* Re: [rt2x00-users] Alignment between rt2500pci and mac80211
[not found] <4A190304.9050002@lente.dyndns.org>
@ 2009-05-24 9:43 ` Ivo van Doorn
0 siblings, 0 replies; only message in thread
From: Ivo van Doorn @ 2009-05-24 9:43 UTC (permalink / raw)
To: users; +Cc: Eric, linux-wireless, Johannes Berg
Hi,
My knowledge about this part of mac80211 is very limited,
so perhaps Johannes or somebody else on the linux-wireless mailinglist
can answer below questions (I already added them to the CC list). :)
> As posted already in:
> http://rt2x00.serialmonkey.com/phpBB/viewtopic.php?f=5&t=5150&sid=ba55e2676ff5ff92ab36e7e16c29e7de
> Rafaello discovered an alignment problem between rt2500pci and mac80211
> while operating with hostapd in master mode.
> I did some debugging to find the cause of this problem and discovered
> that inside the net/mac80211/tx.c:__ieee80211_tx_prepare() function
> __ieee80211_parse_tx_radiotap() is being called, which in itself does an
> skb_pull with iterator.max_length (being 14 in my case).
> I think this is one of the causes of an alignment offset of 2 bytes of
> the skb->data pointer (IF !IS_ALIGNED((int)skb->data,4)), as
> dma_map_single requests require at least alignment at the 4 byte
> boundary (on x86 at least, corrrect me if I'm wrong, as
> http://www.mjmwired.net/kernel/Documentation/DMA-API.txt only talks
> about the cache line boundary).
> Another cause is the __skb_queue_tail() call inside
> __ieee80211_tx_prepare(), unfortunuatly I couldnt always reproduce the
> latter one.
>
> I have a few questions:
> Is there any agreement between mac80211 and maintaing alignment at all?
> Should anything coming back from mac80211 be rechecked for device
> specific alignment ?
>
> According to the comment inside __ieee80211_parse_tx_radiotap(), that
> function should only be called for injected packages, however I couldn't
> discover (yet) where exactly the flag IEE80211_TX_CTL_INJECTED is
> set.Forcefully skipping that part causes the frame to fail, so
> apparently it's neccesary, however I fail to see the logic behind it.
> Is this something the driver explicitly flags / needs?
>
> All the the debugging was done on the daily wireless-testing tree from
> linuxwireless.org.
>
>
> Any comments / insights would be appreciated :-)
>
>
> kind regards,
>
> Eric
>
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