linux-kernel - Re: [PATCH] irqchip/gic-v3: Workaround for GIC-700 erratum 2941627

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Date:   Tue, 4 Jul 2023 17:27:45 +0200
From:   Lorenzo Pieralisi <lpieralisi@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Hanks Chen <hanks.chen@...iatek.com>,
        Cheng-Yuh.Wu@...iatek.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] irqchip/gic-v3: Workaround for GIC-700 erratum 2941627

On Tue, Jul 04, 2023 at 03:44:50PM +0100, Marc Zyngier wrote:

[...]

> > +	return !((gic_irq_in_rdist(d)) || gic_irq(d) >= 8192 ||
> > +		  cpumask_equal(irq_data_get_effective_affinity_mask(d),
> > +				cpumask_of(smp_processor_id())));
> 
> I dislike this statement for multiple reasons:
> 
> - it is written as a negation, making it harder than strictly
>   necessary to parse as it is the opposite of the comment above
> 
> - gic_irq_in_rdist() and gic_irq(d) >= 8192 are two ways of checking
>   the interrupt range -- maybe we should just do that
> 
> - cpumask_equal() is *slow* if you have more that 64 CPUs, something
>   that is increasingly common -- a better option would be to check
>   whether the current CPU is in the mask or not, which would be enough
>   as we only have a single affinity bit set
> 
> - smp_processor_id() can check for preemption, which is pointless
>   here, as we're doing things under the irq_desc raw spinlock.
> 
> I would expect something like:
> 
> 	enum gic_intid_range range = get_intid_range(d);
> 
> 	return (range == SGI_RANGE || range == ESPI_RANGE) &&
> 	       !cpumask_test_cpu(raw_smp_processor_id(),
> 				 irq_data_get_effective_affinity_mask(d));
> 

s/SGI/SPI - just noticed, for the records.

Thanks,
Lorenzo