net: Implement random ethaddr fallback in eth.c

Implement the random ethaddr fallback in eth.c so it is in a common
place and not reimplemented in each board or driver that wants this
behavior.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

README

@@ -5623,7 +5623,8 @@ o If both the SROM and the environment contain a MAC address, and the
   warning is printed.
 
 o If neither SROM nor the environment contain a MAC address, an error
-  is raised.
+  is raised. If CONFIG_NET_RANDOM_ETHADDR is defined, then in this case
+  a random, locally-assigned MAC is used.
 
 If Ethernet drivers implement the 'write_hwaddr' function, valid MAC addresses
 will be programmed into hardware as part of the initialization process.	 This

doc/README.enetaddr

@@ -37,6 +37,8 @@ Correct flow of setting up the MAC address (summarized):
    environment variable will be used unchanged.
    If the environment variable is not set, it will be initialized from
    eth_device->enetaddr, and a warning will be printed.
+   If both are invalid and CONFIG_NET_RANDOM_ETHADDR is defined, a random,
+   locally-assigned MAC is written to eth_device->enetaddr.
 4. Program the address into hardware if the following conditions are met:
 	a) The relevant driver has a 'write_addr' function
 	b) The user hasn't set an 'ethmacskip' environment variable

net/Kconfig

@@ -7,4 +7,12 @@ menuconfig NET
 
 if NET
 
+config NET_RANDOM_ETHADDR
+	bool "Random ethaddr if unset"
+	help
+	  Selecting this will allow the Ethernet interface to function
+	  even when the ethaddr variable for that interface is unset.
+	  A new MAC address will be generated on every boot and it will
+	  not be added to the environment.
+
 endif   # if NET

net/eth.c

@@ -538,9 +538,15 @@ static int eth_post_probe(struct udevice *dev)
 		printf("\nWarning: %s using MAC address from ROM\n",
 		       dev->name);
 	} else if (is_zero_ethaddr(pdata->enetaddr)) {
+#ifdef CONFIG_NET_RANDOM_ETHADDR
+		net_random_ethaddr(pdata->enetaddr);
+		printf("\nWarning: %s (eth%d) using random MAC address - %pM\n",
+		       dev->name, dev->seq, pdata->enetaddr);
+#else
 		printf("\nError: %s address not set.\n",
 		       dev->name);
 		return -EINVAL;
+#endif
 	}
 
 	return 0;
@@ -666,9 +672,15 @@ int eth_write_hwaddr(struct eth_device *dev, const char *base_name,
 		printf("\nWarning: %s using MAC address from net device\n",
 		       dev->name);
 	} else if (is_zero_ethaddr(dev->enetaddr)) {
+#ifdef CONFIG_NET_RANDOM_ETHADDR
+		net_random_ethaddr(dev->enetaddr);
+		printf("\nWarning: %s (eth%d) using random MAC address - %pM\n",
+		       dev->name, eth_number, dev->enetaddr);
+#else
 		printf("\nError: %s address not set.\n",
 		       dev->name);
 		return -EINVAL;
+#endif
 	}
 
 	if (dev->write_hwaddr && !eth_mac_skip(eth_number)) {