Re: [PATCH v2 2/2] hwrng: iproc-rng200 - Add Broadcom IPROC RNG driver

[Scott Branden]

Hi Arnd,

Latency is 32 us for 32bits of data - commented inline.  What delay call 
do you recommend in this case?

On 15-02-26 12:15 PM, Arnd Bergmann wrote:
> On Thursday 26 February 2015 11:37:20 Scott Branden wrote:
> > Response inline.
> >
> > On 15-02-25 11:17 AM, Arnd Bergmann wrote:
> > > On Wednesday 25 February 2015 10:16:24 Scott Branden wrote:
> > > > This adds a driver for random number generator present on Broadcom
> > > > IPROC devices.
> > > >
> > > > Reviewed-by: Ray Jui <rjui@xxxxxxxxxxxx>
> > > > Signed-off-by: Scott Branden <sbranden@xxxxxxxxxxxx>
> > >
> > > The driver looks reasonable overall, I have just one question about
> > > something that sticks out:
> > >
> > > > +    while ((num_remaining > 0) && time_before(jiffies, idle_endtime)) {
> > > ...
> > > > +
> > > > +            /* Are there any random numbers available? */
> > > > +            if ((ioread32(rng_base + RNG_FIFO_COUNT_OFFSET) &
> > > > +                            RNG_FIFO_COUNT_RNG_FIFO_COUNT_MASK) > 0) {
> > > ...
> > > > +            } else {
> > > > +                    if (!wait)
> > > > +                            /* Cannot wait, return immediately */
> > > > +                            return max - num_remaining;
> > > > +
> > > > +                    /* Can wait, give others chance to run */
> > > > +                    cpu_relax();
> > > > +            }
> > > > +    }
> > > > +
> > >
> > > It looks like you do a busy-loop around cpu_relax here if asked to wait.
> > > Is this intentional? I would normally expect either cond_resched() or
> > > some msleep() instead.
> >
> > This code was following examples of other open source drivers - bcm2835
> > and exynos both use cpu_relax.  I'll have to look into this more to
> > understand.
>
> The majority of the driver apparently use udelay(10) to wait, which is
> not much better but at least consistent. The cpu_relax() call probably
> gives better throughput.
>
> I don't understand why none of the drivers actually attempts to
> msleep(), but that may be because the delay is much too long.
>
> Can you find out what the expected latency is for new data to
> become available on your hardware?
RNG generates at a nominal 1 Mbps.  So to generate 32 bits of data takes
approximately 32 us.

> 	Arnd

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