Re: [PATCH V3 08/10] irqchip: Add LoongArch CPU interrupt controller support

[Marc Zyngier]

On Wed, 25 Aug 2021 07:11:50 +0100,
Huacai Chen <chenhuacai@xxxxxxxxxxx> wrote:
> 
> We are preparing to add new Loongson (based on LoongArch, not MIPS)

You keep saying "not MIPS", and yet all I see is a blind copy of the
MIPS code.

> support. This patch add LoongArch CPU interrupt controller support.
> 
> Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
> ---
>  drivers/irqchip/Kconfig             | 10 ++++
>  drivers/irqchip/Makefile            |  1 +
>  drivers/irqchip/irq-loongarch-cpu.c | 76 +++++++++++++++++++++++++++++
>  3 files changed, 87 insertions(+)
>  create mode 100644 drivers/irqchip/irq-loongarch-cpu.c
> 
> diff --git a/drivers/irqchip/Kconfig b/drivers/irqchip/Kconfig
> index 084bc4c2eebd..443c3a7a0cc1 100644
> --- a/drivers/irqchip/Kconfig
> +++ b/drivers/irqchip/Kconfig
> @@ -528,6 +528,16 @@ config EXYNOS_IRQ_COMBINER
>  	  Say yes here to add support for the IRQ combiner devices embedded
>  	  in Samsung Exynos chips.
>  
> +config IRQ_LOONGARCH_CPU
> +	bool
> +	select GENERIC_IRQ_CHIP
> +	select IRQ_DOMAIN
> +	select GENERIC_IRQ_EFFECTIVE_AFF_MASK
> +	help
> +	  Support for the LoongArch CPU Interrupt Controller. For details of
> +	  irq chip hierarchy on LoongArch platforms please read the document
> +	  Documentation/loongarch/irq-chip-model.rst.
> +
>  config LOONGSON_LIOINTC
>  	bool "Loongson Local I/O Interrupt Controller"
>  	depends on MACH_LOONGSON64
> diff --git a/drivers/irqchip/Makefile b/drivers/irqchip/Makefile
> index f88cbf36a9d2..4e34eebe180b 100644
> --- a/drivers/irqchip/Makefile
> +++ b/drivers/irqchip/Makefile
> @@ -105,6 +105,7 @@ obj-$(CONFIG_LS1X_IRQ)			+= irq-ls1x.o
>  obj-$(CONFIG_TI_SCI_INTR_IRQCHIP)	+= irq-ti-sci-intr.o
>  obj-$(CONFIG_TI_SCI_INTA_IRQCHIP)	+= irq-ti-sci-inta.o
>  obj-$(CONFIG_TI_PRUSS_INTC)		+= irq-pruss-intc.o
> +obj-$(CONFIG_IRQ_LOONGARCH_CPU)		+= irq-loongarch-cpu.o
>  obj-$(CONFIG_LOONGSON_LIOINTC)		+= irq-loongson-liointc.o
>  obj-$(CONFIG_LOONGSON_HTPIC)		+= irq-loongson-htpic.o
>  obj-$(CONFIG_LOONGSON_HTVEC)		+= irq-loongson-htvec.o
> diff --git a/drivers/irqchip/irq-loongarch-cpu.c b/drivers/irqchip/irq-loongarch-cpu.c
> new file mode 100644
> index 000000000000..8e9e8d39cb22
> --- /dev/null
> +++ b/drivers/irqchip/irq-loongarch-cpu.c
> @@ -0,0 +1,76 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2020-2021 Loongson Technology Corporation Limited
> + */
> +
> +#include <linux/init.h>
> +#include <linux/kernel.h>
> +#include <linux/interrupt.h>
> +#include <linux/irq.h>
> +#include <linux/irqchip.h>
> +#include <linux/irqdomain.h>
> +
> +#include <asm/loongarch.h>
> +#include <asm/setup.h>
> +
> +static struct irq_domain *irq_domain;
> +
> +static inline void enable_loongarch_irq(struct irq_data *d)

Why 'inline' given that it is used as a function pointer?

> +{
> +	set_csr_ecfg(ECFGF(d->hwirq));
> +}
> +
> +#define eoi_loongarch_irq enable_loongarch_irq

NAK. EOI and enable cannot be the same operation.

> +
> +static inline void disable_loongarch_irq(struct irq_data *d)
> +{
> +	clear_csr_ecfg(ECFGF(d->hwirq));
> +}
> +
> +#define ack_loongarch_irq disable_loongarch_irq

Same thing. Either you have different operations, or this only
supports mask/unmask.

> +
> +static struct irq_chip loongarch_cpu_irq_controller = {
> +	.name		= "LoongArch",
> +	.irq_ack	= ack_loongarch_irq,
> +	.irq_eoi	= eoi_loongarch_irq,
> +	.irq_enable	= enable_loongarch_irq,
> +	.irq_disable	= disable_loongarch_irq,
> +};
> +
> +asmlinkage void default_handle_irq(int irq)
> +{
> +	do_IRQ(irq_linear_revmap(irq_domain, irq));

This looks both wrong and short sighted:

- irq_linear_revmap() is now another name for irq_find_mapping().
  Which means it uses a RCU read critical section. If, as I expect,
  this is just a blind copy of the MIPS code, do_IRQ() will not do
  anything with respect to irq_enter()/irq_exit(), which will result
  in something pretty bad on the exit from idle path. Lockdep will
  probably shout at you pretty loudly.

- A single root interrupt controller is, in my modest experience,
  something that rarely happen. You will eventually have a variety of
  them, and you will have to join the other arches such as arm, arm64,
  riscv and csky that use CONFIG_GENERIC_IRQ_MULTI_HANDLER instead of
  following the existing MIPS model.

You can solve this by:

- Move over to CONFIG_GENERIC_IRQ_MULTI_HANDLER so that the interrupt
  controller can register itself with the core, rather than being
  defined at compile time.

- Drop the do_IRQ() madness. Perform whenever stuff you need to do in
  the arch code *before* calling into the interrupt controller code.

- Use generic_handle_irq() to call into the irq stack. It will handle
  all the irq_enter()/irq_exit() correctly. It will also avoid the
  silly double lookup of the irq_desc on interrupt handling.

> +}
> +
> +static int loongarch_cpu_intc_map(struct irq_domain *d, unsigned int irq,
> +			     irq_hw_number_t hwirq)
> +{
> +	struct irq_chip *chip;
> +
> +	irq_set_noprobe(irq);
> +	chip = &loongarch_cpu_irq_controller;
> +	set_vi_handler(EXCCODE_INT_START + hwirq, default_handle_irq);

What is that? Yet another MIPS legacy? Why does it have to be per
interrupt if it obviously apply to each and every root interrupt?

Given that 'vi' probably stands for "vectored interrupt", why isn't
that the irq_enable() code?

> +	irq_set_chip_and_handler(irq, chip, handle_percpu_irq);
> +
> +	return 0;
> +}
> +
> +static const struct irq_domain_ops loongarch_cpu_intc_irq_domain_ops = {
> +	.map = loongarch_cpu_intc_map,
> +	.xlate = irq_domain_xlate_onecell,
> +};
> +
> +int __init loongarch_cpu_irq_init(void)
> +{
> +	/* Mask interrupts. */
> +	clear_csr_ecfg(ECFG0_IM);
> +	clear_csr_estat(ESTATF_IP);
> +
> +	irq_domain = irq_domain_add_simple(NULL, EXCCODE_INT_NUM,
> +		     LOONGSON_CPU_IRQ_BASE, &loongarch_cpu_intc_irq_domain_ops, NULL);

NAK. You still obviously have some static partitioning of the
interrupt space, which is not acceptable for a new architecture.

> +
> +	if (!irq_domain)
> +		panic("Failed to add irqdomain for LoongArch CPU");
> +
> +	return 0;
> +}

I haven't seen much progress from the first version I reviewed. This
is still the same antiquated, broken MIPS code, only with a different
name.

	M.

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