irqchip/msi-lib: Honour the MSI_FLAG_NO_AFFINITY flag

Bad MSI implementations multiplex MSIs onto a single downstream interrupt,
meaning they have no concept of individual affinity.

The old MSI code did a reasonable job at this by honouring the
MSI_FLAG_NO_AFFINITY, but the new shiny device MSI code doesn't.

Teach it about the sad reality of existing hardware.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250513172819.2216709-7-maz@kernel.org

Author: Marc Zyngier

drivers/irqchip/irq-msi-lib.c

@@ -105,8 +105,13 @@ bool msi_lib_init_dev_msi_info(struct device *dev, struct irq_domain *domain,
 	 * MSI message into the hardware which is the whole purpose of the
 	 * device MSI domain aside of mask/unmask which is provided e.g. by
 	 * PCI/MSI device domains.
+	 *
+	 * The exception to the rule is when the underlying domain
+	 * tells you that affinity is not a thing -- for example when
+	 * everything is muxed behind a single interrupt.
 	 */
-	chip->irq_set_affinity = msi_domain_set_affinity;
+	if (!chip->irq_set_affinity && !(info->flags & MSI_FLAG_NO_AFFINITY))
+		chip->irq_set_affinity = msi_domain_set_affinity;
 	return true;
 }
 EXPORT_SYMBOL_GPL(msi_lib_init_dev_msi_info);