From: Michael Cronenworth <mike@cchtml.com>
Subject: Re: e1000e max frame calculation
Date: Mon, 06 Apr 2015 16:45:40 -0500
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Preface: I am not here to start a discussion about header sizes or hardware limits. 
This is solely about the calculation used to limit a user when they pass a MTU to a 
driver.

On 04/06/2015 11:38 AM, Michael Cronenworth wrote:
> Other drivers appear to be using the "old" maximum frame calculation and can
> continue to use 9000.
>
> Can a standard be decided upon that all drivers use or is it acceptable that one
> driver "does its own thing?"

To be specific here are some examples of maximum MTU calculation and their tests:

sky2: user_mtu > ETH_JUMBO_MTU

r8169: user_mtu < ETH_ZLEN || user_mtu > (ETH_DATA_LEN||x*1024 - ETH_HLEN - 2)

igb:   user_mtu + ETH_HLEN + ETH_FCS_LEN + VLAN_HLEN > MAX_JUMBO_FRAME_SIZE

e1000e <3.15:  user_mtu + ETH_HLEN + ETH_FCS_LEN > adapter->max_hw_frame_size;

e1000e >=3.15: user_mtu + VLAN_HLEN + ETH_HLEN + ETH_FCS_LEN >
                   adapter->max_hw_frame_size;

Let's say the hardware has a limit of 4096 bytes. The user wishes to set an MTU of 
4078 bytes.

sky2: false, mtu is set

r8169: false, mtu is set

igb: true, mtu is not set

e1000e <3.15: false, mtu is set

e1000e <3.15: true, mtu is not set

So some drivers work with the 4078 MTU and some do not. They all had the same 
hardware limit, but their driver's software calculated limit was different. Am I crazy?

Thanks,
Michael