Message-ID: <386c21d475215c95966bbac6b0853e2715081789.camel@sipsolutions.net>
Subject: Re: [PATCH] wifi: mac80211: check the control channel before
 downgrading the bandwidth
From:   Johannes Berg <johannes@sipsolutions.net>
To:     Michael-cy Lee =?UTF-8?Q?=28=E6=9D=8E=E5=B3=BB=E5=AE=87=29?= 
        <Michael-cy.Lee@mediatek.com>,
        "linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>
Cc:     Evelyn Tsai =?UTF-8?Q?=28=E8=94=A1=E7=8F=8A=E9=88=BA=29?= 
        <Evelyn.Tsai@mediatek.com>,
        "linux-mediatek@lists.infradead.org" 
        <linux-mediatek@lists.infradead.org>,
        Shayne Chen =?UTF-8?Q?=28=E9=99=B3=E8=BB=92=E4=B8=9E=29?= 
        <Shayne.Chen@mediatek.com>, "nbd@nbd.name" <nbd@nbd.name>,
        Money Wang =?UTF-8?Q?=28=E7=8E=8B=E4=BF=A1=E5=AE=89=29?= 
        <Money.Wang@mediatek.com>,
        "lorenzo@kernel.org" <lorenzo@kernel.org>,
        Ryder Lee <Ryder.Lee@mediatek.com>
Date:   Tue, 14 Mar 2023 08:42:59 +0100
In-Reply-To: <70f19f86043750c8f164c316496a8690709fc062.camel@mediatek.com>
References: <20221222091354.14050-1-michael-cy.lee@mediatek.com>
         <277f8db1f790931aaac0f94ba0ea69efcb6ba3ac.camel@sipsolutions.net>
         <70f19f86043750c8f164c316496a8690709fc062.camel@mediatek.com>
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Hi,

> To deal with such an error case, Extender=E2=80=99s STA interface would
> downgrade the channel bandwidth (from 80MHz) and run
> ieee80211_link_use_channel to check again until the bandwidth becomes
> the minimum 20MHz.=20

Correct, and then it fails.

> Since the control channels are different, downgrading the bandwidth
> cannot make ieee80211_link_use_channel to return 0.

Right.

> Finally, Extender=E2=80=99s
> STA interface bandwidth (ifmgd->flags) would use 20MHz afterward. (In
> the current kernel version, the flag is link->u.mgd.conn_flags)

Also correct, but does it matter?

> At this moment, a hostapd_cli channel switch command might be issued,
> making Extender's AP interface switch to RootAP's channel (ch36).
> Because the control channel now synchronizes, Extender's STA
> association attempt succeeds. However, Extender=E2=80=99s STA ifmgd->flag=
s
> remains in the fallback state and thus the operating bandwidth is 20MHz
> instead of 80MHz we expect.

So ... still not sure I understand.

Are you saying that

 1) you have a race, and hostapd switches channels while the client-side
    code is in the middle of this loop?

or

 2) the ifmgd->flags are not reset correctly for a new association?


johannes