linux-kernel - Re: Fw: Arm CryptoCell driver -- default Y, even on machines where it is obviously useless

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Date:   Sun, 19 Apr 2020 10:55:00 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Gilad Ben-Yossef <gilad@...yossef.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Hadar Gat <hadar.gat@....com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>
Subject: Re: Fw: Arm CryptoCell driver -- default Y, even on machines where it
 is obviously useless

On Sun, 19 Apr 2020 at 08:33, Gilad Ben-Yossef <gilad@...yossef.com> wrote:
>
> > > -----Original Message-----
> > > From: Pavel Machek <pavel@....cz>
> > > Sent: Saturday, 18 April 2020 13:44
> > > To: kernel list <linux-kernel@...r.kernel.org>; Hadar Gat
> > > <Hadar.Gat@....com>; herbert@...dor.apana.org.au
> > > Subject: Arm CryptoCell driver -- default Y, even on machines where it is
> > > obviously useless
> > >
> > > Hi!
> > >
> > > I'm configuring kernel for x86, and I get offered HW_RANDOM_CCTRNG with
> > > default=Y, and help text suggesting I should enable it.
> > >
> > > That's... two wrong suggestions, right?
> > >
> > > Best regards,
> > > Pavel
> ...
> > ________________________________________
> > From: Hadar Gat <Hadar.Gat@....com>
> > Sent: Saturday, April 18, 2020 11:31 PM
> > To: Pavel Machek; kernel list; herbert@...dor.apana.org.au
> > Cc: Ofir Drang; Gilad Ben Yossef; nd
> > Subject: RE: Arm CryptoCell driver -- default Y, even on machines where it is obviously useless
> >
> > Hi Pavel,
> > I think you got it right..
> > Indeed, Arm CryptoCell CCTRNG driver couldn't be used and obviously useless if the Arm CryptoCell HW does not exist in the system.
>
> There's a delicate point here though - CryptoCell is an independent
> hardware block, it is not tied to a particular CPU architecture.
> There are SoCs with none-Arm architecture CPU using it.
>
> So I would say whatever the answer is, it should be the same for any
> generic embedded style HW block.
>
> And the help text is not architecture specific anyway, is it not..?
>

Both the default y and and the help text are indeed incorrect. This
should be fixed. We don't enable device drivers by default, and
definitely not as as builtins. A conditional default m could be
acceptable if the condition is sufficiently narrow.

While at it, could we add a depends on CONFIG_OF since this code is
definitely unusable on non-DT systems.