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

[David Riley]

On Tue, May 6, 2014 at 9:19 PM, Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
> John,
>
> On Tue, May 6, 2014 at 5:25 PM, John Stultz <john.stultz@xxxxxxxxxx> 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.

This message from Thomas Gleixner seems to back up udelay guaranteeing
a minimum delay (as compared to ktime_get) :
http://lkml.iu.edu//hypermail/linux/kernel/1203.1/01034.html  So it
feels right that that anything shorter should be considered a failure.
 If the system still works, that's fine, but udelay() isn't meeting
it's guarantees.

>
>
>> 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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