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Subject: Re: [PATCH v2] brcmfmac: add support for CQM RSSI notifications
To:     =?UTF-8?Q?Alvin_=c5=a0ipraga?= <ALSI@bang-olufsen.dk>,
        Arend van Spriel <aspriel@gmail.com>,
        Franky Lin <franky.lin@broadcom.com>,
        Hante Meuleman <hante.meuleman@broadcom.com>,
        Chi-hsien Lin <chi-hsien.lin@infineon.com>,
        Wright Feng <wright.feng@infineon.com>,
        Chung-hsien Hsu <chung-hsien.hsu@infineon.com>,
        Kalle Valo <kvalo@codeaurora.org>,
        Andrew Zaborowski <andrew.zaborowski@intel.com>
Cc:     "linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>,
        "brcm80211-dev-list.pdl@broadcom.com" 
        <brcm80211-dev-list.pdl@broadcom.com>,
        "SHA-cyfmac-dev-list@infineon.com" <SHA-cyfmac-dev-list@infineon.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20210114163641.2427591-1-alsi@bang-olufsen.dk>
From:   Arend Van Spriel <arend.vanspriel@broadcom.com>
Message-ID: <2adec5d6-fbc9-680c-01d6-25f83327bf21@broadcom.com>
Date:   Fri, 15 Jan 2021 15:10:57 +0100
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+ Johannes
- netdevs

On 1/14/2021 5:36 PM, 'Alvin =C5=A0ipraga' via BRCM80211-DEV-LIST,PDL wrote=
:
> Add support for CQM RSSI measurement reporting and advertise the
> NL80211_EXT_FEATURE_CQM_RSSI_LIST feature. This enables a userspace
> supplicant such as iwd to be notified of changes in the RSSI for roaming
> and signal monitoring purposes.

The more I am looking into this API the less I understand it or at least=20
it raises a couple of questions. Looking into nl80211_set_cqm_rssi() [1]=20
two behaviors are supported: 1) driver is provisioned with a threshold=20
and hysteresis, or 2) driver is provisioned with high and low threshold.

The second behavior is used when the driver advertises=20
NL80211_EXT_FEATURE_CQM_RSSI_LIST *and* user-space provides more than=20
one RSSI threshold. In both cases the same driver callback is being used=20
so I wonder what is expected from the driver. Seems to me the driver=20
would need to be able to distinguish between the two behavioral=20
scenarios. As there is no obvious way I assume the driver should behave=20
the same for both cases, but again it is unclear to me what that=20
expected/required behavior is.

With behavior 2) some processing is done in cfg80211 itself by=20
cfg80211_cqm_rssi_update() which is called from nl80211_set_cqm_rssi()=20
upon NL80211_CMD_SET_CQM and cfg80211_cqm_rssi_notify() called by=20
driver. If I look at that it matches pretty close what our firmware is=20
doing. The difference is that our firmware avoids RSSI oscillation with=20
a time constraint between RSSI events whereas cfg80211 uses the hysteresis.

So before moving forward, I hope Johannes can chime in and clarify=20
things. Added the commit message introducing the extended feature below.=20
It mentions backward compatibility, but it only considers the extended=20
feature setting when user-space provides more than one threshold.=20
However, when the drivers set the extended feature is expects (low,=20
high) and (threshold, hysteresis) if not. So it seems the extended=20
feature should have precedence over the number of thresholds provided by=20
user-space.

Regards,
Arend

[1]=20
https://elixir.bootlin.com/linux/v5.10.7/source/net/wireless/nl80211.c#L114=
79

---8<-----------------------------------------------------------------
commit 4a4b8169501b18c3450ac735a7e277b24886a651
Author: Andrew Zaborowski <andrew.zaborowski@intel.com>
Date:   Fri Feb 10 10:02:31 2017 +0100

     cfg80211: Accept multiple RSSI thresholds for CQM

     Change the SET CQM command's RSSI threshold attribute to accept any
     number of thresholds as a sorted array.  The API should be backwards
     compatible so that if one s32 threshold value is passed, the old
     mechanism is enabled.  The netlink event generated is the same in both
     cases.

     cfg80211 handles an arbitrary number of RSSI thresholds but drivers=20
have
     to provide a method (set_cqm_rssi_range_config) that configures a rang=
e
     set by a high and a low value.  Drivers have to call back when the RSS=
I
     goes out of that range and there's no additional event for each=20
time the
     range is reconfigured as there was with the current one-threshold API.

     This method doesn't have a hysteresis parameter because there's no
     benefit to the cfg80211 code from having the hysteresis be handled by
     hardware/driver in terms of the number of wakeups.  At the same time
     it would likely be less consistent between drivers if offloaded or
     done in the drivers.

     Signed-off-by: Andrew Zaborowski <andrew.zaborowski@intel.com>
     Signed-off-by: Johannes Berg <johannes.berg@intel.com>

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