netdev - Debugging Ethernet issues

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Date:   Sun, 13 Nov 2016 01:01:14 +0100
From:   Mason <slash.tmp@...e.fr>
To:     netdev <netdev@...r.kernel.org>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Mans Rullgard <mans@...sr.com>,
        Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Zach Brown <zach.brown@...com>, Andrew Lunn <andrew@...n.ch>,
        Shaohui Xie <shaohui.xie@....com>,
        Tim Beale <tim.beale@...iedtelesis.co.nz>,
        Brian Hill <brian@...ston-radar.com>,
        Vince Bridgers <vbridgers2013@...il.com>,
        Balakumaran Kannan <kumaran.4353@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Sebastian Frias <sf84@...oste.net>,
        Kirill Kapranov <kapranoff@...ox.ru>
Subject: Debugging Ethernet issues

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.