linux-kernel - Re: [PATCH v13 4/7] RISC-V: Treat IPIs as normal Linux IRQs

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Date:   Wed, 30 Nov 2022 18:02:29 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Anup Patel <anup@...infault.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 4/7] RISC-V: Treat IPIs as normal Linux IRQs

On Wed, 30 Nov 2022 17:14:09 +0000,
Anup Patel <anup@...infault.org> wrote:
> 
> On Wed, Nov 30, 2022 at 9:48 PM Marc Zyngier <maz@...nel.org> wrote:
> >
> > On Tue, 29 Nov 2022 14:24:46 +0000,
> > Anup Patel <apatel@...tanamicro.com> wrote:
> > >
> > > Currently, the RISC-V kernel provides arch specific hooks (i.e.
> > > struct riscv_ipi_ops) to register IPI handling methods. The stats
> > > gathering of IPIs is also arch specific in the RISC-V kernel.
> > >
> > > Other architectures (such as ARM, ARM64, and MIPS) have moved away
> > > from custom arch specific IPI handling methods. Currently, these
> > > architectures have Linux irqchip drivers providing a range of Linux
> > > IRQ numbers to be used as IPIs and IPI triggering is done using
> > > generic IPI APIs. This approach allows architectures to treat IPIs
> > > as normal Linux IRQs and IPI stats gathering is done by the generic
> > > Linux IRQ subsystem.
> > >
> > > We extend the RISC-V IPI handling as-per above approach so that arch
> > > specific IPI handling methods (struct riscv_ipi_ops) can be removed
> > > and the IPI handling is done through the Linux IRQ subsystem.
> > >
> > > Signed-off-by: Anup Patel <apatel@...tanamicro.com>
> > > ---
> > >  arch/riscv/Kconfig                |   2 +
> > >  arch/riscv/include/asm/sbi.h      |  10 +-
> > >  arch/riscv/include/asm/smp.h      |  35 ++++---
> > >  arch/riscv/kernel/Makefile        |   1 +
> > >  arch/riscv/kernel/cpu-hotplug.c   |   3 +-
> > >  arch/riscv/kernel/irq.c           |   3 +-
> > >  arch/riscv/kernel/sbi-ipi.c       |  81 ++++++++++++++++
> > >  arch/riscv/kernel/sbi.c           | 106 +++-----------------
> > >  arch/riscv/kernel/smp.c           | 155 +++++++++++++++---------------
> > >  arch/riscv/kernel/smpboot.c       |   5 +-
> > >  drivers/clocksource/timer-clint.c |  65 ++++++++++---
> > >  drivers/irqchip/Kconfig           |   1 +
> > >  drivers/irqchip/irq-riscv-intc.c  |  55 +++++------
> > >  13 files changed, 287 insertions(+), 235 deletions(-)
> > >  create mode 100644 arch/riscv/kernel/sbi-ipi.c
> > >
> >
> > [...]
> >
> > > diff --git a/arch/riscv/kernel/sbi-ipi.c b/arch/riscv/kernel/sbi-ipi.c
> > > new file mode 100644
> > > index 000000000000..6466706b03a7
> > > --- /dev/null
> > > +++ b/arch/riscv/kernel/sbi-ipi.c
> > > @@ -0,0 +1,81 @@
> > > +// SPDX-License-Identifier: GPL-2.0-only
> > > +/*
> > > + * Multiplex several IPIs over a single HW IPI.
> > > + *
> > > + * Copyright (c) 2022 Ventana Micro Systems Inc.
> > > + */
> > > +
> > > +#define pr_fmt(fmt) "riscv: " fmt
> > > +#include <linux/cpu.h>
> > > +#include <linux/init.h>
> > > +#include <linux/irq.h>
> > > +#include <linux/irqdomain.h>
> > > +#include <linux/percpu.h>
> > > +#include <asm/sbi.h>
> > > +
> > > +static int sbi_ipi_virq;
> > > +static DEFINE_PER_CPU_READ_MOSTLY(int, sbi_ipi_dummy_dev);
> > > +
> > > +static irqreturn_t sbi_ipi_handle(int irq, void *dev_id)
> > > +{
> > > +     csr_clear(CSR_IP, IE_SIE);
> > > +     ipi_mux_process();
> > > +     return IRQ_HANDLED;
> >
> > Urgh... I really wish I hadn't seen this. This requires a chained
> > handler. You had it before, and yet you dropped it. Why?
> >
> > Either you call ipi_mux_process() from your root interrupt controller,
> > or you implement a chained handler. But not this.
> >
> > Same thing about the clint stuff.
> 
> We had chained handler all along but there is problem (which
> was pointed to us) in using chained handler because the parent
> RISC-V INTC irqchip driver does not have irq_eoi() so the
> chained_irq_enter() and chained_irq_exit() will do the interrupt
> mask/unmask dance which seems unnecessary.
> 
> Is there a better way to avoid the interrupt mask/unmask dance ?

Well, you could have an IPI-specific irqchip, with an empty EOI
callback. Or something. But not *that*.

And next time you change something of that importance, add it to your
change log.

	M.

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