Re: [Qemu-devel] [PATCH v2] hw/misc: Add Exynos4210 Pseudo Random Number Generator

From: Peter Maydell <peter.maydell@linaro.org>
To: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Igor Mitsyanko <i.mitsyanko@gmail.com>,
	qemu-arm <qemu-arm@nongnu.org>,
	QEMU Developers <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] [PATCH v2] hw/misc: Add Exynos4210 Pseudo Random Number Generator
Date: Tue, 4 Apr 2017 15:05:09 +0100	[thread overview]
Message-ID: <CAFEAcA9UGEVVUfL3--goP4mVU+ezegWPcoka41XM8d_E2yyJkQ@mail.gmail.com> (raw)
In-Reply-To: <20170404134402.GA4196@kozik-book>

On 4 April 2017 at 14:44, Krzysztof Kozlowski <krzk@kernel.org> wrote:
> On Tue, Apr 04, 2017 at 01:09:08PM +0100, Peter Maydell wrote:
>> On 18 March 2017 at 19:25, Krzysztof Kozlowski <krzk@kernel.org> wrote:
>> > Add emulation for Exynos4210 Pseudo Random Number Generator which could
>> > work on fixed seeds or with seeds provided by True Random Number
>> > Generator block inside the SoC.
>> >
>> > Implement only the fixed seeds part of it in polling mode (no
>> > interrupts).  Simple testing:
>> >     # echo "exynos" > /sys/class/misc/hw_random/rng_current
>> >     # dd if=/dev/hwrng of=/dev/null bs=1 count=16
>> >
>> > Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
>> >
>> > ---
>> >
>> > Changes since v1:
>> > 1. Use GRand-like functions to fix build on MingW32 (this adds also
>> >    finalize).
>> > 2. Add DPRINTF macro.
>> > 3. Use HWADDR_PRIx and family for printing values.
>>
>> Is there a data sheet that describes this RNG? I had a quick google
>> but couldn't find anything in the 4210 manual you can get from Samsung.
>
> Official and public datasheet - I never heard about it... AFAIK, Samsung
> never released any datasheet... But recently I found a copy of
> Exynos4412 datasheet published on FriendlyArm website:
> http://wiki.friendlyarm.com/wiki/index.php/NanoPC-T1
> (at the bottom in "Resources").
>
> Some blocks in Exynos4412, including the RNG, are the same as in
> Exynos4210. However, you should not expect too much data about the RNG
> in the datasheet...
>
>> In particular I'm not sure we want to use GRand here.
>
> Now, I am not sure neither. :)

The last RNG we added was hw/misc/bcm2835_rng.c, which uses
qcrypto_random_bytes() to get cryptographically-random bytes
from the host. On the other hand it sounds like this Exynos
hardware is a PRNG without true-random input...

> During the discussions about new kernel driver, I found that this RNG
> module (at least on Exynos4412 board) requires seeding but it is not
> following the Pseudo RNG behavior - seeding with the same value produces
> different results. This means that GRand would not be needed here.

That's rather odd. You're right that the datasheet is hopelessly
uninformative about what's actually going on.

thanks
-- PMM