bmips: enet: handle EPROBE_DEFER for the MAC address

Both bmips ethernet drivers call of_get_mac_address() and throw the
return value away. They then decide from the contents of an
uninitialised stack buffer whether a MAC address was found, and fall
back to eth_hw_addr_random() when it does not look valid. An
-EPROBE_DEFER is therefore turned into a random MAC address instead of
into a retry.

This did not matter while fixed-layout NVMEM cells were parsed
synchronously by the NVMEM core. Linux 6.12.104 picks up 751cc1534316
("nvmem: layouts: Add fixed-layout driver"), which drops the
"fixed-layout" entry from of_nvmem_layout_skip_table and turns it into
an ordinary NVMEM layout bus driver: nvmem_register() now only creates
the layout device and sets nvmem->layout, and the cells show up later,
when fixed_layout_probe() runs. Until then
nvmem_layout_module_get_optional() sees no bound driver and
of_nvmem_cell_get() returns -EPROBE_DEFER.

That window is hit on every boot rather than occasionally, because it
is link order and not a race. nvmem_init() is a subsys_initcall, but
the layout driver is a device_initcall and drivers/Makefile places
nvmem/ after both mtd/ and net/. The flash therefore registers its
NVMEM device first, the ethernet driver probes next and defers, and
only afterwards does the fixed-layout driver bind and add the cells.

28 bmips boards take their MAC address from a "fixed-layout" node and
bmips has no base-files fallback, so all of them would come up with a
random MAC address that changes on every boot. Propagate -EPROBE_DEFER
so that the probe is retried once the cell exists, the same way this
was already fixed for lantiq in 50e7e5c80d ("lantiq: xrx200: handle
EPROBE_DEFER for MAC address").

Fixes: 91a52f22a1 ("treewide: backport support for nvmem on non platform devices")
Fixes: 1c552eb44d ("bmips: add basic BCM6358 ethernet support")
Assisted-by: Claude:claude-opus-5
Link: https://github.com/openwrt/openwrt/pull/24808
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/bmips/files/drivers/net/ethernet/broadcom/bcm6348-enet.c

@@ -1565,7 +1565,10 @@ static int bcm6348_emac_probe(struct platform_device *pdev)
 	emac->old_duplex = -1;
 	emac->old_pause = -1;
 
-	of_get_mac_address(node, dev_addr);
+	ret = of_get_mac_address(node, dev_addr);
+	if (ret == -EPROBE_DEFER)
+		return ret;
+
 	if (is_valid_ether_addr(dev_addr)) {
 		dev_addr_set(ndev, dev_addr);
 		dev_info(dev, "mtd mac %pM\n", dev_addr);

target/linux/bmips/files/drivers/net/ethernet/broadcom/bcm6368-enetsw.c

@@ -1005,7 +1005,10 @@ static int bcm6368_enetsw_probe(struct platform_device *pdev)
 	priv->tx_ring_size = ENETSW_DEF_TX_DESC;
 	priv->copybreak = ENETSW_DEF_CPY_BREAK;
 
-	of_get_mac_address(node, dev_addr);
+	ret = of_get_mac_address(node, dev_addr);
+	if (ret == -EPROBE_DEFER)
+		return ret;
+
 	if (is_valid_ether_addr(dev_addr)) {
 		dev_addr_set(ndev, dev_addr);
 		dev_info(dev, "mtd mac %pM\n", dev_addr);