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Re: [RFC/RFT 1/2] mac80211: Add NEED_ALIGNED4_SKBS hw flag

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On 2015-12-17 11:04, Johannes Berg wrote:
> On Thu, 2015-12-17 at 10:20 +0100, Janusz Dziedzic wrote:
>> HW/driver should set NEED_ALIGNED4_SKBS flag in case require
>> aligned skbs to four-byte boundaries.
>> 
>> Before we have to do memmove() in the driver before
>> pass this to HW and memmove() back in tx completion.
>> This patch allow to save CPU and skip such memmoves.
>> For each skb we called memmove(ieee80211_hdrsize()) twice.
> 
> IMHO this is pretty awful from a code complexity POV. You also forgot
> to update fast-xmit maximum header length.
> 
> Note that we (iwlwifi) also kinda need this, but essentially solve it
> with the DMA engine. Can't ath9k do the same?
I tried that approach a few years ago, but it turned out to make the
hardware unstable, causing random DMA lockups.

- Felix
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