netdev - Re: [PATCH] ucc_geth: Add 8 bytes to max TX frame for VLANs

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Date:	Sat, 22 Mar 2008 12:51:41 +0100
From:	"Joakim Tjernlund" <joakim.tjernlund@...nsmode.se>
To:	"Li Yang" <LeoLi@...escale.com>, "Netdev" <netdev@...r.kernel.org>,
	"Linuxppc-Embedded@...abs.Org" <linuxppc-embedded@...abs.org>
Subject: Re: [PATCH] ucc_geth: Add 8 bytes to max TX frame for VLANs

>> -----Original Message-----
>> From: netdev-owner@...r.kernel.org
>> [mailto:netdev-owner@...r.kernel.org] On Behalf Of Joakim Tjernlund
> Sent: Tuesday, March 18, 2008 11:11 PM
>> To: Netdev; Li Yang
>> Cc: Joakim Tjernlund
>>Subject: [PATCH] ucc_geth: Add 8 bytes to max TX frame for VLANs
>>
>> Creating a VLAN interface on top of ucc_geth adds 4 bytes to
>> the frame and the HW controller is not prepared to TX a frame
>> bigger than 1518 bytes which is 4 bytes too small for a full
>> VLAN frame. Also add 4 extra bytes for future expansion.
> 
> IMO, VLAN and Jumbo packet support is not general case of Ethernet.
> Could you make this change optional?  Thanks.
> 
> - Leo 

hmm, I do not agree. VLAN is common today.

If you were to enable HW support for VLAN then the ethernet controller would inject an extra 4 bytes into the frame. 
This change does not change the visible MTU for the user. As is now, soft VLAN is silently broken. Do you
really want the user to find and turn on a controller specific feature to use VLAN?

What does netdev people think? 

 Jocke