netdev - Re: [PATCH] e1000e: Cleanup handling of VLAN

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Message-ID: <alpine.NEB.2.11.1504081714400.3388@chris.i8u.org>
Date:	Wed, 8 Apr 2015 17:26:05 -0700 (PDT)
From:	Hisashi T Fujinaka <htodd@...fifty.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
cc:	Alexander Duyck <alexander.h.duyck@...hat.com>,
	intel-wired-lan@...ts.osuosl.org, jeffrey.t.kirsher@...el.com,
	netdev@...r.kernel.org, mike@...tml.com, david.m.ertman@...el.com
Subject: Re: [PATCH] e1000e: Cleanup handling of VLAN_HLEN as a part of max
 frame size

On Wed, 8 Apr 2015, Hisashi T Fujinaka wrote:

> On Wed, 8 Apr 2015, Alexander Duyck wrote:
>> 
>> It is but it isn't.  If you look in e1000_change_mtu you will find the
>> node about "Jumbo frame workaround on 82579 and newer requires CRC be
>> stripped".  With that being the case I'm wondering if the 9018 doesn't
>> include CRC but instead includes VLAN header.  So as a result the actual
>> hardware is processing frames that are 9022 in size, but the buffer only
>> ever receives at most 9018 since the CRC is stripped.
>> 
>> I suspect that is why the Windows driver has had no issues reporting
>> support for a size of 9014 (excluding VLAN and CRC) on these parts and
>> hasn't had any issues.
>
> I can only tell you what was told to me by Dave Ertman, which is that
> there is a hard hardware limit of 9018 bytes. I wouldn't know if we do
> have Windows issues because it's a completely different division and
> there's no reason for any of those issues to be routed to us.
>
> In fact, the problem with different max MTU across the drivers in Linux
> has only ever been reported by one user.
>
> I'm still looking for the HW documentation and would like Jeff to hold
> off until we find it.

OK. So the data sheet states:

LPE controls whether long packet reception is permitted. Hardware
discards long packets if LPE is 0. A long packet is one longer than 1522
bytes. If LPE is 1, the maximum packet size that the device can receive
is 9018 bytes.

So if you're pre-stripping the CRC, then it will be less than 9018.

I guess I'd like to hear what Dave thinks.
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