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

["Tomas Winkler" <tomasw@gmail.com>]

On Mon, May 5, 2008 at 6:22 PM, Johannes Berg <johannes@sipsolutions.net> 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
>