Messages in this thread | | | Date | Sat, 28 Feb 2015 08:01:11 -0800 | From | Scott Branden <> | Subject | Re: [PATCH v2 2/2] hwrng: iproc-rng200 - Add Broadcom IPROC RNG driver |
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On 15-02-27 01:14 AM, Arnd Bergmann wrote: > On Thursday 26 February 2015 14:26:02 Scott Branden wrote: >> On 15-02-26 12:15 PM, Arnd Bergmann wrote: >>>> On 15-02-25 11:17 AM, Arnd Bergmann wrote: >>>>> On Wednesday 25 February 2015 10:16:24 Scott Branden wrote: >>>> 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. > > The udelay(10) that the other drivers have seems about appropriate then, > and we can independently think of a way to refine the interface. > Please add a comment that explains the rate. Also, is there some kind > of FIFO present in the hwrng device? If it can store close to 1ms work > of data (1000 bits), you can just use an msleep(1) to wait for the > pool to recover. FIFO is 512 bits. I will look as to whether we can live with 1/2 throughput. > > Another option would be to use usleep_range() with the exact amount > of time to wait for, the lower bound being the minimum number of > bytes asked for and the fifo size, the upper bound being the fifo > size. > > Arnd >
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