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

From: Geert Uytterhoeven <geert@linux-m68k.org>
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>
Subject: Re: [PATCH v19 7/7] ptp: arm/arm64: Enable ptp_kvm for arm/arm64
Date: Wed, 26 May 2021 10:18:27 +0200	[thread overview]
Message-ID: <CAMuHMdUBwcB-v0ugCPB3D6dbttiSbqQeHgNrr+r331ntRrfiAw@mail.gmail.com> (raw)
In-Reply-To: <8735u9x6sb.wl-maz@kernel.org>

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.

Thanks!

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