Hey sorry to revive this old thread but I wanted to ask what is the current state of OpenWRT. Was any of the fixes mentioned above integrated? I am using an MT7915 and I have the exact same problem. Trying to use a 160Mhz wide channel in the UK (all of them need dfs it seems) the driver fails immediately (start_dfs_cac() failed, -1) and the device gets disabled.
To answer myself, turns out the MT7915 DBDC card I had does not support 160Mhz on 5Ghz. It is the case because it does only 2T2R on both 5Ghz and 2.4Ghz simultaneously and you need 4T4R for 160Mhz.
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