netdev - RE: Question of Network 1Gbps Intel Packet Split

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Date:	Wed, 23 Apr 2008 08:46:15 -0700
From:	"Brandeburg, Jesse" <jesse.brandeburg@...el.com>
To:	"Raz" <raziebe@...il.com>
Cc:	<netdev@...r.kernel.org>
Subject: RE: Question of Network 1Gbps Intel Packet Split

Raz wrote:
> I have been wondering whether anyone can answer the bellow two
> questions. Intel PCI-E 1Gbps cards have a feature for rx mode :
> "packet split". 

If you'd included something about e1000/e1000e in the message our
filters would have marked this so we could reply to you :-)

> so:
> 1. Is it true that when "packet split" is enabled, **ALL** packets
> will be splitted , header / payload ?

All packets that are 
a) recognized, i.e. IPv4, TCP, UDP, IPv6, etc
b) not too short (sometimes packets < 70 bytes or so are not split, at
least on 82571)
c) sometimes the header after the TCP/UDP header can be split too, like
maybe for NFS, you're welcome to read the manual for PCIe parts posted
at e1000.sf.net

> 2. Is is true that there is exactly one fragment when splitted ?

nr_frags will typically be 1, unless the frame is too short ( < 128 )
and then it is "unsplit" into the header buffer to avoid the page
free/allocation.

In the case of e1000/e1000e, splitting the packet is best used for jumbo
frames, and in that case multiple pages can be used to hold the packet,
with a single header buffer at skb->data, and nr_frags > 1

Hope this helps, and in the future you're welcome to post to
e1000-devel@...ts.sf.net for these kinds of specific to Intel hardware
questions.

  Jesse
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