linux-kernel - Re: [PATCH v2 2/3] hwrng: add Rockchip SoC hwrng driver

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Date:   Mon, 5 Dec 2022 14:13:45 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Aurelien Jarno <aurelien@...el32.net>
Cc:     Olivia Mackall <olivia@...enic.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Heiko Stuebner <heiko@...ech.de>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Lin Jinhan <troy.lin@...k-chips.com>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "moderated list:ARM/Rockchip SoC support" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list:ARM/Rockchip SoC support" 
        <linux-rockchip@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] hwrng: add Rockchip SoC hwrng driver

On Mon, Nov 28, 2022 at 07:47:17PM +0100, Aurelien Jarno wrote:
> The TRNG device does not seem to have a signal conditionner and the FIPS
> 140-2 test returns a lot of failures. They can be reduced by increasing
> RK_RNG_SAMPLE_CNT, in a tradeoff between quality and speed. This value
> has been adjusted to get ~90% of successes and the quality value has
> been set accordingly.

Can't you reduce it even more to get 100%? All we need is 32 bytes every
once in a while.

> +	rk_rng->rng.quality = 900;

If your intention is "90%", this should be 921 or 922, because the
quality knob is out of 1024, not 1000.

Herbert - this seems like a fairly common pitfall I've seen all over the
place. It might be worth making a mental memo to reject or ask questions
about numbers that seem "too round", when you look at these sorts of
patches.

Jason