linux-kernel - Re: [RFC PATCH 7/7] irqchip/apple-aic: add SMP support to the Apple AIC driver.

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Message-ID: <CAL_JsqJq_wfLrJm8rwspkgz81Be2V9WTudRjMAXSrZD50ewFYA@mail.gmail.com>
Date:   Thu, 21 Jan 2021 10:40:17 -0600
From:   Rob Herring <robh@...nel.org>
To:     Mohamed Mediouni <mohamed.mediouni@...amail.com>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Hector Martin <marcan@...can.st>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Stan Skowronek <stan@...ellium.com>
Subject: Re: [RFC PATCH 7/7] irqchip/apple-aic: add SMP support to the Apple
 AIC driver.

On Thu, Jan 21, 2021 at 6:52 AM Mohamed Mediouni
<mohamed.mediouni@...amail.com> wrote:
>
>
>
> > On 21 Jan 2021, at 13:44, Arnd Bergmann <arnd@...nel.org> wrote:
> >
> > On Wed, Jan 20, 2021 at 2:27 PM Mohamed Mediouni
> > <mohamed.mediouni@...amail.com> wrote:
> >
> >> +#ifdef CONFIG_SMP
> >> +static void apple_aic_ipi_send_mask(struct irq_data *d,
> >> +                                   const struct cpumask *mask)
> >
> > Not sure we care about the #ifdef here, given that arch/arm64 does not
> > allow building a kernel without CONFIG_SMP.
> >
> >> +       /*
> >> +     * Ensure that stores to Normal memory are visible to the
> >> +     * other CPUs before issuing the IPI.
> >> +     */
> >> +       wmb();
> >> +
> >> +       for_each_cpu (cpu, mask) {
> >> +               smp_mb__before_atomic();
> >> +               atomic_or(1u << irqnr, per_cpu_ptr(&aic_ipi_mask, cpu));
> >> +               smp_mb__after_atomic();
> >> +               lcpu = get_cpu();
> >> +               if (aic.fast_ipi) {
> >> +                       if ((lcpu >> 2) == (cpu >> 2))
> >> +                               write_sysreg(cpu & 3, SR_APPLE_IPI_LOCAL);
> >> +                       else
> >> +                               write_sysreg((cpu & 3) | ((cpu >> 2) << 16),
> >> +                                            SR_APPLE_IPI_REMOTE);
> >> +               } else
> >> +                       writel(lcpu == cpu ? REG_IPI_FLAG_SELF :
> >> +                                                  (REG_IPI_FLAG_OTHER << cpu),
> >> +                              aic.base + REG_IPI_SET);
> >> +               put_cpu();
> >> +       }
> >> +
> >> +       /* Force the above writes to be executed */
> >> +       if (aic.fast_ipi)
> >> +               isb();
> >> +}
> >
> > Since this just loops over all CPUs, I'd probably just turn it into
> > an ipi_send_single() callback and have the caller do the
> > loop for simplicity.
> >
> > I also have the feeling that splitting one hardware IPI into multiple
> > logical interrupts, which are then all registered by the same irq
> > handler adds a little more complexity than necessary.
> >
> > Changing this would of course require modifications to
> > arch/arm64/kernel/smp.c, which is hardwired to use
> > CONFIG_GENERIC_IRQ_IPI in smp_cross_call(), and allowing
> > a different code path there may be worse than emulating an
> > irqchip.
> >
> >> @@ -186,8 +325,11 @@ static int __init apple_aic_init(struct device_node *node,
> >>       if (WARN(!aic.base, "unable to map aic registers\n"))
> >>               return -EINVAL;
> >>
> >> +       aic.fast_ipi = of_property_read_bool(node, "fast-ipi");
> >
> > Where is this property documented, and what decides which one to use?
> It’s getting documented in the next patch set.
>
> This property is there to enable support for older iPhone processors
> later on, some of which do not have fast IPI support.
>
> On Apple M1, fast-ipi is always on.

This should be implied by the compatible string which needs to be more
specific and include the SoC name.

Rob