On 23 May 2014 10:44, Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote: > On Fri, 2014-05-23 at 08:49 +0200, Michal Kazior wrote: > >> > But you'll WARN_ON() if they're actually not at the same time and you >> > grab a beacon (or for the template case, call csa_update) in the >> > meantime, right? I'd really like to have all those driver requirements >> > (e.g. to stop beaconing) better documented. >> >> Good point. I suppose it should be stated in the docs that once you >> reach ieee80211_csa_is_complete() being true you must not call >> ieee80211_beacon_get() nor ieee80211_csa_update_counter(). ath9k and >> ath10k conform to this. > > Right, it's still a bit strange that you just have to stop beaconing. > Dunno. I guess I don't care all that much since our driver won't support > multiple AP interfaces anyway :-) If you keep on beaconing for too long on a DFS channel after detecting a radar you risk breaking local law (ETSI and FCC specify quiescing periods). >> I wonder what driver should be supposed to look at before starting to >> beacon again? csa_active isn't well protected to be depended upon. If >> we should create a ieee80211_csa_is_active() that just checks if >> beacon->csa_counter_offset[0] != 0 (assuming my other csa counter >> patches are applied) then it's still racy: >> a) rcu_dereference() across ieee80211_csa_is_active(), _is_complete() >> and _beacon_get() can yield different beacon pointers >> b) cs_count <= 1 yields no beacon update (thus no counters/offsets, >> meaning both _csa_is_complete and _csa_is_active() are `false` thus >> suggesting driver can beacon as if nothing happened) >> >> We could fix (b) by simply not treating cs_count <= 1 so special and >> update the beacon anyway. For (a) to work we'd need either make a >> single-call do-all function or introduce an additional call and a >> generic pointer/structure to be passed to other functions so that a >> beacon pointer is consistent across calls. > > You could technically just return NULL in this period, but I don't know > how well drivers would cope with that (though technically they have to > cope with it due to memory allocation failures anyway) Sounds good to me. I guess you'd use ieee80211_csa_is_complete (or ieee80211_csa_update_counter) for template-based drivers to stop beaconing. Michał -- 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
