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

[Anup Patel <anup@brainfault.org>]

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,
Anup

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