Re: [PATCH v2 2/4] crypto: exynos - Improve performance of PRNG

[Åukasz Stelmach]

It was <2017-12-11 pon 15:54>, when Krzysztof Kozlowski wrote:
> On Mon, Dec 11, 2017 at 3:06 PM, Åukasz Stelmach <l.stelmach@xxxxxxxxxxx> wrote:
>> Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>, Bartlomiej
>> Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>
>
> This should not appear here.

A glitch in a scripted invocation of git-format-patch, fixed.

>> Use memcpy_fromio() instead of custom exynos_rng_copy_random() function
>> to retrieve generated numbers from the registers of PRNG.
>>
>> Rearrange the loop around cpu_relax(). In a loop with while() at the
>> beginning and the cpu_relax() removed the retry variable is decremented
>> twice (down to 98).
>
> I had troubles with understanding this sentence... and then I figured
> out that you are referring to some case without cpu_relax(). I do not
> see how it is relevant to this case. Compare the new code with old,
> not with some imaginary case without barriers (thus maybe reordered?).
>
> Your solution is strictly performance oriented so it would be nice to
> see here the exact difference in numbers justifying the change. But
> only the change for while() -> do-while(), not mixed with
> memcpy_fromio.

Apparently, after trhough tests, I must admit, the way the status
register is being polled is insignificant for the performance. I will
remove from the patch any changes in the loop.

It is the way, the random bytes are copied from the regiesteres, that
makes the difference (5.9 MB/s vs 7.1 MB/s)

Thank you very much for your assistance in reaching this conclusion.

-- 
Åukasz Stelmach
Samsung R&D Institute Poland
Samsung Electronics

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