[PATCH RFC 3/2] ARM: improve arch_irq_work_has_interrupt()

[linux@armlinux.org.uk (Russell King - ARM Linux)]

On Mon, Nov 14, 2016 at 04:30:57PM +0000, Marc Zyngier wrote:
> Hi Russell,
> 
> On 14/11/16 15:36, Russell King - ARM Linux wrote:
> > Following on from the previous patch, I think this makes more sense to
> > determine whether we can support IRQ work interrupts.
> > 
> > Whether we can support them or not depends on two things:
> > 
> > (a) whether the kernel has support for receiving IPIs
> > (b) whether it's possible to send an IPI to CPUs including the raising CPU.
> > 
> > (a) is a function of how the kernel is built - and in the case of ARM, it
> > depends whether the kernel is built with SMP enabled or not.
> > (b) is a property of the interrupt controller.
> > 
> > It hasn't ever been a function of the CPU or architecture.
> > 
> > Commit 059e232089e4 ("irqchip/gic: Allow self-SGIs for SMP on UP
> > configurations") changes the GIC IPI code such that we can raise
> > SGIs on uniprocessor systems running on a SMP kernel, which means
> > we can support IRQ work interrupts here as well.
> > 
> > So, we shouldn't be using cpu_smp() (or its previous is_smp() here
> > at all.  Use a flag to indicate whether we can IPI and use that to
> > indicate whether we support irq work interrupts.
> > 
> > Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
> > --- 
> >  arch/arm/include/asm/irq_work.h | 11 +++++++++--
> >  arch/arm/kernel/irq.c           |  0
> >  arch/arm/kernel/smp.c           |  3 +++
> >  drivers/irqchip/irq-gic.c       | 17 +++++++++++++----
> >  4 files changed, 25 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/arm/include/asm/irq_work.h b/arch/arm/include/asm/irq_work.h
> > index 2dc8d7995b48..d7262a3c2f2e 100644
> > --- a/arch/arm/include/asm/irq_work.h
> > +++ b/arch/arm/include/asm/irq_work.h
> > @@ -1,11 +1,18 @@
> >  #ifndef __ASM_ARM_IRQ_WORK_H
> >  #define __ASM_ARM_IRQ_WORK_H
> >  
> > -#include <asm/smp_plat.h>
> > +extern bool irq_controller_can_ipi;
> > +#define irq_controller_can_ipi irq_controller_can_ipi
> >  
> >  static inline bool arch_irq_work_has_interrupt(void)
> >  {
> > -	return cpu_smp();
> > +#ifdef CONFIG_SMP
> > +	/* This depends on the IRQ controller */
> > +	return irq_controller_can_ipi;
> > +#else
> > +	/* The kernel is not built to support IPIs */
> > +	return false;
> > +#endif
> >  }
> >  
> >  #endif /* _ASM_ARM_IRQ_WORK_H */
> > diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> > index 7dd14e8395e6..1fa9412cc4aa 100644
> > --- a/arch/arm/kernel/smp.c
> > +++ b/arch/arm/kernel/smp.c
> > @@ -473,6 +473,9 @@ void __init set_smp_cross_call(void (*fn)(const struct cpumask *, unsigned int))
> >  		__smp_cross_call = fn;
> >  }
> >  
> > +/* This indicates whether the IRQ controller can IPI (including self-IPI) */
> > +bool irq_controller_can_ipi;
> 
> We probably need to initialize this to false, since we have at least 4
> other users of set_smp_cross_call() in the tree.

C programming 101: BSS variables are initialised to zero at the start
of the program.

> > diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
> > index d6c404b3584d..abe8d5807c0f 100644
> > --- a/drivers/irqchip/irq-gic.c
> > +++ b/drivers/irqchip/irq-gic.c
> > @@ -1187,9 +1187,6 @@ static int __init __gic_init_bases(struct gic_chip_data *gic,
> >  		 */
> >  		for (i = 0; i < NR_GIC_CPU_IF; i++)
> >  			gic_cpu_map[i] = 0xff;
> > -#ifdef CONFIG_SMP
> > -		set_smp_cross_call(gic_raise_softirq);
> > -#endif
> >  		cpuhp_setup_state_nocalls(CPUHP_AP_IRQ_GIC_STARTING,
> >  					  "AP_IRQ_GIC_STARTING",
> >  					  gic_starting_cpu, NULL);
> > @@ -1207,8 +1204,20 @@ static int __init __gic_init_bases(struct gic_chip_data *gic,
> >  	}
> >  
> >  	ret = gic_init_bases(gic, irq_start, handle);
> > -	if (ret)
> > +	if (ret) {
> >  		kfree(name);
> > +		return ret;
> > +	}
> > +
> > +	if (gic == &gic_data[0]) {
> > +#ifdef CONFIG_SMP
> > +		set_smp_cross_call(gic_raise_softirq);
> > +#ifdef irq_controller_can_ipi
> > +		if (nr_cpu_ids == 1 || hweight8(gic_cpu_map[0]) == 1)
> > +			irq_controller_can_ipi = true;
> 
> Am I missing something, or is there any sane configuration where this 
> isn't true?

I hope not, but I want to duplicate here the conditions where
gic_raise_softirq() actually _works_ so we stop running into corner
cases where we get "irq work fails" etc.

> Also, maybe it would make some sense to have a more 
> streamlined interface to the architecture code. Something along the 
> lines of:
> 
> diff --git a/arch/arm/include/asm/smp.h b/arch/arm/include/asm/smp.h
> index 3d6dc8b..45612d2 100644
> --- a/arch/arm/include/asm/smp.h
> +++ b/arch/arm/include/asm/smp.h
> @@ -48,6 +48,16 @@ extern void smp_init_cpus(void);
>   */
>  extern void set_smp_cross_call(void (*)(const struct cpumask *, unsigned int));
>  
> +#ifdef CONFIG_SMP
> +#define setup_smp_ipi(f,i)			\
> +	do {					\
> +		set_smp_cross_call(f);		\
> +		irq_controller_can_ipi = (i);	\
> +	} while(0)
> +#else
> +#define setup_smp_ipi(f,i)	do { } while (0)
> +#endif
> +
>  /*
>   * Called from platform specific assembly code, this is the
>   * secondary CPU entry point.
> 
> with the similar entry point for arm64?

I'd prefer to keep the two things separate, but we should definitely
provide a stub for set_smp_cross_call() for when SMP is disabled.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.