netdev - Re: [PATCH v4 net-next 01/15] nfp: correct RX buffer length calculation

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Message-ID: <20160405182456.1dfbb3e7@jkicinski-Precision-T1700>
Date:	Tue, 5 Apr 2016 18:24:56 +0100
From:	Jakub Kicinski <jakub.kicinski@...ronome.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH v4 net-next 01/15] nfp: correct RX buffer length
 calculation

On Tue, 05 Apr 2016 11:39:17 -0400 (EDT), David Miller wrote:
> From: Jakub Kicinski <jakub.kicinski@...ronome.com>
> Date: Fri,  1 Apr 2016 22:06:37 +0100
> 
> > When calculating the RX buffer length we need to account for
> > up to 2 VLAN tags and up to 8 MPLS labels.  Rounding up to 1k
> > is an relic of a distant past and can be removed.  While at
> > it also remove trivial print statement.
> > 
> > Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>  
> 
> I disagree with the MPLS aspect of this change.
> 
> VLAN is special, in that when the hardware supports VLAN properly, the
> VLAN header doesn't eat into the MTU and is sort of "transparent".
> 
> But MPLS doesn't work that way.
> 
> MPLS is in the main frame and takes up MTU space.

Makes sense.  RFC3032 counts MPLS label stack as Frame Payload.
 
> Therefore I see no reason to increase the buffer length by 8 * MPLS
> which is just a rediculous amount of wasted space.
> 
> I'm not applying this without at least some more explanations about
> why exactly you need to account for these values in the commit message.

It's just what FW guys asked me for.  I'll try to find out what their
reasoning was.  

I have a patch queued up which gets rid of unconditionally reserving
64B for firmware prepend, I guess that made me too excited to pay
attention to the fact the accounting for MPLS is indeed questionable...