I am trying to get HT40 @2.4GHz to work on an ar71xx platform and no matter what I try it always seems to limit to HT20. The exact same configs on an older build (early 2017) works perfectly but any later version, right up to current trunk does not work. FYI this is being used in the middle of nowhere with no other wifi signals detected (no other networks in range).
Also worth mentioning that I have confirmed that the 40MHz network is working in the 2017 build correctly and throughput tests also back it up. The same/opposite can be said for the recent builds, confirmed that it is only 20MHz and throughput is in line.
Might also be that mac80211 refuses HT40 in IBSS mode. Could be that one of the updates made the regdomain enforcement more strict, preventing the use of HT40.
Same problem when interface is an AP or a STA (to a 40MHz AP from 2017 build). Connects, but only at HT20. Whereas exact same config on 2017 build connects at 40MHz and data throughput speed confirms
I had a similar problem with enabling HT40 on ar71 in IBSS WPA2 mode.
I solved it by switching to Kernel 4.19.66 with latest snapshots using ath79 target by setting up an 802.11s mesh interface @HT20, then adding a vif AP @HT20. That comes up normally. If i then set the AP to HT40, the mesh interface will follow suit, and also come up with HT40. This sometimes even works automtically after reboot.
But performance is unfortunately limited to 6-9MiB/s on 11s on ath79 currently.
Have you tried setting an interface combination including an AP@HT40?
Just confirming that I have the same problem on 4.19.66 with ath79 - no 40MHz. I don't think it's the kernel, I tried 4.19.67 to 68. What is the best way to diagnose this?