The fullmac firmware doesn't seem to populate a useful rssi indicator
in the RX descriptor, so if one plotted said values, they'd basically
look like garbage.
The reference driver implements a "get current rssi" firmware command
which I guess is really meant for station operation only (as hostap
operation would need rssi per station, not a single firmware read.)
So:
* populate sc_currssi during each calibration run;
* use this in the RX path instead of trying to reconstruct the RSSI
value and passing it around as a pointer;
* do up a quick hack to map the rssi hardware value to some useful
signal level;
* the survey results provide an RSSI value between 0..100, so just
do another quick hack to map it into some usefulish signal level;
* supply a faked noise floor - I haven't yet found how to pull it
out of the firmware.
The scan results and the station RSSI information is now more useful
for indicating signal strength / distance.
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