linux-kernel - Re: [PATCH] irqchip/msi-lib: Honor the MSI_FLAG_PCI_MSI_MASK

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Message-ID: <875xhzhuup.ffs@tglx>
Date: Sat, 17 May 2025 21:59:10 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Marc Zyngier <maz@...nel.org>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] irqchip/msi-lib: Honor the MSI_FLAG_PCI_MSI_MASK_PARENT
 flag

On Sat, May 17 2025 at 11:30, Marc Zyngier wrote:
> +	/*
> +	 * If the parent domain insists on being in charge of masking, obey
> +	 * blindly. The default mask/unmask become the shutdown/enable
> +	 * callbacks, ensuring that we correctly start/stop the interrupt.
> + 	 * We make a point in not using the irq_disable() in order to
> +	 * preserve the "lazy disable" behaviour.
> +	 */
> +	if (info->flags & MSI_FLAG_PCI_MSI_MASK_PARENT) {
> +		chip->irq_shutdown	= chip->irq_mask;
> +		chip->irq_enable	= chip->irq_unmask;

This is only correct, when the chip does not have dedicated
irq_shutdown/enable callbacks. And I really hate the asymmetry of this.

> +		chip->irq_mask		= irq_chip_mask_parent;
> +		chip->irq_unmask	= irq_chip_unmask_parent;
> +	}

I'm still trying to understand, what's the actual problem is you are
trying to solve.

MSIs are edge type interrupts, so the interrupt handling hotpath usually
does not mask at all. The only time masking happens is when it's lazy
disabled or during affinity changes, which is not the end of the world.

Thanks,

        tglx