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 > > _______________________________________________ > users mailing list > users@xxxxxxxxxxxxxxxxxxxxxxx > http://rt2x00.serialmonkey.com/mailman/listinfo/users_rt2x00.serialmonkey.com > -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
