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

diff --git a/drivers/irqchip/irq-msi-lib.c b/drivers/irqchip/irq-msi-lib.c
index 2a61c06c4da07e21297ef3b00157179c5aad4d50..246c30205af40f7c296ed756d3add89015a0ec7d 100644 (file)
--- a/drivers/irqchip/irq-msi-lib.c
+++ b/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);