linux-kernel - Re: [PATCH v14 3/8] genirq: Add mechanism to multiplex a single HW IPI

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Message-ID: <87v8mvqbvq.ffs@tglx>
Date:   Thu, 01 Dec 2022 18:20:25 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Anup Patel <apatel@...tanamicro.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Marc Zyngier <maz@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Atish Patra <atishp@...shpatra.org>,
        Alistair Francis <Alistair.Francis@....com>,
        Anup Patel <anup@...infault.org>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Anup Patel <apatel@...tanamicro.com>
Subject: Re: [PATCH v14 3/8] genirq: Add mechanism to multiplex a single HW IPI

On Thu, Dec 01 2022 at 18:31, Anup Patel 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

s/we//

> mechanism which help us create multiple virtual IPIs using a single
> HW IPI. This generic IPI multiplexing is inspired from the Apple AIC

s/from/by/

> irqchip driver and it is shared by various RISC-V irqchip drivers.

Sure, but now we have two copies of this. One in the Apple AIC and one
here. The obvious thing to do is:

   1) Provide generic infrastructure

   2) Convert AIC to use it

   3) Add RISCV users

No?

> +static void ipi_mux_mask(struct irq_data *d)
> +{
> +	struct ipi_mux_cpu *icpu = this_cpu_ptr(ipi_mux_pcpu);
> +
> +	atomic_andnot(BIT(irqd_to_hwirq(d)), &icpu->enable);
> +}
> +
> +static void ipi_mux_unmask(struct irq_data *d)
> +{
> +	u32 ibit = BIT(irqd_to_hwirq(d));
> +	struct ipi_mux_cpu *icpu = this_cpu_ptr(ipi_mux_pcpu);

The AIC code got the variable ordering correct ...

https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#variable-declarations

> +	atomic_or(ibit, &icpu->enable);
> +
> +	/*
> +	 * The atomic_or() above must complete before the atomic_read()
> +	 * below to avoid racing ipi_mux_send_mask().
> +	 */
> +	smp_mb__after_atomic();
> +
> +	/* If a pending IPI was unmasked, raise a parent IPI immediately. */
> +	if (atomic_read(&icpu->bits) & ibit)
> +		ipi_mux_send(smp_processor_id());
> +}
> +
> +static void ipi_mux_send_mask(struct irq_data *d, const struct cpumask *mask)
> +{
> +	u32 ibit = BIT(irqd_to_hwirq(d));
> +	struct ipi_mux_cpu *icpu = this_cpu_ptr(ipi_mux_pcpu);
> +	unsigned long pending;
> +	int cpu;
> +
> +	for_each_cpu(cpu, mask) {
> +		icpu = per_cpu_ptr(ipi_mux_pcpu, cpu);
> +		pending = atomic_fetch_or_release(ibit, &icpu->bits);
> +
> +		/*
> +		 * The atomic_fetch_or_release() above must complete
> +		 * before the atomic_read() below to avoid racing with
> +		 * ipi_mux_unmask().
> +		 */
> +		smp_mb__after_atomic();
> +
> +		/*
> +		 * The flag writes must complete before the physical IPI is
> +		 * issued to another CPU. This is implied by the control
> +		 * dependency on the result of atomic_read() below, which is
> +		 * itself already ordered after the vIPI flag write.
> +		 */
> +		if (!(pending & ibit) && (atomic_read(&icpu->enable) & ibit))
> +			ipi_mux_send(cpu);
> +	}
> +}
> +
> +static const struct irq_chip ipi_mux_chip = {
> +	.name		= "IPI Mux",
> +	.irq_mask	= ipi_mux_mask,
> +	.irq_unmask	= ipi_mux_unmask,
> +	.ipi_send_mask	= ipi_mux_send_mask,
> +};
> +
> +static int ipi_mux_domain_alloc(struct irq_domain *d, unsigned int virq,
> +				unsigned int nr_irqs, void *arg)
> +{
> +	int i;
> +
> +	for (i = 0; i < nr_irqs; i++) {
> +		irq_set_percpu_devid(virq + i);
> +		irq_domain_set_info(d, virq + i, i, &ipi_mux_chip, NULL,
> +				    handle_percpu_devid_irq, NULL, NULL);
> +	}
> +
> +	return 0;
> +}
> +
> +static const struct irq_domain_ops ipi_mux_domain_ops = {
> +	.alloc		= ipi_mux_domain_alloc,
> +	.free		= irq_domain_free_irqs_top,
> +};
> +
> +/**
> + * ipi_mux_process - Process multiplexed virtual IPIs
> + */
> +void ipi_mux_process(void)
> +{
> +	struct ipi_mux_cpu *icpu = this_cpu_ptr(ipi_mux_pcpu);
> +	irq_hw_number_t hwirq;
> +	unsigned long ipis;
> +	unsigned int en;
> +
> +	/*
> +	 * Reading enable mask does not need to be ordered as long as
> +	 * this function called from interrupt handler because only
> +	 * the CPU itself can change it's own enable mask.
> +	 */
> +	en = atomic_read(&icpu->enable);
> +
> +	/*
> +	 * Clear the IPIs we are about to handle. This pairs with the
> +	 * atomic_fetch_or_release() in ipi_mux_send_mask().

The comments in the AIC code where you copied from are definitely
better...

Thanks,

        tglx