linux-kernel - Re: [PATCH 1/3] wifi: mt76: mt7915: remove VHT160 capability on MT7915

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Date:   Fri, 29 Sep 2023 14:59:26 +0200
From:   Nicolas Cavallari <Nicolas.Cavallari@...en-communications.fr>
To:     Oleksandr Natalenko <oleksandr@...alenko.name>,
        linux-wireless@...r.kernel.org
Cc:     Felix Fietkau <nbd@....name>, Sasha Levin <sashal@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Linux regression tracking (Thorsten Leemhuis)" 
        <regressions@...mhuis.info>
Subject: Re: [PATCH 1/3] wifi: mt76: mt7915: remove VHT160 capability on
 MT7915

On 21/09/2023 07:02, Oleksandr Natalenko wrote:
> Hello Felix.
> 
> On středa 26. července 2023 11:17:02 CEST Felix Fietkau wrote:
>> The IEEE80211_VHT_CAP_EXT_NSS_BW value already indicates support for half-NSS
>> 160 MHz support, so it is wrong to also advertise full 160 MHz support.
>>
>> Fixes: c2f73eacee3b ("wifi: mt76: mt7915: add back 160MHz channel width support for MT7915")
>> Signed-off-by: Felix Fietkau <nbd@....name>
>> ---
>>   drivers/net/wireless/mediatek/mt76/mt7915/init.c | 1 -
>>   1 file changed, 1 deletion(-)
>>
>> diff --git a/drivers/net/wireless/mediatek/mt76/mt7915/init.c b/drivers/net/wireless/mediatek/mt76/mt7915/init.c
>> index ee976657bfc3..78552f10b377 100644
>> --- a/drivers/net/wireless/mediatek/mt76/mt7915/init.c
>> +++ b/drivers/net/wireless/mediatek/mt76/mt7915/init.c
>> @@ -414,7 +414,6 @@ mt7915_init_wiphy(struct mt7915_phy *phy)
>>   			if (!dev->dbdc_support)
>>   				vht_cap->cap |=
>>   					IEEE80211_VHT_CAP_SHORT_GI_160 |
>> -					IEEE80211_VHT_CAP_SUPP_CHAN_WIDTH_160MHZ |
>>   					FIELD_PREP(IEEE80211_VHT_CAP_EXT_NSS_BW_MASK, 1);
>>   		} else {
>>   			vht_cap->cap |=
>>
> 
> For some reason this got backported into the stable kernel:
> 
> ```
> $ git log --oneline v6.5.2..v6.5.4 -- drivers/net/wireless/mediatek/mt76/mt7915/
> c43017fbebcc3 wifi: mt76: mt7915: fix power-limits while chan_switch
> edb1afe042c74 wifi: mt76: mt7915: fix tlv length of mt7915_mcu_get_chan_mib_info
> 9ec0dec0baea3 wifi: mt76: mt7915: remove VHT160 capability on MT7915
> 0e61f73e6ebc0 wifi: mt76: mt7915: fix capabilities in non-AP mode
> 6bce28ce28390 wifi: mt76: mt7915: fix command timeout in AP stop period
> 7af917d4864c6 wifi: mt76: mt7915: rework tx bytes counting when WED is active
> feae00c6468ce wifi: mt76: mt7915: rework tx packets counting when WED is active
> 70bbcc4ad6544 wifi: mt76: mt7915: fix background radar event being blocked
> ```
> 
> and this broke my mt7915-based AP.
> 
> However, if I remove `[VT160]` capability from the hostapd config, things go back to normal. It does seem that 160 MHz still works even.
> 
> Is this expected?

I would say it is expected.

hostapd seems to solely rely on the VHT Supported Channel Width and does not 
seem to support the VHT Extended NSS BW stuff.  So it only knows about full VHT 
160 MHz support and not about half NSS VHT 160 MHz.

The hardware does not actually support full 160 MHz (despite the driver 
erroneously claiming support for it before this patch) so it make sense that 
hostapd fails to start the AP if the config file requests (full) VHT 160 MHz.

However, hostapd knows about half NSS HE 160 MHz and I suspect your 
configuration also requests HE 160 MHz, so 160 MHz works fine in HE but not in VHT.

In any case, it would help to know the hostapd version and your configuration file.