netdev - Re: [PATCH v2] net: add Fast Ethernet driver for PXA168.

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Date:	Tue, 10 Aug 2010 19:30:52 +0200
From:	Lennert Buytenhek <buytenh@...tstofly.org>
To:	Philip Rakity <prakity@...vell.com>
Cc:	Sachin Sanap <ssanap@...vell.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Ashish Karkare <akarkare@...vell.com>,
	Prabhanjan Sarnaik <sarnaik@...vell.com>,
	"eric.y.miao@...il.com" <eric.y.miao@...il.com>,
	Mark Brown <markb@...vell.com>
Subject: Re: [PATCH v2]  net: add Fast Ethernet driver for PXA168.

On Tue, Aug 10, 2010 at 09:24:23AM -0700, Philip Rakity wrote:

> >> * Headroom in SKB for 802.11 not included in the patch since that
> >>  varies based on 802.11 a/b/g/n.
> > 
> > I don't think this is true?
> > 
> > (The 11a/b/n on-the-air preambles are of different lengths (and are
> > sent at different rates), but that isn't visible to software.)
> 
> 802.11b/g headers are larger and if not handled by firmware in an
> offload engine are programmed by the driver.  There needs to be room
> for 
> 
> a) Frame Control (2 bytes)
> b) AID (2 bytes)
> c) address-1 (6 bytes)
> d) address-2 (6 bytes)
> e) address-3 (6 bytes)
> f) seqfrag  (2 bytes)
> g) address-4 (6 bytes) 
> 
> or 30 bytes.

Depends on the driver and/or whether you are doing WDS.

mwl8k always needs 30 bytes due to hardware pecularities (while the
4-address-with-QoS header is 32 bytes, the QoS field is always passed
in the TX descriptor), but other drivers might need 32 bytes (and not
30 bytes) for 4-address-with-QoS headers, or might not need 30 or 32
bytes for transmitting 3-address or 2-address frames.


> 11n needs more.

Really?  Pointer?


> Some of these fields ARE visible to s/w.

Where did I say that they are not?

What I said was that the _preambles_ are not visible to software.
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