Re: [PATCH v19 7/7] ptp: arm/arm64: Enable ptp_kvm for arm/arm64

From: Marc Zyngier <maz@kernel.org>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: jianyong.wu@arm.com, netdev <netdev@vger.kernel.org>,
	Yangbo Lu <yangbo.lu@nxp.com>,
	John Stultz <john.stultz@linaro.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Paolo Bonzini <pbonzini@redhat.com>,
	seanjc@google.com, Richard Cochran <richardcochran@gmail.com>,
	Mark Rutland <Mark.Rutland@arm.com>,
	Will Deacon <will@kernel.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Andre Przywara <Andre.Przywara@arm.com>,
	Steven Price <steven.price@arm.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Sudeep Holla <sudeep.holla@arm.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	kvmarm@lists.cs.columbia.edu, KVM list <kvm@vger.kernel.org>,
	Steve Capper <Steve.Capper@arm.com>,
	justin.he@arm.com, Android Kernel Team <kernel-team@android.com>
Subject: Re: [PATCH v19 7/7] ptp: arm/arm64: Enable ptp_kvm for arm/arm64
Date: Wed, 26 May 2021 09:32:01 +0100	[thread overview]
Message-ID: <871r9tx5dq.wl-maz@kernel.org> (raw)
In-Reply-To: <CAMuHMdUBwcB-v0ugCPB3D6dbttiSbqQeHgNrr+r331ntRrfiAw@mail.gmail.com>

On Wed, 26 May 2021 09:18:27 +0100,
Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> 
> Hi Marc,
> 
> On Wed, May 26, 2021 at 10:01 AM Marc Zyngier <maz@kernel.org> wrote:
> > On Wed, 26 May 2021 08:52:42 +0100,
> > Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > On Tue, May 11, 2021 at 11:13 AM Marc Zyngier <maz@kernel.org> wrote:
> > > > On 2021-05-11 10:07, Geert Uytterhoeven wrote:
> > > > > On Tue, Mar 30, 2021 at 4:56 PM Marc Zyngier <maz@kernel.org> wrote:
> > > > >> From: Jianyong Wu <jianyong.wu@arm.com>
> > > > >
> > > > >> --- a/drivers/ptp/Kconfig
> > > > >> +++ b/drivers/ptp/Kconfig
> > > > >> @@ -108,7 +108,7 @@ config PTP_1588_CLOCK_PCH
> > > > >>  config PTP_1588_CLOCK_KVM
> > > > >>         tristate "KVM virtual PTP clock"
> > > > >>         depends on PTP_1588_CLOCK
> > > > >> -       depends on KVM_GUEST && X86
> > > > >> +       depends on (KVM_GUEST && X86) || (HAVE_ARM_SMCCC_DISCOVERY &&
> > > > >> ARM_ARCH_TIMER)
> > > > >
> > > > > Why does this not depend on KVM_GUEST on ARM?
> > > > > I.e. shouldn't the dependency be:
> > > > >
> > > > >     KVM_GUEST && (X86 || (HAVE_ARM_SMCCC_DISCOVERY && ARM_ARCH_TIMER))
> > > > >
> > > > > ?
> > > >
> > > > arm/arm64 do not select KVM_GUEST. Any kernel can be used for a guest,
> > > > and KVM/arm64 doesn't know about this configuration symbol.
> > >
> > > OK.
> > >
> > > Does PTP_1588_CLOCK_KVM need to default to yes?
> > > Perhaps only on X86, to maintain the status quo?
> >
> > I think I don't really understand the problem you are trying to
> > solve. Is it that 'make oldconfig' now asks you about this new driver?
> > Why is that an issue?
> 
> My first "problem" was that it asked about this new driver on
> arm/arm64, while I assumed there were some missing dependencies
> (configuring a kernel should not ask useless questions).  That turned
> out to be a wrong assumption, so there is no such problem here.
> 
> The second problem is "default y": code that is not critical should
> not be enabled by default.  Hence my last question.

I think consistency between architectures is important. Certainly,
distributions depend on that, and we otherwise end-up with distro
kernels missing functionalities.

The notion of "critical" is also pretty relative. defconfig contains a
gazillion of things that are not critical to most people, for example,
and yet misses a bunch of things that are needed to boot on some of my
systems.

That's just to say that I find it difficult to make that choice from
the PoV of a kernel hacker. I'm personally more inclined to leave
things enabled and let people *disable* things if they want to reduce
the footprint of their kernel.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.