Re: Query on skb headroom and bridging ethernet and wlan

[Stephen Hemminger <shemminger@xxxxxxxxxx>]

On Wed, 20 Apr 2011 15:12:55 +0530
Yogesh Ashok Powar <yogeshp@xxxxxxxxxxx> wrote:

> Hello David/Johannes,
> 
>       Any thoughts/inputs on this?
> 
> Thanks
> Yogesh
> On Thu, Apr 14, 2011 at 01:46:45AM -0700, Yogesh Ashok Powar wrote:
> > Hello All,
> > 	We have observed significant throughput drop on TX path on
> > embedded system when bridging packets from ethernet to wireless side.
> > Our drivers on ethernet and wireless are MV643XX and MWL8K, respectively.
> > 
> > We found that mac80211 end up calling ieee80211_skb_resize (from
> > net/mac80211/tx.c) for every data packet as the skb_headroom was few
> > bytes short than the actual requirement.
> > 
> > We are considering two possible solutions to the problem:
> > 1. Use the scheme developed by Johannes and David.
> > 2. Address the issue by tuning the required extra tx headroom in the mwl8k
> > wireless driver.
> > 
> > The scheme developed by Johannes and David is discussed at the thread started by
> > Johannes on "mac80211: assign needed_headroom/tailroom for netdevs".
> > 
> > Can be seen at
> > http://kerneltrap.com/mailarchive/linux-netdev/2008/5/4/1719104
> > 
> > In one of the reply, David Miller has proposed a solution for such cases; please
> > see http://lists.openwall.net/netdev/2008/05/05/133
> > 
> > For completeness I am adding David's patch after few modifications, mainly
> > replacing old variables with the updated ones to make compilation error free.
> > 
> > With the patch below and a change in ethernet driver to use netdev_alloc_skb, 
> > overhead of ieee80211_skb_resize can be avoided on our setup.
> > 
> > We would like to know, why is this patch (or something similar) not upstream?
> > Or, are there issues that are difficult to resolve?
> > 
> > Thanks
> > Yogesh Powar

You are on wrong list for driver questions. This should go to netdev@xxxxxxxxxxxxxxx
If you have a closed system (ie no other drivers), then just increasing tx headroom
would be simplest. Having too much headroom is harmless.


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