Debugging Ethernet issues

[Mason <slash.tmp@free.fr>]

Hello everyone,

In a past thread ("Ethernet not working on a different SoC with
same eth HW") I was struggling getting Ethernet to work at all on
a new board using a recent 4.7 kernel.

After much hair-pulling, it turned out that *some* of the breakage
was caused by a local patch which should have been guarded by a
preprocessor macro.

But even after reverting that patch, Ethernet does not work well
on this board with kernel 4.7 whereas if I use an ancient 3.4 kernel,
then Ethernet works much better.

Differences:

When connected to a 100 Mbps switch
3.4 negotiates a LAN DHCP setup instantly
4.7 times out

When connected to a Gigabit switch
3.4 negotiates a LAN DHCP setup instantly
4.7 requires over 5 seconds to do so

(In case it matters, my board is using an Atheros 8035 PHY.)

I am aware that there have been hundreds of patches to the phy
and net frameworks in the 3.4 to 4.7 time-frame. I'm wondering
if there are important events I can log, to see what is going
wrong in the 4.7 case?

Are there kernel debugging options I might turn on, to better
understand what is going wrong?

I would be extremely grateful for any insight on this subject.

Regards.