linux-kernel - Re: [PATCH v2 4/6] genirq: Provide an IRQ affinity mask in non-SMP configs

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Date:   Sat, 18 Jun 2022 10:01:55 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Samuel Holland <samuel@...lland.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Guo Ren <guoren@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>,
        Wei Xu <xuwei5@...ilicon.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v2 4/6] genirq: Provide an IRQ affinity mask in non-SMP configs

Hi Samuel,

On Thu, 16 Jun 2022 07:40:26 +0100,
Samuel Holland <samuel@...lland.org> wrote:
> 
> IRQ affinity masks are not allocated in uniprocessor configurations.
> This requires special case non-SMP code in drivers for irqchips which
> have per-CPU enable or mask registers.
> 
> Since IRQ affinity is always the same in a uniprocessor configuration,
> we can still provide the correct affinity mask without allocating one
> per IRQ. We can reuse the system-wide cpu_possible_mask.
> 
> By returning a real cpumask from irq_data_get_affinity_mask even when
> SMP is disabled, irqchip drivers which iterate over that mask will
> automatically do the right thing.
> 
> Signed-off-by: Samuel Holland <samuel@...lland.org>
> ---
> 
> (no changes since v1)
> 
>  include/linux/irq.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/include/linux/irq.h b/include/linux/irq.h
> index 69ee4e2f36ce..d5e958b026aa 100644
> --- a/include/linux/irq.h
> +++ b/include/linux/irq.h
> @@ -151,7 +151,9 @@ struct irq_common_data {
>  #endif
>  	void			*handler_data;
>  	struct msi_desc		*msi_desc;
> +#ifdef CONFIG_SMP
>  	cpumask_var_t		affinity;
> +#endif
>  #ifdef CONFIG_GENERIC_IRQ_EFFECTIVE_AFF_MASK
>  	cpumask_var_t		effective_affinity;
>  #endif
> @@ -881,7 +883,11 @@ static inline int irq_data_get_node(struct irq_data *d)
>  
>  static inline struct cpumask *irq_data_get_affinity_mask(struct irq_data *d)
>  {
> +#ifdef CONFIG_SMP
>  	return d->common->affinity;
> +#else
> +	return &__cpu_possible_mask;
> +#endif

I have a bad feeling about this one. Being in a !SMP configuration
doesn't necessarily mean that __cpu_possible_mask only contains a
single CPU, specially with things like CONFIG_INIT_ALL_POSSIBLE. I can
also imagine an architecture populating this bitmap from firmware
tables irrespective of the SMP status of the kernel.

Can't you use something like:

	return cpumask_of(0);

which is guaranteed to be the right thing on !SMP configuration?

Thanks,

	M.

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