Re: [RFC v2] mac80211: assign needed_headroom/tailroom for netdevs

Tomas Winkler
2008-05-05T17:15:23Z

On Mon, May 5, 2008 at 6:22 PM, Johannes Berg <johannes-cdvu00un1VgdHxzADdlk8Q@public.gmane.org> wrote:
>
> > I've did some measurement of the TX path on an embedded system (2.6.23 kernel)
> > When bridging packets from an ethernet device to wireless there is
> > loss of 12% in the CPU utilization and equivalent throughput reduction
> > in data packets that are checked and expanded in
> > ieee80211_subif_start_xmit function.
>
> Yeah, I figured.
>
>
> > The expansion is just due to bigger size of the 80211 header size, yet
> > the whole packet is reallocated and copied. As we are reaching 11n
> > rates 200bps and up this starts to be visible.
> > When header was reserved to the proper side already in the ethernet
> > driver the overhead was gone. Yet this doesn't seems to be a correct
> > solution for bridging or forwarding.
> > For example Iwlwifi HW supports scattered packets this would allow
> > just reallocate the header, providing data portion is aligned.
>
> Right. Not all hardware supports this though,
I would set NETIF_F_SG OR FRAGLIS to features...? (What actually is
the difference?)

but even when it does I
> don't see what we can do unless we want to do all this inside mac80211
> which I'd rather not.

Like Sending mac80211 header OOB as xmit function argument? This will
probably affect all the wme code as well...
Not good.

> SKBs don't have a way to say "I need N bytes writable headroom but I can
> do s/g operation for the rest" which is what we'd need. If that was
> available, we could even work with clones, then we could pull away the
> ethernet header and build the 802.11/device headers in separate buffers,
> demoting the old 'skb head' buffer to a data buffer...
>

Isn't this a requirement that header fits into a continuous buffer ?
It's sounds strange to me that there is no solution for efficient
bridging... Can bridging code handle this if we have native interface?

Thanks
Tomas

> johannes
>
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