netdev - Re: Question regarding protocol specific mtu for FCoE

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Date:	Wed, 3 Jun 2009 14:17:21 -0700 (Pacific Daylight Time)
From:	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
To:	Rick Jones <rick.jones2@...com>
cc:	"Zou, Yi" <yi.zou@...el.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"Leech, Christopher" <christopher.leech@...el.com>,
	"Dev, Vasu" <vasu.dev@...el.com>,
	"Love, Robert W" <robert.w.love@...el.com>,
	"Ma, Steve" <steve.ma@...el.com>,
	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>
Subject: Re: Question regarding protocol specific mtu for FCoE

> 1) the "HW MTU" used by the NIC and the guts of the driver
> 2) the legacy networking (eg IP etc) MTU passed up through the legacy path
> 3) the fcoe MTU passed up through the fcoe path
> 
> where 1 is the max of 2,3

Whatever is decided to do is going to be targeted at ixgbe first.  The 
scenario you outlined here is exactly how ixgbe hardware works:

- Set HLREG0 to enable jumbo frames.  This allows larger frames to be 
accepted, but doesn't limit them to any minimum size.

- Set MHADD (for 82598) or MAXFRS (for 82599 - same register, new name) to 
indicate what the absolute maximum frame size you will receive.  Again, 
doesn't limit the minimum size.

- The networking software can send any size up to MAXFRS (maximum frame 
size) and the hardware will happily receive it and pass it up the stack.

Cheers,
-PJ Waskiewicz
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