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

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Message-ID: <CAMj1kXECTnTg+2V+oh1+ONca9mUi14CJ1QXoaktM4N2rVs_+EQ@mail.gmail.com>
Date:   Sun, 26 Apr 2020 14:56:51 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Hadar Gat <Hadar.Gat@....com>
Cc:     Gilad Ben-Yossef <gilad@...yossef.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        nd <nd@....com>
Subject: Re: Fw: Arm CryptoCell driver -- default Y, even on machines where it
 is obviously useless

On Sun, 26 Apr 2020 at 14:50, Hadar Gat <Hadar.Gat@....com> wrote:
>
> Hi Ard,
>
> > -----Original Message-----
> > From: Ard Biesheuvel <ardb@...nel.org>
> > Sent: Sunday, 19 April 2020 11:55
> >
> > > > > -----Original Message-----
> > > > > From: Pavel Machek <pavel@....cz>
> > > > > Sent: Saturday, 18 April 2020 13:44
> > > > >
> > > > > 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
> > > >
> > > > 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.
>
> On one hand I totally agree with that and think the default should be N.
> On the other hand, most of the HW_RANDOM drivers set the default to HW_RANDOM
> and it doesn't make sense to me to do something different than almost every other HW RANDOM device.
> Do I miss something here?
>

Yes. First of all, using 'default HW_RANDOM' everywhere makes no sense
at all, but that is not your fault.

If you look at drivers/char/hw_random/Kconfig, you will see that most
drivers have additional depends lines, which means that 'default m'
(or 'default y' in case CONFIG_HW_RANDOM=y) will only take affect if
the dependency is fulfilled.

It makes no sense to enable this driver on *every* single Linux
system, right? Especially given that many architectures do not even
support device tree, which is a prerequisite to be able to even probe.




drivers/char/hw_random/Kconfig-config HW_RANDOM_INTEL
drivers/char/hw_random/Kconfig- depends on (X86 || IA64) && PCI
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_AMD
drivers/char/hw_random/Kconfig- depends on (X86 || PPC_MAPLE) && PCI
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_ATMEL
drivers/char/hw_random/Kconfig- depends on ARCH_AT91 && HAVE_CLK && OF
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_BCM2835
drivers/char/hw_random/Kconfig- depends on ARCH_BCM2835 ||
ARCH_BCM_NSP || ARCH_BCM_5301X || \
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_IPROC_RNG200
drivers/char/hw_random/Kconfig- depends on ARCH_BCM_IPROC ||
ARCH_BCM2835 || ARCH_BRCMSTB
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_GEODE
drivers/char/hw_random/Kconfig- depends on X86_32 && PCI
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_N2RNG
drivers/char/hw_random/Kconfig- depends on SPARC64
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_VIA
drivers/char/hw_random/Kconfig- depends on X86
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_IXP4XX
drivers/char/hw_random/Kconfig- depends on ARCH_IXP4XX
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_OMAP
drivers/char/hw_random/Kconfig- depends on ARCH_OMAP16XX ||
ARCH_OMAP2PLUS || ARCH_MVEBU
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_OMAP3_ROM
drivers/char/hw_random/Kconfig- depends on ARCH_OMAP3
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_OCTEON
drivers/char/hw_random/Kconfig- depends on CAVIUM_OCTEON_SOC
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_PASEMI
drivers/char/hw_random/Kconfig- depends on PPC_PASEMI
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_VIRTIO
drivers/char/hw_random/Kconfig- depends on VIRTIO
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_TX4939
drivers/char/hw_random/Kconfig- depends on SOC_TX4939
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_MXC_RNGA
drivers/char/hw_random/Kconfig- depends on SOC_IMX31
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_IMX_RNGC
drivers/char/hw_random/Kconfig- depends on HAS_IOMEM && HAVE_CLK
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_NOMADIK
drivers/char/hw_random/Kconfig- depends on ARCH_NOMADIK
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_PSERIES
drivers/char/hw_random/Kconfig- depends on PPC64 && IBMVIO
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_POWERNV
drivers/char/hw_random/Kconfig- depends on PPC_POWERNV
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_HISI
drivers/char/hw_random/Kconfig- depends on HW_RANDOM && ARCH_HISI
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_HISI_V2
drivers/char/hw_random/Kconfig- depends on HW_RANDOM && ARM64 && ACPI
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_ST
drivers/char/hw_random/Kconfig- depends on HW_RANDOM && ARCH_STI
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_XGENE
drivers/char/hw_random/Kconfig- depends on HW_RANDOM && ARCH_XGENE
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_STM32
drivers/char/hw_random/Kconfig- depends on HW_RANDOM && (ARCH_STM32 ||
COMPILE_TEST)
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_PIC32
drivers/char/hw_random/Kconfig- depends on HW_RANDOM && MACH_PIC32
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_MESON
drivers/char/hw_random/Kconfig- depends on HW_RANDOM
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_CAVIUM
drivers/char/hw_random/Kconfig- depends on HW_RANDOM && PCI && (ARM64
|| (COMPILE_TEST && 64BIT))
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_MTK
drivers/char/hw_random/Kconfig- depends on HW_RANDOM
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_S390
drivers/char/hw_random/Kconfig- depends on S390
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_EXYNOS
drivers/char/hw_random/Kconfig- depends on ARCH_EXYNOS || COMPILE_TEST
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_OPTEE
drivers/char/hw_random/Kconfig- depends on OPTEE
--
drivers/char/hw_random/Kconfig-config HW_RANDOM_NPCM
drivers/char/hw_random/Kconfig- depends on ARCH_NPCM || COMPILE_TEST