Re: [Gumstix-users] Kernel udelay() accuracy after mpurate change

[James A. <ang...@gm...>]

Thanks Scott!

This is a very useful finding.

James Ang.


On Thu, May 9, 2013 at 7:15 AM, Scott Ellis <sc...@ju...> wrote:

> This is all from an Overo running a 3.2 kernel.
>
> It seems udelay() does not adjust for the mpurate change on the
> kernel command line.
>
> Measurements are taken with a scope watching a gpio line get toggled.
>
> The gpio toggling is not instantaneous, but it is an order of magnitude
> less then the udelay times I'm testing, so I'm ignoring it.
>
> I ran three tests for each udelay.
>
> Clock at 600 MHz
>
> udelay(5)   : 6.1, 5.9, 6.1
> udelay(10)  : 10.9, 10.7, 10.9
> udelay(20)  : 21.0, 20.7, 20.8
> udelay(50)  : 50.6, 50.9, 50.7
> udelay(100) : 100.3, 100.5, 100.5
>
> Clock at 800 MHz
>
> udelay(5)   : 4.6, 4.4, 4.4
> udelay(10)  : 8.4, 8.2, 8.2
> udelay(20)  : 15.5, 15.8, 15.5
> udelay(50)  : 38.3, 38.0, 38.1
> udelay(100) : 75.6, 75.3, 75.4
>
>
> The usual expectation is that udelay(x) will delay at least x, so
> this could be a problem if you are counting on that.
>
> It's easy enough to calculate the value you should use with udelay()
> if you know you are changing the clock, i.e. multiply by 4/3 in this
> case.
>
> hrtimers() are not affected, but then they aren't too accurate below
> 300 usec which leaves a gap where you might need udelay.
>
> This is just a heads up for other Gumstix driver developers out there.
>
> It could be this is obvious or well known, but I didn't see anything
> from a cursory Google. I never noticed it before.
>
> I just happened to stumble across it today working on some hardware
> needing a delay in the 40-60 usec range.
>
> I can post a simple kernel module to demonstrate the problem if
> anyone is interested.
>
>
>
> --
> View this message in context:
> http://gumstix.8.x6.nabble.com/Kernel-udelay-accuracy-after-mpurate-change-tp4967243.html
> Sent from the Gumstix mailing list archive at Nabble.com.
>
>
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-- 
Regards,
James