From mboxrd@z Thu Jan  1 00:00:00 1970
From: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Subject: DSA vs envelope frames
Date: Wed, 30 Nov 2016 17:58:31 +0300
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>> (1) When DSA is in use, frames processed by FEC chip contain DSA tag and
>> thus can be larger than hardcoded limit of 1522. This issue is not
>> FEC-specific, any driver that hardcodes maximum frame size to 1522 (many
>> do) will have this issue if used with DSA.
> 
> BTW I'm trying to introduce envelope frames to solve this kind of problems.
> http://marc.info/?t=147496691500005&r=1&w=2
> http://marc.info/?t=147496691500003&r=1&w=2
> http://marc.info/?t=147496691500002&r=1&w=2
> http://marc.info/?t=147496691500004&r=1&w=2
> http://marc.info/?t=147496691500001&r=1&w=2
> 
> It needs jumbo frame support of NICs though.

Thanks for pointing to this.

Indeed frame with DSA tag conceptually is an envelope frame.

ndev->env_hdr_len introduced by your patches, actually is explicitly
handled difference between (MTU + 18) and frame that HW should allow.
If this is known, hardware can be configured to work with DSA. At least
FEC hardware that can send and receive "slightly larger" frames after
simple register configuration.

Furthermore, since DSA configuration is known statically (it comes from
device tree), ndo_set_env_hdr_len method could be automatically called
at init, making setup working by default if driver supports that. And if
not, perhaps can automatically lower MTU.

Looks like a solution :)

What's current status of this work?

What is not really clear - what if several tagging protocols are used
together. AFAIU, things may be more complex that simple appending of
tags, e.g. EDSA tag can carry VLAN id inside.

Nikita