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Passing MAC address to kernel via Device Tree Blob

Former Member

Good morning everyone,

in our lab, we have several ZedBoards of which some obtain an IP address from a DHCP server based on their MAC address and some of them use a fixed IP address (no DHCP).

Based on my experience with other embedded Linux boards, the straight forward solution to achieve this would be to generate an individual device tree source (DTS) file for each board containing
- the specific MAC address and
- the specific boot arguments for the kernel to either turn on DHCP or to assign a specific fixed IP address.

According to the Embedded Linux Hands-on Tutorial and the Embedded Linux Development Guide, it is recommended to set the MAC address of the ethernet port of the ZedBoard via U-Boot (zynq_zed.h). However, it is also mentioned that the MAC address can be set via DTS file. To do so, the MAC address assignment in the U-Boot configuration needs to be deleted.

Now to my problem:
If I now boot the ZedBoard with such a configuration, the MAC address of the system differs from the address specified in the DTS file. More precisely, the first 4 Bytes are equal to the specified address but the last 2 Bytes are always zero.
Example:
AA:BB:CC:DD:11:FF -> AA:BB:CC:DD:00:00

If the MAC address is set via U-Boot, everything is fine. Except that I then need to generate both a specific BOOT.BIN (containing the U-Boot) and a DTS file for each board.

I think that there is an error in how the MAC address is passed from the DTS to the driver (xemacps). Does anyone of you have an idea how to fix that?

Best regards,

Pirmin

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    Okay, I figured out how to give a Zed board that is running Linaro Ubuntu a unique MAC address.

    As mentioned in a previous post above, the way one would expect this to work is by setting the ethaddr environment variable in uBoot. (This actually works for Petalinux).
    The problem I had when I did that, was that Linux wouldn't create my eth0 interface.
    What I just found out was that it actually DID create an eth4 interface. Of course I didn't have an interface file for it, so it wasn't usable. But after I used ipconfig to give it the right info (IP address, gateway, etc), it worked just fine.

    So then I started to wonder where this eth4 came from.
    Turns out it is created by the udev system.
    By looking at the rules file that creates the interfaces ( /etc/udev/rules.d/70-persistent-net.rules) I noticed that it tries to create, indeed, 5 interfaces, one for each MAC address I ever tried in all my testing. So, had I changed the ethaddr Uboot variable yet again, it would have added a sixth (eth5) interface for it (which would never be configured right, since I did't expect an eth5).
    As it turns out this rules file /etc/udev/rules.d/70-persistent-net.rules is itself created dynamically at boot time by a udev script called /lib/udev/write_net_rules.
    THAT is the file that keeps adding interfaces to the rules file if it gets a MAC address passed that it hasn't used before.
    One way of fixing this it to alter that script to not add new interfaces, but a much simpler solution is to simply delete the rules file!

    So here is the entire procedure:

    1. Boot into Linux
    2. Run:
       rm /etc/udev/rules.d/70-persistent-net.rules
       sync
       boot
    3. Stop uBoot from booting, so you get into the uBoot command line (if you accidently boot into Linux again, remove the rules file again!)
    4. Run:
       setenv ethaddr xx:xx:xx:xx:xx:xx
       saveenv
       reset   (NOT 'boot'!)
    5. Let it boot into Linux .. now you have an eth0 with the new MAC address

    Hope this is helpful to someone .. (the critical step is to remove the rules file! You only have to do all this once for each board/device)


    ~ Paul

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