I have a number of wireless repeaters in operation for 18 months or so. I use the Huawei HG556a, a bcm63xx box with an Atheros AR9223 wifi radio. Typically the WWAN connection is the slowest, and weakest, link as the router providing Internet access is furthest away. Seems to me that the Airtime Fairness patch will result in the WWAN link getting less time to talk, not a good idea in a wireless repeater.
In any case, a build of trunk with the airtime fairness patch applied will reliable fail (reboot) within minutes if the WWAN source router broadcasts legacy wifi modes (b & g). The problem is worse if I build SMP rather than generic. Removing the Airtime Fairness patch resolves the issue.
I did a make of trunk two days ago and flashed it on 4 HG556a wireless repeaters. I included an init script to disable ATF at runtime as suggested here by @jow. The problem has vanished. Very many thanks.
Current builds (with the latest wireless-testing and this patch seemed to have made things worse. Where I could first do 5-7 days without clients getting disconnected and not reconnecting anymore, with build r3436-90728c7 this starts happening within 24 hours.
Airtime_flags at 0 makes this issue disappear completely. I would really like to use airtime fairness (it makes a positive difference in my situation), but it has to be reliable as well.