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Date:   Tue, 4 Sep 2018 11:57:55 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Anup Patel <anup@brainfault.org>
Cc:     Palmer Dabbelt <palmer@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jason Cooper <jason@lakedaemon.net>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Atish Patra <atish.patra@wdc.com>,
        Christoph Hellwig <hch@infradead.org>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        Palmer Dabbelt <palmer@dabbelt.com>
Subject: Re: [RFC PATCH 4/5] irqchip: RISC-V Local Interrupt Controller Driver
Message-ID: <20180904185755.GD25119@infradead.org>
References: <20180904124514.6290-1-anup@brainfault.org>
 <20180904124514.6290-5-anup@brainfault.org>
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On Tue, Sep 04, 2018 at 06:15:13PM +0530, Anup Patel wrote:
> Few advantages of this new driver over previous one are:
> 1. It registers all local interrupts as per-CPU interrupts

Please explain why this is an advantage.

> 2. We can develop drivers for devices with per-CPU local interrupts
> without changing arch code or this driver

Please explain why this is a good thing and not just a workaround for
the fact that some moron decided they need non-standard interrupts
and should only be done as a last resort.

> 3. It allows local interrupt controller DT node under each CPU DT node
> as well as single system-wide DT node for local interrupt controller.

Please explain why this is can't happen right now.

Downsides are that it is a heck lot more code and introduces indirect
calls in the interrupt fast path.

So for now a clear NAK.

> The RISC-V INTC driver is compliant with RISC-V Hart-Level Interrupt
> Controller DT bindings located at:
> Documentation/devicetree/bindings/interrupt-controller/riscv,cpu-intc.txt
> 
> Signed-off-by: Anup Patel <anup@brainfault.org>
> Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>
> ---
>  arch/riscv/include/asm/irq.h      |  15 ++-
>  arch/riscv/kernel/irq.c           |  59 +----------
>  drivers/irqchip/Kconfig           |  15 ++-
>  drivers/irqchip/Makefile          |   1 +
>  drivers/irqchip/irq-riscv-intc.c  | 164 ++++++++++++++++++++++++++++++
>  drivers/irqchip/irq-sifive-plic.c |  21 +++-
>  include/linux/cpuhotplug.h        |   1 +
>  7 files changed, 214 insertions(+), 62 deletions(-)
>  create mode 100644 drivers/irqchip/irq-riscv-intc.c
> 
> diff --git a/arch/riscv/include/asm/irq.h b/arch/riscv/include/asm/irq.h
> index 93eb75eac4ff..fe503d71876a 100644
> --- a/arch/riscv/include/asm/irq.h
> +++ b/arch/riscv/include/asm/irq.h
> @@ -15,7 +15,20 @@
>  #ifndef _ASM_RISCV_IRQ_H
>  #define _ASM_RISCV_IRQ_H
>  
> -#define NR_IRQS         0
> +/*
> + * Possible interrupt causes:
> + */
> +#define INTERRUPT_CAUSE_SOFTWARE    1
> +#define INTERRUPT_CAUSE_TIMER       5
> +#define INTERRUPT_CAUSE_EXTERNAL    9
> +
> +/*
> + * The high order bit of the trap cause register is always set for
> + * interrupts, which allows us to differentiate them from exceptions
> + * quickly.  The INTERRUPT_CAUSE_* macros don't contain that bit, so we
> + * need to mask it off.
> + */
> +#define INTERRUPT_CAUSE_FLAG	(1UL << (__riscv_xlen - 1))
>  
>  void riscv_timer_interrupt(void);
>  
> diff --git a/arch/riscv/kernel/irq.c b/arch/riscv/kernel/irq.c
> index c51c9b402e87..46a311e5f0f6 100644
> --- a/arch/riscv/kernel/irq.c
> +++ b/arch/riscv/kernel/irq.c
> @@ -7,69 +7,16 @@
>  
>  #include <linux/interrupt.h>
>  #include <linux/irqchip.h>
> -#include <linux/irqdomain.h>
> -
> -#include <asm/sbi.h>
> -
> -/*
> - * Possible interrupt causes:
> - */
> -#define INTERRUPT_CAUSE_SOFTWARE    1
> -#define INTERRUPT_CAUSE_TIMER       5
> -#define INTERRUPT_CAUSE_EXTERNAL    9
> -
> -/*
> - * The high order bit of the trap cause register is always set for
> - * interrupts, which allows us to differentiate them from exceptions
> - * quickly.  The INTERRUPT_CAUSE_* macros don't contain that bit, so we
> - * need to mask it off.
> - */
> -#define INTERRUPT_CAUSE_FLAG	(1UL << (__riscv_xlen - 1))
>  
>  asmlinkage void __irq_entry do_IRQ(struct pt_regs *regs)
>  {
> -	struct pt_regs *old_regs;
> -
> -	switch (regs->scause & ~INTERRUPT_CAUSE_FLAG) {
> -	case INTERRUPT_CAUSE_TIMER:
> -		old_regs = set_irq_regs(regs);
> -		irq_enter();
> -		riscv_timer_interrupt();
> -		irq_exit();
> -		set_irq_regs(old_regs);
> -		break;
> -#ifdef CONFIG_SMP
> -	case INTERRUPT_CAUSE_SOFTWARE:
> -		/*
> -		 * We only use software interrupts to pass IPIs, so if a non-SMP
> -		 * system gets one, then we don't know what to do.
> -		 */
> -		handle_IPI(regs);
> -		break;
> -#endif
> -	case INTERRUPT_CAUSE_EXTERNAL:
> -		old_regs = set_irq_regs(regs);
> -		irq_enter();
> +	if (handle_arch_irq)
>  		handle_arch_irq(regs);
> -		irq_exit();
> -		set_irq_regs(old_regs);
> -		break;
> -	default:
> -		panic("unexpected interrupt cause");
> -	}
> -}
> -
> -#ifdef CONFIG_SMP
> -static void smp_ipi_trigger_sbi(const struct cpumask *to_whom)
> -{
> -	sbi_send_ipi(cpumask_bits(to_whom));
>  }
> -#endif
>  
>  void __init init_IRQ(void)
>  {
>  	irqchip_init();
> -#ifdef CONFIG_SMP
> -	set_smp_ipi_trigger(smp_ipi_trigger_sbi);
> -#endif
> +	if (!handle_arch_irq)
> +		panic("No interrupt controller found.");
>  }
> diff --git a/drivers/irqchip/Kconfig b/drivers/irqchip/Kconfig
> index 383e7b70221d..885e182586f4 100644
> --- a/drivers/irqchip/Kconfig
> +++ b/drivers/irqchip/Kconfig
> @@ -371,7 +371,18 @@ config QCOM_PDC
>  	  Power Domain Controller driver to manage and configure wakeup
>  	  IRQs for Qualcomm Technologies Inc (QTI) mobile chips.
>  
> -endmenu
> +config RISCV_INTC
> +	bool "RISC-V Interrupt Controller"
> +	depends on RISCV
> +	default y
> +	help
> +	   This enables support for the local interrupt controller found in
> +	   standard RISC-V systems.  The local interrupt controller handles
> +	   timer interrupts, software interrupts, and hardware interrupts.
> +	   Without a local interrupt controller the system will be unable to
> +	   handle any interrupts, including those passed via the PLIC.
> +
> +	   If you don't know what to do here, say Y.
>  
>  config SIFIVE_PLIC
>  	bool "SiFive Platform-Level Interrupt Controller"
> @@ -384,3 +395,5 @@ config SIFIVE_PLIC
>  	   interrupt sources are subordinate to the PLIC.
>  
>  	   If you don't know what to do here, say Y.
> +
> +endmenu
> diff --git a/drivers/irqchip/Makefile b/drivers/irqchip/Makefile
> index fbd1ec8070ef..e638ee5c4452 100644
> --- a/drivers/irqchip/Makefile
> +++ b/drivers/irqchip/Makefile
> @@ -87,4 +87,5 @@ obj-$(CONFIG_MESON_IRQ_GPIO)		+= irq-meson-gpio.o
>  obj-$(CONFIG_GOLDFISH_PIC) 		+= irq-goldfish-pic.o
>  obj-$(CONFIG_NDS32)			+= irq-ativic32.o
>  obj-$(CONFIG_QCOM_PDC)			+= qcom-pdc.o
> +obj-$(CONFIG_RISCV_INTC)		+= irq-riscv-intc.o
>  obj-$(CONFIG_SIFIVE_PLIC)		+= irq-sifive-plic.o
> diff --git a/drivers/irqchip/irq-riscv-intc.c b/drivers/irqchip/irq-riscv-intc.c
> new file mode 100644
> index 000000000000..c80502a1e12b
> --- /dev/null
> +++ b/drivers/irqchip/irq-riscv-intc.c
> @@ -0,0 +1,164 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2012 Regents of the University of California
> + * Copyright (C) 2017-2018 SiFive
> + * Copyright (C) 2018 Anup Patel
> + */
> +
> +#define pr_fmt(fmt) "riscv-intc: " fmt
> +#include <linux/atomic.h>
> +#include <linux/bits.h>
> +#include <linux/cpu.h>
> +#include <linux/irq.h>
> +#include <linux/irqchip.h>
> +#include <linux/irqdomain.h>
> +#include <linux/interrupt.h>
> +#include <linux/of.h>
> +#include <linux/smp.h>
> +#include <asm/sbi.h>
> +
> +static struct irq_domain *intc_domain;
> +static atomic_t intc_init = ATOMIC_INIT(0);
> +
> +static void riscv_intc_irq(struct pt_regs *regs)
> +{
> +	struct pt_regs *old_regs;
> +	unsigned long cause = regs->scause & ~INTERRUPT_CAUSE_FLAG;
> +
> +	if (unlikely(cause >= BITS_PER_LONG))
> +		panic("unexpected interrupt cause");
> +
> +	switch (cause) {
> +	case INTERRUPT_CAUSE_TIMER:
> +		old_regs = set_irq_regs(regs);
> +		irq_enter();
> +		riscv_timer_interrupt();
> +		irq_exit();
> +		set_irq_regs(old_regs);
> +		break;
> +#ifdef CONFIG_SMP
> +	case INTERRUPT_CAUSE_SOFTWARE:
> +		/*
> +		 * We only use software interrupts to pass IPIs, so if a non-SMP
> +		 * system gets one, then we don't know what to do.
> +		 */
> +		handle_IPI(regs);
> +		break;
> +#endif
> +	default:
> +		handle_domain_irq(intc_domain, cause, regs);
> +		break;
> +	}
> +}
> +
> +/*
> + * On RISC-V systems local interrupts are masked or unmasked by writing the SIE
> + * (Supervisor Interrupt Enable) CSR.  As CSRs can only be written on the local
> + * hart, these functions can only be called on the hart that corresponds to the
> + * IRQ chip.  They are only called internally to this module, so they BUG_ON if
> + * this condition is violated rather than attempting to handle the error by
> + * forwarding to the target hart, as that's already expected to have been done.
> + */
> +static void riscv_intc_irq_mask(struct irq_data *d)
> +{
> +	csr_clear(sie, 1 << (long)d->hwirq);
> +}
> +
> +static void riscv_intc_irq_unmask(struct irq_data *d)
> +{
> +	csr_set(sie, 1 << (long)d->hwirq);
> +}
> +
> +#ifdef CONFIG_SMP
> +static void riscv_intc_ipi_trigger(const struct cpumask *to_whom)
> +{
> +	sbi_send_ipi(cpumask_bits(to_whom));
> +}
> +
> +static int riscv_intc_cpu_starting(unsigned int cpu)
> +{
> +	csr_set(sie, 1 << INTERRUPT_CAUSE_SOFTWARE);
> +	return 0;
> +}
> +
> +static int riscv_intc_cpu_dying(unsigned int cpu)
> +{
> +	csr_clear(sie, 1 << INTERRUPT_CAUSE_SOFTWARE);
> +	return 0;
> +}
> +
> +static void riscv_intc_smp_init(void)
> +{
> +	csr_write(sie, 0);
> +	csr_write(sip, 0);
> +
> +	set_smp_ipi_trigger(riscv_intc_ipi_trigger);
> +
> +	cpuhp_setup_state(CPUHP_AP_IRQ_RISCV_STARTING,
> +			  "irqchip/riscv/intc:starting",
> +			  riscv_intc_cpu_starting,
> +			  riscv_intc_cpu_dying);
> +
> +}
> +#else
> +static void riscv_intc_smp_init(void)
> +{
> +	csr_write(sie, 0);
> +	csr_write(sip, 0);
> +}
> +#endif
> +
> +static struct irq_chip riscv_intc_chip = {
> +	.name = "RISC-V INTC",
> +	.irq_mask = riscv_intc_irq_mask,
> +	.irq_unmask = riscv_intc_irq_unmask,
> +};
> +
> +static int riscv_intc_domain_map(struct irq_domain *d, unsigned int irq,
> +				 irq_hw_number_t hwirq)
> +{
> +	irq_set_percpu_devid(irq);
> +	irq_domain_set_info(d, irq, hwirq, &riscv_intc_chip, d->host_data,
> +			    handle_percpu_devid_irq, NULL, NULL);
> +	irq_set_status_flags(irq, IRQ_NOAUTOEN);
> +
> +	return 0;
> +}
> +
> +static const struct irq_domain_ops riscv_intc_domain_ops = {
> +	.map	= riscv_intc_domain_map,
> +	.xlate	= irq_domain_xlate_onecell,
> +};
> +
> +static int __init riscv_intc_init(struct device_node *node,
> +				  struct device_node *parent)
> +{
> +	/*
> +	 * RISC-V device trees can have one INTC DT node under
> +	 * each CPU DT node so INTC init function will be called
> +	 * once for each INTC DT node. We only need to do INTC
> +	 * init once for boot CPU so we use atomic counter to
> +	 * achieve this.
> +	 */
> +	if (atomic_inc_return(&intc_init) > 1)
> +		return 0;
> +
> +	intc_domain = irq_domain_add_linear(node, BITS_PER_LONG,
> +					    &riscv_intc_domain_ops, NULL);
> +	if (!intc_domain)
> +		goto error_add_linear;
> +
> +	set_handle_irq(&riscv_intc_irq);
> +
> +	riscv_intc_smp_init();
> +
> +	pr_info("%lu local interrupts mapped\n", (long)BITS_PER_LONG);
> +
> +	return 0;
> +
> +error_add_linear:
> +	pr_warn("unable to add IRQ domain\n");
> +	return -ENXIO;
> +}
> +
> +IRQCHIP_DECLARE(riscv, "riscv,cpu-intc", riscv_intc_init);
> diff --git a/drivers/irqchip/irq-sifive-plic.c b/drivers/irqchip/irq-sifive-plic.c
> index 532e9d68c704..ab9614d5a2b4 100644
> --- a/drivers/irqchip/irq-sifive-plic.c
> +++ b/drivers/irqchip/irq-sifive-plic.c
> @@ -8,6 +8,7 @@
>  #include <linux/io.h>
>  #include <linux/irq.h>
>  #include <linux/irqchip.h>
> +#include <linux/irqchip/chained_irq.h>
>  #include <linux/irqdomain.h>
>  #include <linux/module.h>
>  #include <linux/of.h>
> @@ -146,14 +147,17 @@ static struct irq_domain *plic_irqdomain;
>   * that source ID back to the same claim register.  This automatically enables
>   * and disables the interrupt, so there's nothing else to do.
>   */
> -static void plic_handle_irq(struct pt_regs *regs)
> +static void plic_handle_irq(struct irq_desc *desc)
>  {
>  	struct plic_handler *handler = this_cpu_ptr(&plic_handlers);
> +	struct irq_chip *chip = irq_desc_get_chip(desc);
>  	void __iomem *claim = plic_hart_offset(handler->ctxid) + CONTEXT_CLAIM;
>  	irq_hw_number_t hwirq;
>  
>  	WARN_ON_ONCE(!handler->present);
>  
> +	chained_irq_enter(chip, desc);
> +
>  	csr_clear(sie, SIE_SEIE);
>  	while ((hwirq = readl(claim))) {
>  		int irq = irq_find_mapping(plic_irqdomain, hwirq);
> @@ -166,6 +170,8 @@ static void plic_handle_irq(struct pt_regs *regs)
>  		writel(hwirq, claim);
>  	}
>  	csr_set(sie, SIE_SEIE);
> +
> +	chained_irq_exit(chip, desc);
>  }
>  
>  /*
> @@ -183,7 +189,7 @@ static int plic_find_hart_id(struct device_node *node)
>  }
>  
>  static int __init plic_init(struct device_node *node,
> -		struct device_node *parent)
> +			    struct device_node *parent)
>  {
>  	int error = 0, nr_handlers, nr_mapped = 0, i;
>  	u32 nr_irqs;
> @@ -218,7 +224,7 @@ static int __init plic_init(struct device_node *node,
>  		struct of_phandle_args parent;
>  		struct plic_handler *handler;
>  		irq_hw_number_t hwirq;
> -		int cpu;
> +		int cpu, parent_irq;
>  
>  		if (of_irq_parse_one(node, i, &parent)) {
>  			pr_err("failed to parse parent for context %d.\n", i);
> @@ -229,6 +235,13 @@ static int __init plic_init(struct device_node *node,
>  		if (parent.args[0] == -1)
>  			continue;
>  
> +		if (irq_find_host(parent.np)) {
> +			parent_irq = irq_of_parse_and_map(node, i);
> +			if (parent_irq)
> +				irq_set_chained_handler(parent_irq,
> +							plic_handle_irq);
> +		}
> +
>  		cpu = plic_find_hart_id(parent.np);
>  		if (cpu < 0) {
>  			pr_warn("failed to parse hart ID for context %d.\n", i);
> @@ -248,7 +261,7 @@ static int __init plic_init(struct device_node *node,
>  
>  	pr_info("mapped %d interrupts to %d (out of %d) handlers.\n",
>  		nr_irqs, nr_mapped, nr_handlers);
> -	set_handle_irq(plic_handle_irq);
> +
>  	return 0;
>  
>  out_iounmap:
> diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
> index caf40ad0bbc6..ca7268a38cef 100644
> --- a/include/linux/cpuhotplug.h
> +++ b/include/linux/cpuhotplug.h
> @@ -100,6 +100,7 @@ enum cpuhp_state {
>  	CPUHP_AP_IRQ_ARMADA_XP_STARTING,
>  	CPUHP_AP_IRQ_BCM2836_STARTING,
>  	CPUHP_AP_IRQ_MIPS_GIC_STARTING,
> +	CPUHP_AP_IRQ_RISCV_STARTING,
>  	CPUHP_AP_ARM_MVEBU_COHERENCY,
>  	CPUHP_AP_PERF_X86_AMD_UNCORE_STARTING,
>  	CPUHP_AP_PERF_X86_STARTING,
> -- 
> 2.17.1
> 
---end quoted text---