From: Lee Jones <lee.jones@linaro.org>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, peter@korsgaard.com,
kernel@stlinux.com, daniel.thompson@linaro.org,
pankaj.dev@st.com, festevam@gmail.com,
herbert@gondor.apana.org.au
Subject: Re: [PATCH 3/3] hwrng: st: Use real-world device timings for timeout
Date: Wed, 7 Oct 2015 08:53:39 +0100 [thread overview]
Message-ID: <20151007075339.GG17172@x1> (raw)
In-Reply-To: <20151006205631.GA21513@n2100.arm.linux.org.uk>
On Tue, 06 Oct 2015, Russell King - ARM Linux wrote:
> On Tue, Oct 06, 2015 at 09:51:22PM +0100, Lee Jones wrote:
> > On Tue, 06 Oct 2015, Russell King - ARM Linux wrote:
> > > On Tue, Oct 06, 2015 at 03:44:00PM +0100, Lee Jones wrote:
> > > > Samples are documented to be available every 0.667us, so in theory
> > > > the 8 sample deep FIFO should take 5.336us to fill. However, during
> > > > thorough testing, it became apparent that filling the FIFO actually
> > > > takes closer to 12us.
> > >
> > > Is that measured?
> >
> > I measured it using ktime. Hopefully that was adequate.
> >
> > > > +/*
> > > > + * Samples are documented to be available every 0.667us, so in theory
> > > > + * the 8 sample deep FIFO should take 5.336us to fill. However, during
> > > > + * thorough testing, it became apparent that filling the FIFO actually
> > > > + * takes closer to 12us.
> > > > + */
> > > > +#define ST_RNG_FILL_FIFO_TIMEOUT 12
> > >
> > > I hope you're not using such a precise figure with udelay(). udelay()
> > > is not guaranteed to give exactly (or even at least) the delay you
> > > request. It's defined to give an approximate delay.
> > >
> > > Many people have a problem understanding that, so I won't explain why
> > > it is that way, just accept that it is and move on... it's not going
> > > to magically get "fixed" because someone has just learnt about this. :)
> >
> > Thanks for the info. I did do testing, again using ktime, to make
> > sure and on our platform (is it platform specific?) I measured
> > udelay(1) to be ~1100ns. After moving to a 12us timeout and reading
> > many MBs of randomness I am yet to receive any more timeouts.
>
> If you happen to fall back to the software timing loop, udelay(1) will not
> be >=1us anymore, but will be slightly shorter.
>
> That's because the loops_per_jiffy value is calculated as the number of
> loops between each timer interrupt - so the period being measured is the
> timer period, minus the time it takes for the timer interrupt to run.
> The latter is indeterminant. Consequently, the loops_per_jiffy estimate
> is always slightly under the real number of loops-per-jiffy, so delays
> generated by udelay() and friends will always be slightly short.
>
> The faster your HZ value, the bigger the error. The longer the interrupt
> handler takes, the bigger the error.
Thanks for taking the time to explain.
> IIRC, Linus recommends a x2 factor on delays, especially timeouts generated
> by these functions.
In this implementation it shouldn't matter too much either way. Even
when the timeouts were prolific, bandwidth was not reduced due to the
quick turn-round of the subsystem. I don't foresee any impact on
bandwidth if we were to raise the timeout either; in fact, I doubt
we'd ever see a timeout again.
--
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
Follow Linaro: Facebook | Twitter | Blog
next prev parent reply other threads:[~2015-10-07 7:53 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-06 14:43 [RESEND 1/3] hwrng: st: dt: Fix trivial typo in node address Lee Jones
2015-10-06 14:43 ` [PATCH 2/3] hwrng: st: Report correct FIFO size Lee Jones
2015-10-06 15:48 ` Daniel Thompson
2015-10-06 15:23 ` Lee Jones
2015-10-06 14:44 ` [PATCH 3/3] hwrng: st: Use real-world device timings for timeout Lee Jones
2015-10-06 19:37 ` Russell King - ARM Linux
2015-10-06 20:51 ` Lee Jones
2015-10-06 20:56 ` Russell King - ARM Linux
2015-10-07 7:53 ` Lee Jones [this message]
2015-10-06 15:24 ` [RESEND 1/3] hwrng: st: dt: Fix trivial typo in node address Herbert Xu
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