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Date: Tue, 06 May 2014 17:25:33 -0700 From: John Stultz <john.stultz@...aro.org> To: David Riley <davidriley@...omium.org>, Thomas Gleixner <tglx@...utronix.de> CC: Doug Anderson <dianders@...omium.org>, linux-kernel@...r.kernel.org Subject: Re: [PATCH 0/2] Add test to validate udelay 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. 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? thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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