linux-kernel - RE: [PATCH v2 2/3] hwrng: starfive - Add TRNG driver for StarFive SoC

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Date:   Mon, 9 Jan 2023 04:33:26 +0000
From:   JiaJie Ho <jiajie.ho@...rfivetech.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
CC:     Olivia Mackall <olivia@...enic.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Emil Renner Berthing <kernel@...il.dk>,
        Conor Dooley <conor.dooley@...rochip.com>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>
Subject: RE: [PATCH v2 2/3] hwrng: starfive - Add TRNG driver for StarFive SoC



> -----Original Message-----
> From: Herbert Xu <herbert@...dor.apana.org.au>
> Sent: 9 January, 2023 12:00 PM
> To: JiaJie Ho <jiajie.ho@...rfivetech.com>
> Cc: Olivia Mackall <olivia@...enic.com>; Rob Herring <robh+dt@...nel.org>;
> Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>; Emil Renner
> Berthing <kernel@...il.dk>; Conor Dooley <conor.dooley@...rochip.com>;
> linux-crypto@...r.kernel.org; devicetree@...r.kernel.org; linux-
> kernel@...r.kernel.org; linux-riscv@...ts.infradead.org
> Subject: Re: [PATCH v2 2/3] hwrng: starfive - Add TRNG driver for StarFive
> SoC
> 
> On Mon, Jan 09, 2023 at 03:41:14AM +0000, JiaJie Ho wrote:
> >
> > The average wait time is around 20 microseconds.
> > I measured from writel cmd to wait_for_completion done.
> 
> Do you know of an upper bound? E.g., if we limit it to 40us how many
> requests would fail on average?
> 

I don't have the upper bound. 
I ran the rngtest with 1000 blocks, none of the wait time exceeded 30us.

> Having a maximum delay of 40us would be OK with wait == 0.  So you could
> implement it in a way such that if the wait time exceeded 40us then you
> return if wait == 0, otherwise you can wait longer.
> 

I'll do this then.

Thanks
Jia Jie