Re: wireless vs. alignment requirements

On Sat, Nov 24, 2007 at 10:13:19PM +0100, Johannes Berg wrote:

Eight bytes really sucks for wireless, many things are multiples of four
and QoS vs. non-QoS frames have a multiple of four and common hardware
only adds two padding bytes to get it aligned on four bytes so there's
no easy way to get hardware to align it properly. Hmm.

Sorry I was wrong about the 8 bytes requirement.  Although the
IPv6 protocol does try to maintain an 8-byte alignment the Linux
stack never does anything that requires that.

So 4 bytes is enough.

However, the wireless core is definitely not out of the woods.
It needs to support variable hardware header lengths that are
not always a multiple of 4.

So here's my suggestion.  Modify the wireless core to fix up any
packets which aren't aligned correctly.  That should make it
work albeit in a way that's less than optimal.

Then for each driver where you care about this performance
(seriously I wouldn't for the speeds these things run at :),
pick the most common wireless hardware header length and have
the IP (or any other upper-level protocol) header aligned to
at least 4 bytes.  Or better if you know what hardware header
length that you're going to get (e.g., based on what mode you're
in) then do the skb_reserve accordingly.

It's a good thing these things aren't running at 10Gb :)

Cheers,
-- 
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