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Subject: Re: [PATCH] wifi: mac80211: Allow NSS change only up to capability
From:   Johannes Berg <johannes@sipsolutions.net>
To:     Rameshkumar Sundaram <quic_ramess@quicinc.com>
Cc:     linux-wireless@vger.kernel.org
Date:   Fri, 21 Oct 2022 13:42:35 +0200
In-Reply-To: <20221018044341.5453-1-quic_ramess@quicinc.com>
References: <20221018044341.5453-1-quic_ramess@quicinc.com>
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On Tue, 2022-10-18 at 10:13 +0530, Rameshkumar Sundaram wrote:
> Stations can update bandwidth/NSS change in
> VHT action frame with action type Operating Mode Notification.
> (IEEE Std 802.11-2020 - 9.4.1.53 Operating Mode field)
>=20
> For Operating Mode Notification, an RX NSS change to a value
> greater than AP's maximum NSS should not be allowed.
> Hence allow NSS change only up to maximum NSS that is negotiated
> and capped to AP's capability during association.

That seems reasonable. Might be worth noting in a comment in the code
which AP has such bugs though, just so we know it later.

>  	if (link_sta->pub->rx_nss !=3D nss) {
> -		link_sta->pub->rx_nss =3D nss;
> -		sta_opmode.rx_nss =3D nss;
> -		changed |=3D IEEE80211_RC_NSS_CHANGED;
> -		sta_opmode.changed |=3D STA_OPMODE_N_SS_CHANGED;
> +		cur_nss =3D link_sta->pub->rx_nss;
> +		link_sta->pub->rx_nss =3D 0;
> +		ieee80211_sta_set_rx_nss(link_sta);
> +		/* Do not allow an nss change to rx_nss greater than max_nss
> +		 * negotiated and capped to APs capability during association.
> +		 */
> +		if (nss <=3D link_sta->pub->rx_nss) {
> +			link_sta->pub->rx_nss =3D nss;

That, however, doesn't seem right. It means that you can only ever
reduce the RX NSS, not switch it around within the originally negotiated
range.

johannes