[tip: irq/msi] irqchip/msi-lib: Honour the MSI_FLAG_NO_AFFINITY flag

[tip-bot2 for Marc Zyngier]

The following commit has been merged into the irq/msi branch of tip:

Commit-ID:     06526443a34c06879664eb5ae247c5e93dde7ed9
Gitweb:        https://git.kernel.org/tip/06526443a34c06879664eb5ae247c5e93dde7ed9
Author:        Marc Zyngier <maz@xxxxxxxxxx>
AuthorDate:    Tue, 13 May 2025 18:28:16 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Fri, 16 May 2025 21:32:20 +02:00

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@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/all/20250513172819.2216709-7-maz@xxxxxxxxxx

---
 drivers/irqchip/irq-msi-lib.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/irqchip/irq-msi-lib.c b/drivers/irqchip/irq-msi-lib.c
index 2a61c06..246c302 100644
--- 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);