RaspberryPi3+: can't set mac address from device tree

[Paolo Pisati]

Impact:

The RaspberryPi3+ board came with an usb's to ethernet lan78xx nic adapter - thhis chip has no external eeprom, nor internal preprogrammed mac address and as such, after every reboot, it generates a random new ethernet mac address.

Supposedly, the lan78xx driver is able to read its mac address from the device tree passed to the kernel (see d093067 "lan78xx: Read MAC address from DT if present"), making it possible to have reproducible/fixed mac addresses, but unfortunately this feature isn't working in our Xenial/raspi2 kernel (while it's working fine in Bionc/raspi2).

After some debugging i found that the usb_net struct backing the lan78xx driver, had no clue about the dt node it was associated, making this piece of code constantly evaluates to NULL:

static void lan78xx_init_mac_address(struct lan78xx_net *dev)
...
mac_addr = of_get_mac_address(dev->udev->dev.of_node);
...

There "of_node" was always NULL.

After more debugging, i found that the feature to bind usb_net devices and device tree nodes was introduced in the Linux 4.6 cycle, and as such Xenial didn't have it.

Fix:

Apply the patch and recompile

How to test:

Upon boot of the RaspberryPi3+ board (and after the dtb has been loaded into memory), stop at the uboot prompt and check the usbeether device node:

U-Boot> fdt print /soc/usb/usb1@1/usb1_1@1/usbether@1
usbether@1 {
        local-mac-address = [b8 27 eb ce 07 ba];
        compatible = "usb424,7800";
        reg = <0x00000001>;
        microchip,eee-enabled;
        microchip,tx-lpi-timer = <0x00000258>;
        phandle = <0x00000024>;
};

the "local-mac-address" has been filled by the Broadcom bootloader, and contains the lan78xx's mac address.

Then proceed with the boot routine, readh the user prompt, and check eth0's mac address:

$ ifconfig eth0 | grep HWaddr
eth0 Link encap:Ethernet HWaddr b8:27:eb:ce:07:ba

If it's the same as above, then the fix worked.

Regression:

The patch itself has been upstream since v4.6-rc1, and it changes a very small portion of code, so i'll say it's regression potential is very low / none.

[Viktor Petersson (vpetersson)]

@p-pisati what's the ETA on this? We're desperate for this feature. See https://bugs.launchpad.net/ubuntu/+source/linux-raspi2/+bug/1798578

[Launchpad Janitor (janitor)]

This bug was fixed in the package linux-raspi2 - 4.4.0-1100.108

---------------
linux-raspi2 (4.4.0-1100.108) xenial; urgency=medium

  * linux-raspi2: 4.4.0-1100.108 -proposed tracker (LP: #1799409)

  * RaspberryPi3+: can't set mac address from device tree (LP: #1798578)
    - USB: core: let USB device know device node

  [ Ubuntu: 4.4.0-139.165 ]

  * linux: 4.4.0-139.165 -proposed tracker (LP: #1799401)
  * Kernel panic after the ubuntu_nbd_smoke_test on Xenial kernel (LP: #1793464)
    - nbd: Remove signal usage
    - nbd: Timeouts are not user requested disconnects
    - nbd: Cleanup reset of nbd and bdev after a disconnect
    - nbd: don't shutdown sock with irq's disabled
    - nbd: fix race in ioctl
  * fscache: bad refcounting in fscache_op_complete leads to OOPS (LP: #1797314)
    - SAUCE: fscache: Fix race in decrementing refcount of op->npages
  * xenial: virtio-scsi: CPU soft lockup due to loop in
    virtscsi_target_destroy() (LP: #1798110)
    - SAUCE: (no-up) virtio-scsi: Decrement reqs counter before SCSI command
      requeue
  * Error reported when creating ZFS pool with "-t" option, despite successful
    pool creation (LP: #1769937)
    - SAUCE: (noup) Update zfs to 0.6.5.6-0ubuntu26
  * Xenial update: 4.4.160 upstream stable release (LP: #1798770)
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