rt2x00 & mac80211: correct usage of ieee80211_beacon_get_tim?

[Helmut Schaa <helmut.schaa@googlemail.com>]

Hi,

I've just reviewed the beacon handling in rt2x00 in AP mode and experienced
some inconsistencies. The DTIM count is not correctly updated: sometimes
multiple beacons are sent out using the same DTIM count.

rt2x00 calls ieee80211_beacon_get_tim right after the current beacon was
sent out to fetch the next one. However, rt2x00 also implements the set_tim
callback and updates the beacon in each call to set_tim. As far as I
understood the code in mac80211 the set_tim callback is called when the
first frame for a powersaving station gets queued.

Since every call to ieee80211_beacon_get_tim updates the DTIM count the
following can happen (assuming a DTIM period of 2):

- the hw sends out the current beacon (DTIM count == 0)
- call to ieee80211_beacon_get_tim fetches the next beacon (DTIM count == 1)
- the first frame for a PS STA gets queued -> set_tim
- again call ieee80211_beacon_get_tim (DTIM count == 0)
- hw sends out the beacon with incorrect DTIM count

A proper way of fixing this issue would be not to use the set_tim callback but
just fetch the next beacon right before it gets send out (like ath* does).
However, that's not easily possible with rt2x00 devices older then rt2800 as
they only generate beacon_done interrupts (which is obviously too late for
fetching the current beacon ;) ).

So, is the current implementation in rt2x00 supposed to work and mac80211
needs fixing?

Could we add a parameter to ieee80211_beacon_get_tim that indicates if a _new_
beacon should be generated or if the _current_ beacon should be updated in
response to the set_tim callback?

Any other ideas?

Thanks,
Helmut