PCI/MSI: Add helper function pci_msi_domain_get_msi_rid().

Add pci_msi_domain_get_msi_rid() to return the MSI requester id (RID).
Initially needed by gic-v3 based systems. It will be used by follow on
patch to drivers/irqchip/irq-gic-v3-its-pci-msi.c

Initially supports mapping the RID via OF device tree.  In the future,
this could be extended to use ACPI _IORT tables as well.
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

drivers/pci/msi.c

@@ -20,6 +20,7 @@
 #include <linux/io.h>
 #include <linux/slab.h>
 #include <linux/irqdomain.h>
+#include <linux/of_irq.h>
 
 #include "pci.h"
 
@@ -1327,4 +1328,35 @@ struct irq_domain *pci_msi_create_default_irq_domain(struct fwnode_handle *fwnod
 
 	return domain;
 }
+
+static int get_msi_id_cb(struct pci_dev *pdev, u16 alias, void *data)
+{
+	u32 *pa = data;
+
+	*pa = alias;
+	return 0;
+}
+/**
+ * pci_msi_domain_get_msi_rid - Get the MSI requester id (RID)
+ * @domain:	The interrupt domain
+ * @pdev:	The PCI device.
+ *
+ * The RID for a device is formed from the alias, with a firmware
+ * supplied mapping applied
+ *
+ * Returns: The RID.
+ */
+u32 pci_msi_domain_get_msi_rid(struct irq_domain *domain, struct pci_dev *pdev)
+{
+	struct device_node *of_node;
+	u32 rid = 0;
+
+	pci_for_each_dma_alias(pdev, get_msi_id_cb, &rid);
+
+	of_node = irq_domain_get_of_node(domain);
+	if (of_node)
+		rid = of_msi_map_rid(&pdev->dev, of_node, rid);
+
+	return rid;
+}
 #endif /* CONFIG_PCI_MSI_IRQ_DOMAIN */

include/linux/msi.h

@@ -294,6 +294,7 @@ irq_hw_number_t pci_msi_domain_calc_hwirq(struct pci_dev *dev,
 					  struct msi_desc *desc);
 int pci_msi_domain_check_cap(struct irq_domain *domain,
 			     struct msi_domain_info *info, struct device *dev);
+u32 pci_msi_domain_get_msi_rid(struct irq_domain *domain, struct pci_dev *pdev);
 #endif /* CONFIG_PCI_MSI_IRQ_DOMAIN */
 
 #endif /* LINUX_MSI_H */