[Buildroot] Handling delays in network interface visibility in Raspberry Pi 3

[Christopher McCrory]

On phone so please excuse typos and brevity.

This is probably due to spanning tree (stp) on your switch. STP will delay
forwarding traffic from 30 to 55 ish seconds. In this case the pi is just
booting too fast. PCs have enough delays that you probably wouldn't notice
this happening.  Add a sleep 56 before the ifup bit, if that solves
the problem, stp it is.  If so, solved and done. Or if possible enable
portfast on the swichport, this will speed up the stp convergence.

For buildroot, on my raspberry pi builds, I only did wifi. But IIRC, the
dhcp client should have some options to adjust the number of requests
and/or the time between requests. If so the time it waits to receive a
response should be over 55 ish seconds in total to account for stp.
Preferably, the last request should happen after stp has converged.



On Mon, Feb 15, 2021, 9:08 AM Ivo Grondman <buildroot at grondman.net> wrote:

> Hi all,
>
> Today I’ve been trying to debug the following issue. Using Buildroot
> 2020.11.2 and the Vagrantfile that comes with it, I tried to build a
> “vanilla” Buildroot image in the vagrant VM for my Raspberry Pi 3 Model B+
> in the following way:
>
> $ make raspberrypi3_defconfig
> $ make
>
> i.e. without any customisation. The image builds fine and the Pi boots
> without errors or warnings, but the network interface eth0 (configured with
> DHCP) will not have an IPv4 address and I manually have to perform an `ifup
> -a` or `ifup eth0` to get it going. So I went through all the
> initialisation files that have a part in bringing up the network interface
> and soon came to conclude that the script in
> /etc/network/if-pre-up.d/wait_iface has the right intentions, but still
> doesn’t solve the slow-to-appear interface issue.
>
> It looks as if that particular script still needs to wait just that bit
> longer when it sees that /sys/class/net/eth0 exists before actually
> exiting. Using the file as is will not make eth0 retrieve an address from
> the DHCP server, but if I add a bit of extra sleep in the if-statement
> where it detects the presence of /sys/class/net/eth0 then it *will* work.
>
> Now my problem with this approach is that it feels like a hack. Moreover,
> I’d like to know if something else could be the issue here. So:
>
> 1) Did anyone of you experience similar issues with a Raspberry Pi?
> 2) Are there any alternative, better solutions to this problem?
> 3) If the answer to 2) is a “no”, should I submit a patch for this
> particular script? I’m a bit hesitant to put in a patch with an extra delay
> on detection of the interface, as this is obviously not something you want
> to do for *all* possible platforms…
>
> Any help is greatly appreciated.
>
> Best regards,
>
> Ivo Grondman
>
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