linux-kernel - Re: [PATCH v2] hwrng: Add Arm SMCCC TRNG based driver

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Message-ID: <20210721111656.GA4259@sirena.org.uk>
Date:   Wed, 21 Jul 2021 12:16:56 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Andre Przywara <andre.przywara@....com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Matt Mackall <mpm@...enic.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Will Deacon <will@...nel.org>, Ali Saidi <alisaidi@...zon.com>,
        Jon Nettleton <jon@...id-run.com>
Subject: Re: [PATCH v2] hwrng: Add Arm SMCCC TRNG based driver

On Wed, Jul 21, 2021 at 12:18:03AM +0100, Andre Przywara wrote:
> On Wed, 21 Jul 2021 08:02:42 +1000
> Benjamin Herrenschmidt <benh@...nel.crashing.org> wrote:
> > On Tue, 2021-07-20 at 17:16 +0100, Andre Przywara wrote:

> > > Yes, a similar idea was already brought up before. I think there is even
> > > the potential for something like an artificial SMCCC "bus", where those
> > > services presentable as devices could be auto-detected (by checking
> > > known function IDs), the respective drivers would then probe
> > > automatically?  

> > Sounds like a boot time killer...

> How so? To be clear, at the moment there is basically just the TRNG
> service we would probe for, maybe FF-A, then adding as we go. But in
> any case it would be just a handful, and querying is very quick
> (SMC/HVC, then just a switch/case on the other side, and ERET).
> Is there any particular scenario you are concerned about? Quick
> starting guests?

It's also worth pointing out that we're already doing the enumeration
part, making things a bus would just be about reorganising the code that
checks if services are present to a central place so it looks more Linux
style.  If anything I'd guess that if we get to the point where things
are slow enough to worry about having that code in one central place
would make doing something about it easier (eg, adding a "list all
services" service or firmware binding).

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