"channel [21] (157) is disabled for use in AP mode"

[Zack Buhman]

I'd like to use channel 157, as I've done in the past. I've read [1] that
157 is perfectly valid to use in the US. Apparently hostapd disagrees
with this assesment.

I'm using hostapd hostapd v2.0 [2] using the nl80211 driver on ath9k
hardware.

iw reg get: http://ix.io/5o7
iw list: http://ix.io/5o6
hostapd -dd /etc/hostapd/hostapd.conf: http://ix.io/5o8

I'm not quite understanding how hostapd came to the conclusion that I
can't use channel 157, and, more importantly, how I can fix this.

[1] https://en.wikipedia.org/wiki/List_of_WLAN_channels#5.C2.A0GHz_.28802.11a.2Fh.2Fj.2Fn.29.5B13.5D
[2] http://w1.fi/releases/hostapd-2.0.tar.gz