rt2x00: hardy netbook-lpia, signal strength is compressed

[Colin Ian King]

Signal strength range reported by the rt2x00 driver is very compressed and almost useless as a metric of signal strength.

The driver starts out with values of 50% for rx_percentage and tx_percentage. Given that, signal strength should behave as follows:

Network connection w/ 0% RSSI - 40/100, rising to 80/100 assuming 0% frame loss
Network connection w/ 50% RSSI - 50/100, rising to 90/100 assuming 0% frame loss
Network connection w/ 100% RSSI - 60/100, rising to 100/100 assuming 0% frame loss

This calculation is only really useful for a network that you're already connected to. For determining signal strength of other networks, I think the easiest thing to do is recompute the signal strength with the assumption that the rx + tx values are (a) always 50% and (b) not interesting. This effectively recenters the scale as follows:

recalculated strength = (reported strength - 40) * 5

Driver-reported strength: realculated signal strength:
40 0
45 25
50 50
55 75
60 100

(This calculation *doesn't* work for networks you're connected to, because it's not valid to assume that tx/rx success percentage is always 50% in that case)