netdev - Re: [PATCH v3] i40e: Prevent setting MTU if greater than MFS

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Date: Tue, 12 Mar 2024 11:01:48 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Erwan Velu <erwanaliasr1@...il.com>
Cc: Erwan Velu <e.velu@...teo.com>, Jesse Brandeburg
 <jesse.brandeburg@...el.com>, Tony Nguyen <anthony.l.nguyen@...el.com>, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] i40e: Prevent setting MTU if greater than MFS

Hi,

On Tue, 2024-03-12 at 10:42 +0100, Erwan Velu wrote:
> Commit 6871a7de705b6f6a4046f0d19da9bcd689c3bc8e from iPXE project is

Please use the checkpatch-friendly commit reference: <12char hash>
("<title>")

> setting the MFS to 0x600 = 1536.
> 
> At boot time the i40e driver complains about it with
> the following message but continues.
> 
> 	MFS for port 1 has been set below the default: 600
> 
> If the MTU size is increased, the driver accept it but large packets will not
> be processed by the firmware generating tx_errors. The issue is pretty
> silent for users. i.e doing TCP in such context will generates lots of
> retransmissions until the proper window size (below 1500) will be used.
> 
> To fix this case, it would have been ideal to increase the MFS,
> via i40e_aqc_opc_set_mac_config, incoming patch will take care of it.
> 
> At least, this commit prevents setting up an MTU greater than the current MFS.
> It will avoid being in the position of having an MTU set to 9000 on the
> netdev with a firmware refusing packets larger than 1536.

This looks like a legit fix that should target the 'net' tree, @Tony:
do you agree? 

If so, Erwan, please include a suitable fixes tag in the next revision.
Please include into the subj prefix a suitable target tree. I think
this should go first via the intel tree for testing, so 'iwl-net'
should fit.

In any case please respect the 24h grace period when posting on netdev:

https://elixir.bootlin.com/linux/latest/source/Documentation/process/maintainer-netdev.rst#L399

Cheers,

Paolo