irqdomain: Add IRQ_DOMAIN_FLAG_MSI_IMMUTABLE and irq_domain_is_msi_immutable()

Add the flag IRQ_DOMAIN_FLAG_MSI_IMMUTABLE and the API function
irq_domain_is_msi_immutable() to check if the MSI controller retains an
immutable address/data pair during irq_set_affinity().

Ensure compatibility with MSI users like PCIe Endpoint Doorbell, which
require the address/data pair to remain unchanged after setup. Use this
function to verify if the MSI controller is immutable.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250414-ep-msi-v18-2-f69b49917464@nxp.com

Author: nxpfrankli

include/linux/irqdomain.h

@@ -231,6 +231,9 @@ enum {
 	/* Irq domain must destroy generic chips when removed */
 	IRQ_DOMAIN_FLAG_DESTROY_GC	= (1 << 10),
 
+	/* Address and data pair is mutable when irq_set_affinity() */
+	IRQ_DOMAIN_FLAG_MSI_IMMUTABLE	= (1 << 11),
+
 	/*
 	 * Flags starting from IRQ_DOMAIN_FLAG_NONCORE are reserved
 	 * for implementation specific purposes and ignored by the
@@ -691,6 +694,10 @@ static inline bool irq_domain_is_msi_device(struct irq_domain *domain)
 	return domain->flags & IRQ_DOMAIN_FLAG_MSI_DEVICE;
 }
 
+static inline bool irq_domain_is_msi_immutable(struct irq_domain *domain)
+{
+	return domain->flags & IRQ_DOMAIN_FLAG_MSI_IMMUTABLE;
+}
 #else	/* CONFIG_IRQ_DOMAIN_HIERARCHY */
 static inline int irq_domain_alloc_irqs(struct irq_domain *domain,
 			unsigned int nr_irqs, int node, void *arg)