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

[Scott Branden <sbranden@broadcom.com>]

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
>