linux-kernel - Re: [PATCH v6 05/10] arm64: hyperv: Add interrupt handlers for VMbus and stimer

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Date:   Wed, 18 Mar 2020 10:52:30 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Michael Kelley <mikelley@...rosoft.com>
Cc:     Will Deacon <will@...nel.org>, Ard Biesheuvel <ardb@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        gregkh <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "olaf@...fle.de" <olaf@...fle.de>,
        Andy Whitcroft <apw@...onical.com>,
        vkuznets <vkuznets@...hat.com>, Jason Wang <jasowang@...hat.com>,
        "marcelo.cerri@...onical.com" <marcelo.cerri@...onical.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Sunil Muthuswamy <sunilmut@...rosoft.com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH v6 05/10] arm64: hyperv: Add interrupt handlers for VMbus
 and stimer

On Wed, Mar 18, 2020 at 1:16 AM Michael Kelley <mikelley@...rosoft.com> wrote:
> From: Arnd Bergmann <arnd@...db.de>
> > On Sat, Mar 14, 2020 at 4:36 PM Michael Kelley <mikelley@...rosoft.com> wrote:
> > >
> > > Add ARM64-specific code to set up and handle the interrupts
> > > generated by Hyper-V for VMbus messages and for stimer expiration.
> > >
> > > This code is architecture dependent and is mostly driven by
> > > architecture independent code in the VMbus driver and the
> > > Hyper-V timer clocksource driver.
> > >
> > > This code is built only when CONFIG_HYPERV is enabled.
> > >
> > > Signed-off-by: Michael Kelley <mikelley@...rosoft.com>
> >
> > This looks like it should be a nested irqchip driver instead, so your
> > device drivers can use the normal request_irq() functions etc.
> >
> > Is anything preventing you from doing that? If so, please describe
> > that in the changelog and in a comment in the driver.
> >
>
> As mentioned in my reply on Patch 1, Hyper-V offers a limited synthetic
> interrupt controller managed by Linux code that's been around the last
> 10 years on the x86 side.   For reasons that pre-date me, it was not written
> as an irqchip driver.

I think the reason is just that 10 years ago, we did not have the concept
of irqchips as device drivers.

> Modulo the small routines you see in this patch, the code is architecture
> independent, and it seems we ought to keep the high degree of commonality.
> Re-architecting the arch independent code to model as an irqchip driver seems
> to carry some risk to the x86 side that has a lot of real-world usage today, but
> I'll take a look and see what the risks look like and if it adds any clarity.

How many drivers link against the custom interface? If it's less than 10,
making it a real driver is probably not too hard to do.

       Arnd