feat(libfdt): add function to set MAC addresses

The devicetree specification[1] defines the generic DT properties
"mac-address" and "local-mac-address", that allow to set the MAC address
for a network device. This is needed because many platform network
devices do not define a method for obtaining a unique MAC address, and
many devices lack the non-volatile storage to hold such a number.

Some platforms (for instance Allwinner) derive the MAC address from
another unique SoC property, for instance some serial number. To allow
those MAC address to be set by TF-A, add a function that finds the DT
node of a network device (by using the "ethernet<x>" alias), then adding
the "local-mac-address" property into that node, setting it to a user
provided address. Platforms can use this function to generate MAC
addresses in a platform specific way, and store them in the DT.

DT consumers like U-Boot or the Linux kernel will automatically pick up
the address from that property and program the MAC device accordingly.

[1] https://devicetree-specification.readthedocs.io/en/latest/chapter4-device-bindings.html#local-mac-address-property

Change-Id: I3f5766cc575fa9718f9ca23e8269b11495c43be2
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/common/fdt_fixup.c b/common/fdt_fixup.c
index de02b46..0ab50d8 100644
--- a/common/fdt_fixup.c
+++ b/common/fdt_fixup.c
@@ -479,3 +479,44 @@
 						   (ac + sc + ac) * 4,
 						   val, sc * 4);
+ * fdt_set_mac_address () - store MAC address in device tree
+ * @dtb:	pointer to the device tree blob in memory
+ * @eth_idx:	number of Ethernet interface in /aliases node
+ * @mac_addr:	pointer to 6 byte MAC address to store
+ *
+ * Use the generic local-mac-address property in a network device DT node
+ * to define the MAC address this device should be using. Many platform
+ * network devices lack device-specific non-volatile storage to hold this
+ * address, and leave it up to firmware to find and store a unique MAC
+ * address in the DT.
+ * The MAC address could be read from some board or firmware defined storage,
+ * or could be derived from some other unique property like a serial number.
+ *
+ * Return: 0 on success, a negative libfdt error value otherwise.
+ */
+int fdt_set_mac_address(void *dtb, unsigned int ethernet_idx,
+			const uint8_t *mac_addr)
+	char eth_alias[12];
+	const char *path;
+	int node;
+	if (ethernet_idx > 9U) {
+		return -FDT_ERR_BADVALUE;
+	}
+	snprintf(eth_alias, sizeof(eth_alias), "ethernet%d", ethernet_idx);
+	path = fdt_get_alias(dtb, eth_alias);
+	if (path == NULL) {
+		return -FDT_ERR_NOTFOUND;
+	}
+	node = fdt_path_offset(dtb, path);
+	if (node < 0) {
+		ERROR("Path \"%s\" not found in DT: %d\n", path, node);
+		return node;
+	}
+	return fdt_setprop(dtb, node, "local-mac-address", mac_addr, 6);
diff --git a/include/common/fdt_fixup.h b/include/common/fdt_fixup.h
index 7a590b2..0399ff5 100644
--- a/include/common/fdt_fixup.h
+++ b/include/common/fdt_fixup.h
@@ -17,5 +17,7 @@
 		      unsigned int afflv1, unsigned int afflv2);
 int fdt_adjust_gic_redist(void *dtb, unsigned int nr_cores, uintptr_t gicr_base,
 			  unsigned int gicr_frame_size);
+int fdt_set_mac_address(void *dtb, unsigned int ethernet_idx,
+			const uint8_t *mac_addr);
 #endif /* FDT_FIXUP_H */