(I don't seem to be able to log into Flyspray anymore, so I'm posting this here.)
I recently tried to change the wireless interface names of my VAP interfaces and I noticed that using a dot in the option
ifname breaks (some of) my interfaces. The problems seems to be with netifd which treats these interfaces as tagged VLAN interfaces while they are not. I know that it's common to use a dot to designate VLAN interfaces, but technically the dot bears no meaning and is a valid character in non-VLAN interfaces as well.
I don't think this is a huge issue, but maybe it should be documented that dots are only allowed in VLAN interfaces on OpenWrt. What made this a bit more problematic for me was that the breakage was not obvious. hostapd didn't show any error and wifi status showed both radios up. Yet, the SSIDs in question were not broadcasted anymore and the
ip command showed that the interfaces are down.
To complicate this further, the error did not occur on all radios but it seems to depend on the driver or hardware as well. With my ath9k based 2.4GHz radio, the dot in the interface name broke the VAP. But with my ath10k based 5GHz radio it continued to work (or at least broadcast the SSIDs). I would assume that the two drivers or radios handle VLAN interfaces differently (or one doesn't support VLAN tags at all).