linux-kernel - Re: [PATCH v7 3/7] genirq: Add mechanism to multiplex a single HW IPI

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Message-ID: <CAAhSdy2Xz6UOBfb11LMMhS9+eZL54M6asZLH0zvhGxy9Mv6yMg@mail.gmail.com>
Date:   Thu, 21 Jul 2022 17:56:02 +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 <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 3/7] genirq: Add mechanism to multiplex a single HW IPI

On Thu, Jul 21, 2022 at 5:39 PM Marc Zyngier <maz@...nel.org> wrote:
>
> On 2022-07-21 12:44, Anup Patel wrote:
> > On Thu, Jul 21, 2022 at 4:30 PM Marc Zyngier <maz@...nel.org> wrote:
> >>
> >> On Wed, 20 Jul 2022 16:23:44 +0100,
> >> 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 shared among various RISC-V
> >> > irqchip drivers.
> >> >
> >> > Signed-off-by: Anup Patel <apatel@...tanamicro.com>
> >> > ---
> >> >  include/linux/irq.h  |  16 ++++
> >> >  kernel/irq/Kconfig   |   4 +
> >> >  kernel/irq/Makefile  |   1 +
> >> >  kernel/irq/ipi-mux.c | 199 +++++++++++++++++++++++++++++++++++++++++++
> >> >  4 files changed, 220 insertions(+)
> >> >  create mode 100644 kernel/irq/ipi-mux.c
> >> >
> >> > diff --git a/include/linux/irq.h b/include/linux/irq.h
> >> > index 505308253d23..a97bf13a8965 100644
> >> > --- a/include/linux/irq.h
> >> > +++ b/include/linux/irq.h
> >> > @@ -1249,6 +1249,22 @@ int __ipi_send_mask(struct irq_desc *desc, const struct cpumask *dest);
> >> >  int ipi_send_single(unsigned int virq, unsigned int cpu);
> >> >  int ipi_send_mask(unsigned int virq, const struct cpumask *dest);
> >> >
> >> > +/**
> >> > + * struct ipi_mux_ops - IPI multiplex operations
> >> > + *
> >> > + * @ipi_mux_clear:   Optional function to clear parent IPI
> >> > + * @ipi_mux_send:    Trigger parent IPI on target CPUs
> >> > + */
> >> > +struct ipi_mux_ops {
> >> > +     void (*ipi_mux_clear)(unsigned int parent_virq);
> >> > +     void (*ipi_mux_send)(unsigned int parent_virq,
> >> > +                          const struct cpumask *mask);
> >> > +};
> >> > +
> >> > +void ipi_mux_process(void);
> >> > +int ipi_mux_create(unsigned int parent_virq, unsigned int nr_ipi,
> >> > +                const struct ipi_mux_ops *ops);
> >> > +
> >> >  #ifdef CONFIG_GENERIC_IRQ_MULTI_HANDLER
> >> >  /*
> >> >   * Registers a generic IRQ handling function as the top-level IRQ handler in
> >> > diff --git a/kernel/irq/Kconfig b/kernel/irq/Kconfig
> >> > index 10929eda9825..2388e7d40ed3 100644
> >> > --- a/kernel/irq/Kconfig
> >> > +++ b/kernel/irq/Kconfig
> >> > @@ -84,6 +84,10 @@ config GENERIC_IRQ_IPI
> >> >       bool
> >> >       select IRQ_DOMAIN_HIERARCHY
> >> >
> >> > +# Generic IRQ IPI Mux support
> >> > +config GENERIC_IRQ_IPI_MUX
> >> > +     bool
> >> > +
> >> >  # Generic MSI interrupt support
> >> >  config GENERIC_MSI_IRQ
> >> >       bool
> >> > diff --git a/kernel/irq/Makefile b/kernel/irq/Makefile
> >> > index b4f53717d143..f19d3080bf11 100644
> >> > --- a/kernel/irq/Makefile
> >> > +++ b/kernel/irq/Makefile
> >> > @@ -15,6 +15,7 @@ obj-$(CONFIG_GENERIC_IRQ_MIGRATION) += cpuhotplug.o
> >> >  obj-$(CONFIG_PM_SLEEP) += pm.o
> >> >  obj-$(CONFIG_GENERIC_MSI_IRQ) += msi.o
> >> >  obj-$(CONFIG_GENERIC_IRQ_IPI) += ipi.o
> >> > +obj-$(CONFIG_GENERIC_IRQ_IPI_MUX) += ipi-mux.o
> >> >  obj-$(CONFIG_SMP) += affinity.o
> >> >  obj-$(CONFIG_GENERIC_IRQ_DEBUGFS) += debugfs.o
> >> >  obj-$(CONFIG_GENERIC_IRQ_MATRIX_ALLOCATOR) += matrix.o
> >> > diff --git a/kernel/irq/ipi-mux.c b/kernel/irq/ipi-mux.c
> >> > new file mode 100644
> >> > index 000000000000..bd6b31ca588b
> >> > --- /dev/null
> >> > +++ b/kernel/irq/ipi-mux.c
> >> > @@ -0,0 +1,199 @@
> >> > +// SPDX-License-Identifier: GPL-2.0-only
> >> > +/*
> >> > + * Multiplex several virtual IPIs over a single HW IPI.
> >> > + *
> >> > + * Copyright (c) 2022 Ventana Micro Systems Inc.
> >> > + */
> >> > +
> >> > +#define pr_fmt(fmt) "ipi-mux: " fmt
> >> > +#include <linux/cpu.h>
> >> > +#include <linux/init.h>
> >> > +#include <linux/irq.h>
> >> > +#include <linux/irqchip.h>
> >> > +#include <linux/irqchip/chained_irq.h>
> >> > +#include <linux/irqdomain.h>
> >> > +#include <linux/smp.h>
> >> > +
> >> > +static unsigned int ipi_mux_nr;
> >> > +static unsigned int ipi_mux_parent_virq;
> >> > +static struct irq_domain *ipi_mux_domain;
> >> > +static const struct  ipi_mux_ops *ipi_mux_ops;
> >> > +static DEFINE_PER_CPU(unsigned long, ipi_mux_bits);
> >> > +
> >> > +static void ipi_mux_send_mask(struct irq_data *d, const struct cpumask *mask)
> >> > +{
> >> > +     int cpu;
> >> > +
> >> > +     /* Barrier before doing atomic bit update to IPI bits */
> >> > +     smp_mb__before_atomic();
> >> > +
> >> > +     for_each_cpu(cpu, mask)
> >> > +             set_bit(d->hwirq, per_cpu_ptr(&ipi_mux_bits, cpu));
> >> > +
> >> > +     /* Barrier after doing atomic bit update to IPI bits */
> >> > +     smp_mb__after_atomic();
> >> > +
> >> > +     /* Trigger the parent IPI */
> >> > +     ipi_mux_ops->ipi_mux_send(ipi_mux_parent_virq, mask);
> >> > +}
> >> > +
> >> > +static const struct irq_chip ipi_mux_chip = {
> >> > +     .name           = "IPI Mux",
> >> > +     .ipi_send_mask  = ipi_mux_send_mask,
> >>
> >> I've given this a bit more though, and I came to the conclusion that
> >> we really should have the full masking semantics here, even if Linux
> >> currently doesn't really use it.
> >>
> >> It makes the handling a bit more complex, and unmasking a pending IPI
> >> must be handled gracefully, but we already have implemented most of
> >> that code in the irq-apple-aic driver.
> >>
> >> And if we go down this road, such a driver should be very easy to move
> >> over this infrastructure, making the change a lot more palatable.
> >
> > Sounds good.
> >
> > I will send v8 of ipi-mux which can be easily adapted for irq-apple-aic
> > driver
> > and it will have masking semantics as well.
>
> Thanks. No hurry though, as I'm closing the queue for 5.20 (fixes
> only until -rc1).

Okay, I will send after 5.20-rc1.

Regards,
Anup

>
> Thanks,
>
>          M.
> --
> Jazz is not dead. It just smells funny...