[PATCH] ARM: Allow machine to specify it's own IRQ handlers at run-time

[Russell King - ARM Linux]

On Mon, Nov 22, 2010 at 04:26:50PM +0800, Eric Miao wrote:
> I wonder if it's still acceptable for a patch like below to have a dynamic IRQ
> handler at run-time. There is some changes to the irq_handler, esp. some
> SMP changes.
> 
> Magnus had some similar patches to have a run-time irq_handler, which is
> a bit more complicated, but I'm also very happy with.

I'd prefer a simple but efficient approach.  I am worried about
unnecessarily increasing the IRQ latency - and IRQ latency is something
that we should be concerned about.  It has the ability to make platforms
useless.

Eg, we know that SMC91x net interfaces are sensitive to IRQ latency, and
the same is true of serial ports with small FIFOs.

Any attempt to support multiple IRQ handlers is going to increase IRQ
handling latency - I don't think that's something which can be avoided,
and I'm wondering what effect this and the recent genirq changes is going
to have.  Has anyone been keeping an eye on IRQ handling latency?

Lastly, this patch is dependent on your machine class patches, which makes
it unsuitable as-is for mainline.

> commit d57e464929c9b2b50cda07e8683679583adf0fee
> Author: Eric Miao <eric.y.miao at gmail.com>
> Date:   Mon Nov 22 16:13:51 2010 +0800
> 
>     ARM: Allow machine to specify it's own IRQ handlers at run-time
> 
>     Normally different ARM platform has different way to decode the IRQ
>     hardware status and demultiplex to the corresponding IRQ handler.
>     This is highly optimized by macro irq_handler in entry-armv.S, and
>     each machine class defines their own macro to decode the IRQ number.
>     However, this prevents multiple machine classes to be built into a
>     single kernel.
> 
>     By allowing each machine to specify thier own handler, and making
>     function pointer 'handle_arch_irq' to point to it at run time, this
>     can be solved. And introduce CONFIG_MULTI_IRQ_HANDLER to allow both
>     solutions to work.
> 
>     Comparing with the highly optimized macro of irq_handler, the new
>     function must be written with care not to lose too much performance.
>     And the IPI stuff on SMP is expected to move to the provided arch
>     IRQ handler as well.
> 
>     Signed-off-by: Eric Miao <eric.miao at canonical.com>
>     Acked-by: Nicolas Pitre <nicolas.pitre at linaro.org>
> 
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index a19a526..c2c9838 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -1017,6 +1017,11 @@ config CPU_HAS_PMU
>  	default y
>  	bool
> 
> +config MULTI_IRQ_HANDLER
> +	bool
> +	help
> +	  Allow each machine to specify it's own IRQ handler at run time.
> +
>  if !MMU
>  source "arch/arm/Kconfig-nommu"
>  endif
> diff --git a/arch/arm/include/asm/mach/arch.h b/arch/arm/include/asm/mach/arch.h
> index d97a964..7d55356 100644
> --- a/arch/arm/include/asm/mach/arch.h
> +++ b/arch/arm/include/asm/mach/arch.h
> @@ -40,6 +40,9 @@ struct machine_desc {
>  	void			(*init_irq)(void);
>  	struct sys_timer	*timer;		/* system tick timer	*/
>  	void			(*init_machine)(void);
> +#ifdef CONFIG_MULTI_IRQ_HANDLER
> +	void			(*handle_irq)(struct pt_regs *);
> +#endif
>  };
> 
>  /*
> diff --git a/arch/arm/include/asm/mach/irq.h b/arch/arm/include/asm/mach/irq.h
> index ce3eee9..6ecdad9 100644
> --- a/arch/arm/include/asm/mach/irq.h
> +++ b/arch/arm/include/asm/mach/irq.h
> @@ -22,6 +22,10 @@ extern void (*init_arch_irq)(void);
>  extern void init_FIQ(void);
>  extern int show_fiq_list(struct seq_file *, void *);
> 
> +#ifdef CONFIG_MULTI_IRQ_HANDLER
> +extern void (*handle_arch_irq)(struct pt_regs *);
> +#endif
> +
>  /*
>   * This is for easy migration, but should be changed in the source
>   */
> diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> index c09e357..8ed4f7c 100644
> --- a/arch/arm/kernel/entry-armv.S
> +++ b/arch/arm/kernel/entry-armv.S
> @@ -29,6 +29,7 @@
>  /*
>   * Interrupt handling.  Preserves r7, r8, r9
>   */
> +#ifndef CONFIG_MULTI_IRQ_HANDLER
>  	.macro	irq_handler
>  	get_irqnr_preamble r5, lr
>  1:	get_irqnr_and_base r0, r6, r5, lr
> @@ -62,6 +63,14 @@
>  #endif
> 
>  	.endm
> +#else
> +	.macro	irq_handler
> +	ldr	r4, =handle_arch_irq
> +	mov	r0, sp
> +	mov	lr, pc
> +	ldr	pc, [r4]
> +	.endm
> +#endif	/* CONFIG_MULTI_IRQ_HANDLER */
> 
>  #ifdef CONFIG_KPROBES
>  	.section	.kprobes.text,"ax",%progbits
> @@ -1245,3 +1254,9 @@ cr_alignment:
>  	.space	4
>  cr_no_alignment:
>  	.space	4
> +
> +#ifdef CONFIG_MULTI_IRQ_HANDLER
> +	.globl	handle_arch_irq
> +handle_arch_irq:
> +	.long	0
> +#endif
> diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> index 336f14e..a834c6c 100644
> --- a/arch/arm/kernel/setup.c
> +++ b/arch/arm/kernel/setup.c
> @@ -876,6 +876,12 @@ void __init setup_arch(char **cmdline_p)
>  	system_timer = mdesc->timer;
>  	init_machine = mdesc->init_machine;
> 
> +#ifdef CONFIG_MULTI_IRQ_HANDLER
> +	handle_arch_irq = class ? class->handle_irq : NULL;
> +	if (!handle_arch_irq)
> +		panic("No machine class specific IRQ handler defined\n");
> +#endif
> +
>  #ifdef CONFIG_VT
>  #if defined(CONFIG_VGA_CONSOLE)
>  	conswitchp = &vga_con;