netdev - Re: Debugging Ethernet issues

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Date:   Sun, 13 Nov 2016 20:51:46 +0100
From:   Mason <slash.tmp@...e.fr>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev <netdev@...r.kernel.org>,
        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>,
        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: Re: Debugging Ethernet issues

On 13/11/2016 04:09, Andrew Lunn wrote:

> Mason wrote:
>
>> When connected to a Gigabit switch
>> 3.4 negotiates a LAN DHCP setup instantly
>> 4.7 requires over 5 seconds to do so
> 
> When you run tcpdump on the DHCP server, are you noticing the first
> request is missing?
> 
> What can happen is the dhclient gets started immediately and sends out
> its first request before auto-negotiation has finished. So this first packet
> gets lost. The retransmit after a few seconds is then successful.

I will run tcpdump on the server as I run udhcpc on the client
for Linux 3.4 vs 4.7

Do you know what would make auto-negotiation fail at 100 Mbps
on 4.7? (whereas it succeeds on 3.4)

(Thinking out loud) If the problem were in auto-negotiation,
then if should work if I hard-code speed and duplex using
ethtool, right? (IIRC, hard-coding doesn't help.)

Regards.