linux-kernel - Re: [PATCH v13 3/7] genirq: Add mechanism to multiplex a single HW IPI

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Date:   Wed, 30 Nov 2022 22:33:05 +0530
From:   Anup Patel <anup@...infault.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Anup Patel <apatel@...tanamicro.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Atish Patra <atishp@...shpatra.org>,
        Alistair Francis <Alistair.Francis@....com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v13 3/7] genirq: Add mechanism to multiplex a single HW IPI

On Wed, Nov 30, 2022 at 8:17 PM Marc Zyngier <maz@...nel.org> wrote:
>
> On Tue, 29 Nov 2022 14:24:45 +0000,
> Anup Patel <apatel@...tanamicro.com> wrote:
> >
> > All RISC-V platforms have a single HW IPI provided by the INTC local
> > interrupt controller. The HW method to trigger INTC IPI can be through
> > external irqchip (e.g. RISC-V AIA), through platform specific device
> > (e.g. SiFive CLINT timer), or through firmware (e.g. SBI IPI call).
> >
> > To support multiple IPIs on RISC-V, we add a generic IPI multiplexing
> > mechanism which help us create multiple virtual IPIs using a single
> > HW IPI. This generic IPI multiplexing is inspired from the Apple AIC
> > irqchip driver and it is shared by various RISC-V irqchip drivers.
> >
> > Signed-off-by: Anup Patel <apatel@...tanamicro.com>
> > ---
> >  include/linux/irq.h  |   3 +
> >  kernel/irq/Kconfig   |   5 ++
> >  kernel/irq/Makefile  |   1 +
> >  kernel/irq/ipi-mux.c | 193 +++++++++++++++++++++++++++++++++++++++++++
> >  4 files changed, 202 insertions(+)
> >  create mode 100644 kernel/irq/ipi-mux.c
>
> [...]
>
> This is finally starting to look acceptable. My only changes are:
>
> diff --git a/kernel/irq/ipi-mux.c b/kernel/irq/ipi-mux.c
> index 626c564233e9..1a6ba19b736c 100644
> --- a/kernel/irq/ipi-mux.c
> +++ b/kernel/irq/ipi-mux.c
> @@ -69,6 +69,12 @@ static void ipi_mux_send_mask(struct irq_data *d, const struct cpumask *mask)
>                  */
>                 smp_mb__after_atomic();
>
> +               /*
> +                * The flag writes must complete before the physical IPI is
> +                * issued to another CPU. This is implied by the control
> +                * dependency on the result of atomic_read() below, which is
> +                * itself already ordered after the vIPI flag write.
> +                */
>                 if (!(pending & ibit) && (atomic_read(&icpu->enable) & ibit))
>                         ipi_mux_send(cpu);
>         }
> @@ -160,7 +166,7 @@ int ipi_mux_create(unsigned int nr_ipi, void (*mux_send)(unsigned int cpu))
>                 goto fail_free_cpu;
>         }
>
> -       domain = irq_domain_create_simple(fwnode, nr_ipi, 0,
> +       domain = irq_domain_create_linear(fwnode, nr_ipi,
>                                           &ipi_mux_domain_ops, NULL);
>         if (!domain) {
>                 pr_err("unable to add IPI Mux domain\n");
>
> The first hunk preserve an important piece of information about how
> delicate the ordering is, while the second only allocates the irqdesc
> structures as needed, not upfront.
>
> I'll shortly go over the rest of the irqchip code and can apply the
> above myself if there is nothing more to fix.
>
> I've also converted the AIC driver over to this[1], and nothing broke
> so far...
>
>         M.
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=irq/ipi-mux

Thanks Marc, I will include your changes in the next patch revision.

>
> --
> Without deviation from the norm, progress is not possible.

Regards,
Anup