linux-kernel - RE: [PATCH 1/2] Drivers: hv: Move Hyper-V clockevents code to new clocksource driver

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Date:   Wed, 13 Mar 2019 18:17:15 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Michael Kelley <mikelley@...rosoft.com>
Cc:     "catalin.marinas\@arm.com" <catalin.marinas@....com>,
        "mark.rutland\@arm.com" <mark.rutland@....com>,
        "will.deacon\@arm.com" <will.deacon@....com>,
        "marc.zyngier\@arm.com" <marc.zyngier@....com>,
        "linux-arm-kernel\@lists.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "gregkh\@linuxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv\@vger.kernel.org" <linux-hyperv@...r.kernel.org>,
        "olaf\@aepfle.de" <olaf@...fle.de>,
        "apw\@canonical.com" <apw@...onical.com>,
        "jasowang\@redhat.com" <jasowang@...hat.com>,
        "marcelo.cerri\@canonical.com" <marcelo.cerri@...onical.com>,
        Sunil Muthuswamy <sunilmut@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>
Subject: RE: [PATCH 1/2] Drivers: hv: Move Hyper-V clockevents code to new clocksource driver

Michael Kelley <mikelley@...rosoft.com> writes:

> From: Vitaly Kuznetsov <vkuznets@...hat.com> Sent: Wednesday, March 13, 2019 7:23 AM
>
>> >> > diff --git a/drivers/clocksource/Makefile b/drivers/clocksource/Makefile
>> >> > index be6e0fb..a887955 100644
>> >> > --- a/drivers/clocksource/Makefile
>> >> > +++ b/drivers/clocksource/Makefile
>> >> > @@ -83,3 +83,4 @@ obj-$(CONFIG_ATCPIT100_TIMER)		+= timer-atcpit100.o
>> >> >  obj-$(CONFIG_RISCV_TIMER)		+= timer-riscv.o
>> >> >  obj-$(CONFIG_CSKY_MP_TIMER)		+= timer-mp-csky.o
>> >> >  obj-$(CONFIG_GX6605S_TIMER)		+= timer-gx6605s.o
>> >> > +obj-$(CONFIG_HYPERV)			+= hyperv_syntimer.o
>> >>
>> >> (just a couple of spare thoughs)
>> >>
>> >> CONFIG_HYPERV can also be a module, are we OK with that? (we'll have to
>> >> support module loading/unloading then and honestly I see no reason for
>> >> that. I would prefer everything but VMBus devices to be in
>> >> kernel.) If we don't want it to be a module we can create a hidden
>> >> CONFIG_HYPERV_STIMER or something like that - just like we already do
>> >> for CONFIG_HYPERV_TSCPAGE.
>> >>
>> >> There is, however, one additional dependency here: when running in
>> >> non-direct mode, Hyper-V clockevent devices require functional Hyper-V
>> >> messaging - which currently lives in VMBus code so that may explain why
>> >> you may want to keep stimer code in the same entity. Or, alternatively,
>> >> we can move Hyper-V messaging out of VMBus code (is it actually
>> >> architecture-agnostic?)
>> >>
>> >
>> > I thought about introducing CONFIG_HYPERV_STIMER, but in my
>> > judgment it was just unnecessary complexity.  The Hyper-V clocksource
>> > driver can't exist independent of Hyper-V, and vice versa.  When both the
>> > clocksource and clockevents code is considered, the VMbus driver and
>> > Hyper-V initialization code has to call directly into the driver since the
>> > Hyper-V synthetic timers and reference time counter aren't independently
>> > enumerated.  Even if we could get the Hyper-V messaging out of VMbus
>> > code, we would still need the clocksource initialization call directly from
>> > hyperv_init(), which is not in a module (see the 2nd patch of the series).
>> >
>> 
>> Right, so hv_init_clocksource() cannot live in hv_vmbus module and we
>> need to somehow prevent hyperv_syntimer.o from going in there. And
>> 
>> +obj-$(CONFIG_HYPERV)                      += hyperv_syntimer.o
>> 
>> will do exactly the opposite - put it in hv_vmbus module. Or am I
>> missing something? (I haven't tried to build your code yet, sorry).
>> 
>
> That line just controls whether hyperv_syntimer.o is built.  It doesn't put
> it in the hv_vmbus module.  All of the clocksource .o files that are built go
> into the kernel, not in a module.  But thinking about it more, the above works
> correctly when CONFIG_HYPERV=y, but not when CONFIG_HYPERV=m.

Yes, that's what I meant.

> I'll have to introduce CONFIG_HYPERV_TIMER after all.  Will fix this in v2.  Thanks
> for the discussion!

Thanks!

-- 
Vitaly