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From:   Chris Chiu <chiu@endlessm.com>
Date:   Mon, 27 May 2019 14:38:38 +0800
Message-ID: <CAB4CAwf7O9tyUwc+gPSZrBES+Bt7iTjhE1fbbVxYKqzjtmZBxw@mail.gmail.com>
Subject: Re: [RFC PATCH 2/2] rtl8xxxu: Add watchdog to update rate mask by
 signal strength
To:     Daniel Drake <drake@endlessm.com>
Cc:     Jes Sorensen <jes.sorensen@gmail.com>,
        Kalle Valo <kvalo@codeaurora.org>,
        David Miller <davem@davemloft.net>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        Linux Upstreaming Team <linux@endlessm.com>,
        Larry Finger <Larry.Finger@lwfinger.net>
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On Wed, May 22, 2019 at 2:38 AM Daniel Drake <drake@endlessm.com> wrote:
>
> On Fri, May 10, 2019 at 2:37 AM Chris Chiu <chiu@endlessm.com> wrote:
> > I've verified that multiple virtual interface can not work simultaneously in
> > STA mode. I assigned different mac address for different vifs, I can only
> > bring only one interface up. If I want to bring the second vif up, it always
> > complains "SIOCSIFFLAGS: Device or resource busy".
>
> Interesting. Can you go deeper into that so that we can be more
> confident of this limitation?
>
> ieee80211_open() is the starting point.
> ieee80211_check_concurrent_iface() is one candidate to generate -EBUSY
> but from inspection, I don't think that's happening in this case,
> perhaps you can keep following through in order to figure out which
> part of the code is not allowing the 2nd STA interface to come up.
>
> Daniel

The -EBUSY is returned by the ieee80211_check_combinations() in the
ieee80211_check_concurrent_iface() function which is invoked each time
doing ieee80211_open().
The ieee80211_check_combinations() returns the -EBUSY because of
cfg80211_check_combinations() will iterate all interfaces of different types
then checks the combination is valid or not, which in this case the number
of interface combination accumulated by cfg80211_iter_sum_ifcombos is 0
when I'm trying to bring up the second station interface.

Chris