netdev - Re: [PATCH 0/10 REV5] Implement skb batching and support in IPoIB/E1000

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Date:	Mon, 17 Sep 2007 09:37:15 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	hadi@...erus.ca
Cc:	johnpol@....mipt.ru, herbert@...dor.apana.org.au, kaber@...sh.net,
	shemminger@...ux-foundation.org, jagana@...ibm.com,
	Robert.Olsson@...a.slu.se, rick.jones2@...com, xma@...ibm.com,
	gaagaan@...il.com, netdev@...r.kernel.org, rdreier@...co.com,
	peter.p.waskiewicz.jr@...el.com, mcarlson@...adcom.com,
	mchan@...adcom.com, general@...ts.openfabrics.org,
	kumarkr@...ux.ibm.com, tgraf@...g.ch, randy.dunlap@...cle.com,
	sri@...ibm.com
Subject: Re: [PATCH 0/10 REV5] Implement skb batching and support in
 IPoIB/E1000

From: jamal <hadi@...erus.ca>
Date: Mon, 17 Sep 2007 08:51:40 -0400

> On Sun, 2007-16-09 at 20:13 -0700, David Miller wrote:
> 
> > This only makes sense for devices which can 1) scatter-gather
> > and 2) checksum on transmit.  
> 
> If you have knowledge there are enough descriptors in the driver to
> cover all skbs you are passing, do you need to have #1? 
> Note i dont touch fragments, i am assuming the driver is smart enough to
> handle them otherwise it wont advertise it can handle scatter-gather

Yes, because you can have multiple descriptors per SKB
because we have the head part in skb->data and the rest
in the page vector.

Thus the device must be able to handle multiple descriptors
representing one packet.

> > Otherwise we make too many copies and/or passes over the data.
> 
> I didnt understand this last bit - you are still going to go over the
> list regardless of whether you call ->hard_start_xmit() once or
> multiple times over the same list, no? In the later case i am assuming
> a trimmed down ->hard_start_xmit()

If the device can't checksum, we have to pass over the data to
compute the checksum and stick it into the headers.

If the device can't scatter-gather, we have to allocate and
copy into a linear buffer.

Otherwise it's just bumping page reference counts and adjusting
offsets, no data touching at all.
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