[PATCH 3/3] hwrng: st: Use real-world device timings for timeout

[Lee Jones]

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
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