netdev - Re: [RFC] qeth: exploit gro for layer 3 driver

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Message-ID: <20100121134845.GA28645@tuxmaker.boeblingen.de.ibm.com>
Date:	Thu, 21 Jan 2010 14:48:45 +0100
From:	Blaschka <frank.blaschka@...ibm.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [RFC] qeth: exploit gro for layer 3 driver

On Thu, Jan 21, 2010 at 05:11:33AM -0800, David Miller wrote:
> From: Blaschka <frank.blaschka@...ibm.com>
> Date: Thu, 21 Jan 2010 13:37:22 +0100
> 
> > Since the qeth driver can not use NAPI I would like to hear
> > your opinion about following approach to exploit GRO.
> 
> Can you remind us why it can't use NAPI?
> 
> I personally find it hard to believe... :-)
Sure, let me try ...

on s/390 we neither have have a PCI bus nor a DMA transfer. We have
a common software component called qdio which does a DMA like data
transfer to/from the hardware. The inbound interface between qeth
and qdio is a tasklet running under the control of qdio. So the qeth
driver can not disable qdio IRQs (I guess this is a major reason why
not using NAPI) and poll the data.
qdio is also used for other s/390 non network drivers (e.g. fcp) so
changing this component is rather difficult and out of qeth scope.
The qdio framework already provides a bottomhalf so todays approach
to call netif_rx seems to be questionable anyway ...
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