Date:   Wed, 26 Jan 2022 21:08:49 +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>,
        Rob Herring <robh+dt@...nel.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 2/6] irqchip/riscv-intc: Set intc domain as the default host

On Wed, Jan 26, 2022 at 4:17 PM Marc Zyngier <maz@...nel.org> wrote:
>
> On Wed, 26 Jan 2022 10:12:25 +0000,
> Anup Patel <apatel@...tanamicro.com> wrote:
> >
> > On Wed, Jan 26, 2022 at 2:31 PM Marc Zyngier <maz@...nel.org> wrote:
> > >
> > > On Wed, 26 Jan 2022 03:16:55 +0000,
> > > Anup Patel <anup@...infault.org> wrote:
> > > >
> > > > On Tue, Jan 25, 2022 at 11:47 PM Marc Zyngier <maz@...nel.org> wrote:
> > > > >
> > > > > On Tue, 25 Jan 2022 05:42:13 +0000,
> > > > > Anup Patel <apatel@...tanamicro.com> wrote:
> > > > > >
> > > > > > We have quite a few RISC-V drivers (such as RISC-V SBI IPI driver,
> > > > > > RISC-V timer driver, RISC-V PMU driver, etc) which do not have a
> > > > > > dedicated DT/ACPI fwnode. This patch makes intc domain as the default
> > > > > > host so that these drivers can directly create local interrupt mapping
> > > > > > using standardized local interrupt numbers
> > > > > >
> > > > > > Signed-off-by: Anup Patel <apatel@...tanamicro.com>
> > > > > > ---
> > > > > >  drivers/clocksource/timer-riscv.c | 17 +----------------
> > > > > >  drivers/irqchip/irq-riscv-intc.c  |  9 +++++++++
> > > > > >  2 files changed, 10 insertions(+), 16 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/clocksource/timer-riscv.c b/drivers/clocksource/timer-riscv.c
> > > > > > index 1767f8bf2013..dd6916ae6365 100644
> > > > > > --- a/drivers/clocksource/timer-riscv.c
> > > > > > +++ b/drivers/clocksource/timer-riscv.c
> > > > > > @@ -102,8 +102,6 @@ static irqreturn_t riscv_timer_interrupt(int irq, void *dev_id)
> > > > > >  static int __init riscv_timer_init_dt(struct device_node *n)
> > > > > >  {
> > > > > >       int cpuid, hartid, error;
> > > > > > -     struct device_node *child;
> > > > > > -     struct irq_domain *domain;
> > > > > >
> > > > > >       hartid = riscv_of_processor_hartid(n);
> > > > > >       if (hartid < 0) {
> > > > > > @@ -121,20 +119,7 @@ static int __init riscv_timer_init_dt(struct device_node *n)
> > > > > >       if (cpuid != smp_processor_id())
> > > > > >               return 0;
> > > > > >
> > > > > > -     domain = NULL;
> > > > > > -     child = of_get_compatible_child(n, "riscv,cpu-intc");
> > > > > > -     if (!child) {
> > > > > > -             pr_err("Failed to find INTC node [%pOF]\n", n);
> > > > > > -             return -ENODEV;
> > > > > > -     }
> > > > > > -     domain = irq_find_host(child);
> > > > > > -     of_node_put(child);
> > > > > > -     if (!domain) {
> > > > > > -             pr_err("Failed to find IRQ domain for node [%pOF]\n", n);
> > > > > > -             return -ENODEV;
> > > > > > -     }
> > > > > > -
> > > > > > -     riscv_clock_event_irq = irq_create_mapping(domain, RV_IRQ_TIMER);
> > > > > > +     riscv_clock_event_irq = irq_create_mapping(NULL, RV_IRQ_TIMER);
> > > > > >       if (!riscv_clock_event_irq) {
> > > > > >               pr_err("Failed to map timer interrupt for node [%pOF]\n", n);
> > > > > >               return -ENODEV;
> > > > > > diff --git a/drivers/irqchip/irq-riscv-intc.c b/drivers/irqchip/irq-riscv-intc.c
> > > > > > index b65bd8878d4f..9f0a7a8a5c4d 100644
> > > > > > --- a/drivers/irqchip/irq-riscv-intc.c
> > > > > > +++ b/drivers/irqchip/irq-riscv-intc.c
> > > > > > @@ -125,6 +125,15 @@ static int __init riscv_intc_init(struct device_node *node,
> > > > > >               return rc;
> > > > > >       }
> > > > > >
> > > > > > +     /*
> > > > > > +      * Make INTC as the default domain which will allow drivers
> > > > > > +      * not having dedicated DT/ACPI fwnode (such as RISC-V SBI IPI
> > > > > > +      * driver, RISC-V timer driver, RISC-V PMU driver, etc) can
> > > > > > +      * directly create local interrupt mapping using standardized
> > > > > > +      * local interrupt numbers.
> > > > > > +      */
> > > > > > +     irq_set_default_host(intc_domain);
> > > > >
> > > > > No, please. This really is a bad idea. This sort of catch-all have
> > > > > constantly proven to be a nuisance, because they discard all the
> > > > > topology information. Eventually, you realise that you need to know
> > > > > where this is coming from, but it really is too late.
> > > > >
> > > > > I'd rather you *synthesise* a fwnode (like ACPI does) rather then do
> > > > > this.
> > > >
> > > > In absence of INTC as the default domain, currently we have various
> > > > drivers looking up INTC irq_domain from DT (or ACPI). This is quite a
> > > > bit of duplicate code across various drivers.
> > > >
> > > > How about having a irq_domain lookup routine (riscv_intc_find_irq_domain())
> > > > exported by the RISC-V INTC driver or arch/riscv ?
> > > > OR
> > > > Do you have an alternative suggestion ?
> > >
> > > But *why* don't you provide an interrupt controller node for DT? I
> > > really don't think that's outlandish to require.
> >
> > Historically, all RISC-V SBI related drivers never had any DT/ACPI
> > node because we can always query/discover the SBI functionality
> > at runtime.
> >
> > The mechanism to query/discover SBI IPI, Timer and PMU is
> > through SBI base functions. Also, local interrupts used by these
> > drivers are specified by the RISC-V specification. This means having
> > a DT/ACPI node for these drivers doesn't provide any info.
> >
> > We will be having KVM RISC-V AIA support in future which will not
> > have a DT/ACPI node as well because this can be discovered as a
> > CPU capability and the local interrupt to be used is specified by the
> > RISC-V hypervisor specification.
> >
> > >
> > > For ACPI, we already have an interface that allows a fwnode to be
> > > registered (acpi_set_irq_model) and interrupts mapped
> > > (acpi_register_gsi).
> >
> > The ACPI specification being proposed for RISC-V does not have
> > any details for SBI IPI, Timer, and PMU for the same reasons
> > mentioned above.
>
> Neither does it on the other architectures.
>
> And yet we are able to synthesise fwnodes and use the whole of the
> infrastructure as intended without having to resort to this crap that
> was only introduced to cope with 20 year old PPC board files.
>
> Only dead architectures are using irq_set_default_host().

Okay, I will drop the idea of using irq_set_default_host() in INTC driver.

>
> >
> > >
> > > You should already have all the required tools you need.
> >
> > Are you okay if arch/riscv exports riscv_intc_find_irq_domain() ?
> > OR
> > Maybe export riscv_intc_find_irq_domain() from INTC driver ?
>
> Neither. That's just papering over the core problem.
>
> Either you start creating fwnodes out of thin air, which is what we do
> for both x86 and arm64 when using ACPI, or you add support for SBI (or
> whatever new spec the RISC-V people come up with) as a provider of
> fwnodes.
>
> Anything else looks like a pretty bad regression.

Actually, SBI spec has been used for quite a few years in RISC-V now.
It can be compared with the ARM PSCI spec so it's not a HW description
format like DT or ACPI.

How about arch/riscv creating an exported riscv_intc_fwnode ?
(This riscv_intc_fwnode can be used by various drivers to obtain
the INTC irq_domain)

Regards,
Anup

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

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