DFS start_dfs_cac() failed, -1 / nl80211: Failed to start radar detection: -16 (Device or resource busy)

[Conrad Kostecki]

Dear Mailinglist,
I am trying to run hostapd acting in 5GHz mode with enable DFS. This seems not to be possible for me, as it fails.

I am using ath9k, AR9590. Kernel is 3.16.1 and hostapd version 2.2.

-- snip --
ACS: Ideal channel is 116 (5580 MHz) with total interference factor of 1.14549e-18
wlp7s0: ACS-COMPLETED freq=5580 channel=116
HT40: control channel: 116  secondary channel: 120
Completing interface initialization
Mode: IEEE 802.11a  Channel: 116  Frequency: 5580 MHz
DFS 2 channels required radar detection
DFS all channels available, (SKIP CAC): no
DFS 0 chans unavailable - choose other channel: no
wlp7s0: interface state ACS->DFS
DFS start CAC on 5580 MHz
wlp7s0: DFS-CAC-START freq=5580 chan=116 sec_chan=1, width=0, seg0=0, seg1=0, cac_time=60s
nl80211: Start radar detection (CAC) 5580 MHz (ht_enabled=1, vht_enabled=0, bandwidth=40 MHz, cf1=5590 MHz, cf2=0 MHz)
nl80211: Failed to start radar detection: -16 (Device or resource busy)
DFS start_dfs_cac() failed, -1
-- snip --

Why it does says, that device or resource is busy? When I run hostapd in 2.4GHz mode, it's working perfectly fine for me. Does anybody has some idea? I am not trying to run 2.4GHZ / 5.0GHz simultaneous.

Thanks!
Conrad