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Message-ID: <87h74ipcos.wl-maz@kernel.org>
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.
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