netdev - Re: [RFC] gro: Is it ok to share a single napi from several devs ?

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Message-Id: <20100828.144130.189711704.davem@davemloft.net>
Date:	Sat, 28 Aug 2010 14:41:30 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...tta.com
Cc:	eric.dumazet@...il.com, jarkao2@...il.com,
	herbert@...dor.apana.org.au, netdev@...r.kernel.org
Subject: Re: [RFC] gro: Is it ok to share a single napi from several devs ?

From: Stephen Hemminger <shemminger@...tta.com>
Date: Sat, 28 Aug 2010 10:14:24 -0700

> The Marvell Yukon2 hardware supports two interfaces sharing a common interrupt.
> Therfore the sky2 driver has up to two net devices and a single NAPI per board.
> It is possible in a single invocation of the poll loop to process frames
> for both ports.  GRO works by combining received packets from identical
> flows over one NAPI interval. It is possible on sky2 that one packet
> could be processed for the first port, and the next packet processed was for
> second port and the two packets were related so that GRO would combine them.
> The check for the same dev is required to prevent this. Yes it is an unlikely
> corner case, but the purpose of GRO is to do aggregation but preserve the
> flow characteristics of the incoming traffic.

If that is true then GRO is going to refuse to merge every single
frame that arrives on the second port of a SKY2 device. :-)

This is because for the two ports, you allocate and register one NAPI
instance which uses only the first port's netdev pointer.

Therefore, when GRO compares napi->dev to skb->dev it will always not
match for packets coming from the second port since the netdev pointer
in skb->dev will be different.

Since netpoll does similar things, this means both NAPI and netpoll
cannot function properly with SKY2's second port.  It will only work
right on the first port.


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