backstory that made you generate the test?
> I assume something bit you where udelay was way off? Or were you using
> udelay for some sort of accuracy sensitive use?

Several times we've seen cases where udelay() was pretty broken with
cpufreq if you were actually implementing udelay() with
loops_per_jiffy.  I believe it may also be broken upstream on
multicore systems, though now that ARM arch timers are there maybe we
don't care as much?

Specifically, there is a lot of confusion between the global loops per
jiffy and the per CPU one.  On ARM I think we always use the global
one and we attempt to scale it as cpufreq changes.  ...but...

* cores tend scale together and there's a single global.  That means
you might have started the delay loop at one freq and ended it at
another (if another CPU changes the freq).

* I believe there's some strange issues in terms of how the loops per
jiffy variable is initialized and how the "original CPU freq" is.  I
know we ran into issues on big.LITTLE where the LITTLE cores came up
and clobbered the loops_per_jiffy variable but it was still doing math
based on the big cores.

-Doug
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