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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Wed, 26 May 2021 10:50:47 +0200
Message-ID: <CAMuHMdVFE=RB5wPUi9PH5WeQPSYfcg+ugKigB4nAo9yotO+WYg@mail.gmail.com>
Subject: Re: [PATCH v19 7/7] ptp: arm/arm64: Enable ptp_kvm for arm/arm64
To:     Marc Zyngier <maz@kernel.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>
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Hi Marc,

On Wed, May 26, 2021 at 10:32 AM Marc Zyngier <maz@kernel.org> wrote:
> On Wed, 26 May 2021 09:18:27 +0100,
> Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > 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

I'm not talking about defconfig, but about "default y" in defconfig.

> 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.

Perhaps those should be added, so those systems can be tested using
defconfig?  At least for arm64, I think that's aligned with the
arm64 defconfig policy.

> 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.

The standard question to respond to w.r.t. "default y" is: "Why is
your feature so special that it needs to be enabled by default?",
which implies "default y" is the exception, not the rule.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds