netdev - Re: [PATCH] r8152: limit MAC pass-through to one device

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Date:   Wed, 10 Oct 2018 10:30:26 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     oneukum@...e.com
Cc:     Mario.Limonciello@...l.com, jkohoutek@...e.com,
        netdev@...r.kernel.org
Subject: Re: [PATCH] r8152: limit MAC pass-through to one device

From: Oliver Neukum <oneukum@...e.com>
Date: Wed, 10 Oct 2018 19:17:05 +0200

> On Mi, 2018-10-10 at 17:18 +0000, Mario.Limonciello@...l.com wrote:
>> > 
>> > MAC address having to be unique, a MAC coming from the host
>> > must be used at most once at a time. Hence the users must
>> > be recorded and additional users must fall back to conventional
>> > methods.
>> 
>> I checked with the internal team and actually applies pass through MAC on both
>> Windows Realtek driver and in UEFI network stack this applies to ALL supported
>> Realtek devices (R8153-AD).
> 
> I may have formulated this badly. What happens if you attach two
> devices of this type to the same host?

If the devices are on different physical network segments it will
work.

This was common back in the day even.  Old Sparc machines had a host
defined MAC address, and even with multi-port cards they would all use
that same MAC address unless the firmware nodes had individual device
MAC addresses defined.

I think it's a valid configuration.

But I can see that elements of userspace such as udev will likely not
be happy because their device naming schemes are built on the
assumption that all MAC addresses are unique.