linux-kernel - Re: [PATCH v3 1/2] kernel: time: Add udelay

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Message-ID: <CAD=FV=WoJXCcx4L7wTWgfX3LkyPBLUzWqB-KCP_fSjnzOx2MXw@mail.gmail.com>
Date:	Thu, 12 Jun 2014 15:54:16 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	David Riley <davidriley@...omium.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/2] kernel: time: Add udelay_test module to validate udelay

David,

On Thu, Jun 12, 2014 at 1:13 PM, David Riley <davidriley@...omium.org> wrote:
> Create a module that allows udelay() to be executed to ensure that
> it is delaying at least as long as requested (with a little bit of
> error allowed).
>
> There are some configurations which don't have reliably udelay
> due to using a loop delay with cpufreq changes which should use
> a counter time based delay instead.  This test aims to identify
> those configurations where timing is unreliable.
>
> Signed-off-by: David Riley <davidriley@...omium.org>
> ---
>  kernel/time/Kconfig       |   7 ++
>  kernel/time/Makefile      |   1 +
>  kernel/time/udelay_test.c | 170 ++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 178 insertions(+)

Looks good to me.  Also properly demonstrates that utter brokenness
that is udelay on exynos5420 and exynos5800 right now upstream, like:

171 usecs x 100: exp=171000 allowed=170145 min=57791 avg=61586
max=435750 FAIL=99

...the same kernel on exynos5250 shows all passes (though I didn't
stress the low speeds where there are known problems due to the
intermediate freq transition).

Reviewed-by: Doug Anderson <dianders@...omium.org>
Tested-by: Doug Anderson <dianders@...omium.org>
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