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

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Message-Id: <20160405.113917.846241936652070162.davem@davemloft.net>
Date:	Tue, 05 Apr 2016 11:39:17 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	jakub.kicinski@...ronome.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH v4 net-next 01/15] nfp: correct RX buffer length
 calculation

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.

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.