netdev - Re: [PATCH] veth: Optionally pad packets to minimum Ethernet length

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Date:   Tue, 12 Dec 2017 10:34:23 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:     Dan Williams <dcbw@...hat.com>,
        Ed Swierk <eswierk@...portsystems.com>, netdev@...r.kernel.org,
        Benjamin Warren <ben@...portsystems.com>,
        Keith Holleman <holleman@...portsystems.com>
Subject: Re: [PATCH] veth: Optionally pad packets to minimum Ethernet length

On Tue, 12 Dec 2017 16:18:17 -0200
Marcelo Ricardo Leitner <marcelo.leitner@...il.com> wrote:

> On Tue, Dec 12, 2017 at 11:32:46AM -0600, Dan Williams wrote:
> > On Tue, 2017-12-12 at 08:13 -0800, Ed Swierk wrote:  
> > > Most physical Ethernet devices pad short packets to the minimum
> > > length
> > > of 64 bytes (including FCS) on transmit. It can be useful to simulate
> > > this behavior when debugging a problem that results from it (such as
> > > incorrect L4 checksum calculation).
> > > 
> > > Padding is unnecessary for most applications so leave it off by
> > > default. Enable padding only when the otherwise unused IFF_AUTOMEDIA
> > > flag is set (e.g. by writing 0x5003 to flags in sysfs).  
> > 
> > This seems like a weird overload of AUTOMEDIA, which no other driver
> > uses for this purpose.  Seems like the only other user of AUTOMEDIA is
> > 8390/etherh.c for some 10BaseT/10Base2 stuff.
> > 
> > I'm not sure what the interface should be, but perhaps a sysfs
> > attribute would be better than overloading IFF_AUTOMEDIA?  
> 
> What about using some tc action (i.e. skbmod) for this?
> 
>   Marcelo

Why not add to netdevsim rather than cluttering up a normal driver
with test support.  We just pulled a bunch of test stuff out of dummy
for the same reason.