Re: [PATCH v2] brcmfmac: add support for CQM RSSI notifications

Hi Arend,

On 1/15/21 3:10 PM, Arend Van Spriel wrote:

+ Johannes
- netdevs

On 1/14/2021 5:36 PM, 'Alvin Šipraga' via BRCM80211-DEV-LIST,PDL wrote:

quoted

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 
it raises a couple of questions. Looking into nl80211_set_cqm_rssi() [1] 
two behaviors are supported: 1) driver is provisioned with a threshold 
and hysteresis, or 2) driver is provisioned with high and low threshold. >
The second behavior is used when the driver advertises 
NL80211_EXT_FEATURE_CQM_RSSI_LIST *and* user-space provides more than 
one RSSI threshold. In both cases the same driver callback is being used 
so I wonder what is expected from the driver. Seems to me the driver 
would need to be able to distinguish between the two behavioral 
scenarios. As there is no obvious way I assume the driver should behave 
the same for both cases, but again it is unclear to me what that 
expected/required behavior is.

It will only provision the driver according to behaviour (1) if 0 or 1 
thresholds are being set AND the driver implements 
set_cqm_rssi_config(). But it says in the documentation for the 
set_cqm_rssi_range_config() callback[1] that it supersedes 
set_cqm_rssi_config() (or at least that there is no point in 
implementing _config if range_config is implemented). In that case, and 
if just one threshold is supplied (with a hysteresis), then a suitable 
range is computed by cfg80211_cqm_rssi_update() and provided to 
set_cqm_rssi_range_config(). I guess the implication here is that the 
two behaviours are functionally equivalent. I'm not sure I can argue for 
or against that because I don't really know what the semantics of the 
original API were supposed to be, but it seems reasonable.

As a starting point - and since the firmware behaviour is very close 
already - I implemented only set_cqm_rssi_range(). I have been testing 
with iwd, which by default sets just a single threshold and hysteresis, 
and the driver was sending notifications as would be expected.

[1] 
https://elixir.bootlin.com/linux/v5.10.7/source/include/net/cfg80211.h#L3780

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

 From what I gathered, the set_cqm_rssi_range_config(low, high) API 
should configure the driver to send a LOW/HIGH event to cfg80211 
whenever the RSSI is outside of the range [low, high]. cfg80211 seems to 
take care of how to deal with multiple thresholds then by calling back 
into _range_config from cfg80211_cqm_rssi_notify() to readjust the 
range. I could be oversimplifying things though and I would be glad to 
get some clarification.

Kind regards,
Alvin

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

[1] 
https://elixir.bootlin.com/linux/v5.10.7/source/net/wireless/nl80211.c#L11479 


---8<-----------------------------------------------------------------
commit 4a4b8169501b18c3450ac735a7e277b24886a651
Author: Andrew Zaborowski [off-list ref]
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 
have
     to provide a method (set_cqm_rssi_range_config) that configures a 
range
     set by a high and a low value.  Drivers have to call back when the 
RSSI
     goes out of that range and there's no additional event for each 
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 [off-list ref]
     Signed-off-by: Johannes Berg [off-list ref]