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

Michal Kazior michal.kazior
Sun Aug 24 23:04:44 PDT 2014

On 24 August 2014 23:51, Conrad Kostecki <ck at conrad-kostecki.de> wrote:
> 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.

I believe ath9k is capable of running only at 20Mhz bandwidth (your
config is using 40Mhz) with radar detection.


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