ethernet: add a helper for assigning port addresses

We have 5 drivers which offset base MAC addr by port id.
Create a helper for them.

This helper takes care of overflows, which some drivers
did not do, please complain if that's going to break
anything!
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Shannon Nelson <snelson@pensando.io>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/etherdevice.h

@@ -551,6 +551,27 @@ static inline unsigned long compare_ether_header(const void *a, const void *b)
 #endif
 }
 
+/**
+ * eth_hw_addr_gen - Generate and assign Ethernet address to a port
+ * @dev: pointer to port's net_device structure
+ * @base_addr: base Ethernet address
+ * @id: offset to add to the base address
+ *
+ * Generate a MAC address using a base address and an offset and assign it
+ * to a net_device. Commonly used by switch drivers which need to compute
+ * addresses for all their ports. addr_assign_type is not changed.
+ */
+static inline void eth_hw_addr_gen(struct net_device *dev, const u8 *base_addr,
+				   unsigned int id)
+{
+	u64 u = ether_addr_to_u64(base_addr);
+	u8 addr[ETH_ALEN];
+
+	u += id;
+	u64_to_ether_addr(u, addr);
+	eth_hw_addr_set(dev, addr);
+}
+
 /**
  * eth_skb_pad - Pad buffer to mininum number of octets for Ethernet frame
  * @skb: Buffer to pad