diff --git a/arch/powerpc/kernel/rtas_pci.c b/arch/powerpc/kernel/rtas_pci.c
index 5a798ac6aecf..b4a0de79c060 100644
--- a/arch/powerpc/kernel/rtas_pci.c
+++ b/arch/powerpc/kernel/rtas_pci.c
@@ -81,8 +81,7 @@ int rtas_read_config(struct pci_dn *pdn, int where, int size, u32 *val)
 	if (!config_access_valid(pdn, where))
 		return PCIBIOS_BAD_REGISTER_NUMBER;
 
-	addr = ((where & 0xf00) << 20) | (pdn->busno << 16) |
-		(pdn->devfn << 8) | (where & 0xff);
+	addr = rtas_config_addr(pdn->busno, pdn->devfn, where);
 	buid = pdn->phb->buid;
 	if (buid) {
 		ret = rtas_call(ibm_read_pci_config, 4, 2, &returnval,
@@ -134,8 +133,7 @@ int rtas_write_config(struct pci_dn *pdn, int where, int size, u32 val)
 	if (!config_access_valid(pdn, where))
 		return PCIBIOS_BAD_REGISTER_NUMBER;
 
-	addr = ((where & 0xf00) << 20) | (pdn->busno << 16) |
-		(pdn->devfn << 8) | (where & 0xff);
+	addr = rtas_config_addr(pdn->busno, pdn->devfn, where);
 	buid = pdn->phb->buid;
 	if (buid) {
 		ret = rtas_call(ibm_write_pci_config, 5, 1, NULL, addr,
diff --git a/include/asm-powerpc/rtas.h b/include/asm-powerpc/rtas.h
index 82a27e9a041f..d34f9e1f242c 100644
--- a/include/asm-powerpc/rtas.h
+++ b/include/asm-powerpc/rtas.h
@@ -230,5 +230,21 @@ extern unsigned long rtas_rmo_buf;
 
 #define GLOBAL_INTERRUPT_QUEUE 9005
 
+/**
+ * rtas_config_addr - Format a busno, devfn and reg for RTAS.
+ * @busno: The bus number.
+ * @devfn: The device and function number as encoded by PCI_DEVFN().
+ * @reg: The register number.
+ *
+ * This function encodes the given busno, devfn and register number as
+ * required for RTAS calls that take a "config_addr" parameter.
+ * See PAPR requirement 7.3.4-1 for more info.
+ */
+static inline u32 rtas_config_addr(int busno, int devfn, int reg)
+{
+	return ((reg & 0xf00) << 20) | ((busno & 0xff) << 16) |
+			(devfn << 8) | (reg & 0xff);
+}
+
 #endif /* __KERNEL__ */
 #endif /* _POWERPC_RTAS_H */