Messages in this thread | | | Date | Thu, 26 Feb 2015 14:26:02 -0800 | From | Scott Branden <> | Subject | Re: [PATCH v2 2/2] hwrng: iproc-rng200 - Add Broadcom IPROC RNG driver |
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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@broadcom.com> >>>> Signed-off-by: Scott Branden <sbranden@broadcom.com> >>> >>> 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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