genirq/msi: Add helper for creating MSI-parent irq domains

Creating an irq domain that serves as an MSI parent requires
a substantial amount of esoteric boiler-plate code, some of
which is often provided twice (such as the bus token).

To make things a bit simpler for the unsuspecting MSI tinkerer,
provide a helper that does it for them, and serves as documentation
of what needs to be provided.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250513172819.2216709-3-maz@kernel.org

Author: Marc Zyngier

include/linux/msi.h

@@ -636,6 +636,10 @@ struct irq_domain *msi_create_irq_domain(struct fwnode_handle *fwnode,
 					 struct msi_domain_info *info,
 					 struct irq_domain *parent);
 
+struct irq_domain_info;
+struct irq_domain *msi_create_parent_irq_domain(struct irq_domain_info *info,
+						const struct msi_parent_ops *msi_parent_ops);
+
 bool msi_create_device_irq_domain(struct device *dev, unsigned int domid,
 				  const struct msi_domain_template *template,
 				  unsigned int hwsize, void *domain_data,

kernel/irq/msi.c

@@ -911,6 +911,32 @@ struct irq_domain *msi_create_irq_domain(struct fwnode_handle *fwnode,
 	return __msi_create_irq_domain(fwnode, info, 0, parent);
 }
 
+/**
+ * msi_create_parent_irq_domain - Create an MSI-parent interrupt domain
+ * @info:		MSI irqdomain creation info
+ * @msi_parent_ops:	MSI parent callbacks and configuration
+ *
+ * Return: pointer to the created &struct irq_domain or %NULL on failure
+ */
+struct irq_domain *msi_create_parent_irq_domain(struct irq_domain_info *info,
+						const struct msi_parent_ops *msi_parent_ops)
+{
+	struct irq_domain *d;
+
+	info->hwirq_max		= max(info->hwirq_max, info->size);
+	info->size		= info->hwirq_max;
+	info->domain_flags	|= IRQ_DOMAIN_FLAG_MSI_PARENT;
+	info->bus_token		= msi_parent_ops->bus_select_token;
+
+	d = irq_domain_instantiate(info);
+	if (IS_ERR(d))
+		return NULL;
+
+	d->msi_parent_ops = msi_parent_ops;
+	return d;
+}
+EXPORT_SYMBOL_GPL(msi_create_parent_irq_domain);
+
 /**
  * msi_parent_init_dev_msi_info - Delegate initialization of device MSI info down
  *				  in the domain hierarchy