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Date:   Thu, 21 Jul 2022 13:09:23 +0100
From:   Marc Zyngier <maz@kernel.org>
To:     Anup Patel <anup@brainfault.org>
Cc:     Anup Patel <apatel@ventanamicro.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Atish Patra <atishp@atishpatra.org>,
        Alistair Francis <Alistair.Francis@wdc.com>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        "linux-kernel@vger.kernel.org List" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v7 3/7] genirq: Add mechanism to multiplex a single HW IPI
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 <8735euzq60.wl-maz@kernel.org>
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On 2022-07-21 12:44, Anup Patel wrote:
> On Thu, Jul 21, 2022 at 4:30 PM Marc Zyngier <maz@kernel.org> wrote:
>> 
>> On Wed, 20 Jul 2022 16:23:44 +0100,
>> Anup Patel <apatel@ventanamicro.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@ventanamicro.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).

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...