Re: [RFC 1/2] wireless: add ieee80211_asmdu_to_8023s

Johannes Berg
2009-11-27T09:20:58Z

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On Fri, 2009-11-27 at 10:52 +0800, Zhu Yi wrote:

> > Also -- you lost the extra TX headroom which I think mac80211 as an AP
> > requires since it could forward these frames? Or does that not happen?
> > Not sure right now why that was there to start with.
>
> Yes, I'd like to hear more feedback on this. I think it's a trade off
> between performance optimization and clean interface. As we already use
> dev_alloc_skb to reserve 32 bytes headroom, it should be enough for most
> of the current drivers.

Is it? I don't think so. Many drivers go up beyond that as far as I
know. Then some do different things like putting it in a different DMA
block.

> While for those drivers really need a bigger
> extra headroom and support Rx aggregation, this probably means
> ieee80211_skb_resize. But the resize should always happen for every
> packet from the IP stack, right?

No, davem and I optimised that away a long time ago via using
netdev->needed_headroom and netdev->needed_tailroom. It even works for
bridges and their slave devices, iirc.