wpa_supplicant seems to prefer 2.4GHz over 5GHz

[John Frankish]

> > According to the wpa_supplicant AP scoring, it would indeed seem that the 2.4GHz AP
> > is proffered since it has better SNR (although the SNR for both is good.
> > Generally, wpa_s considers any SNR above 30 as good).
> > 
> > a0:55:4f:66:d5:18 freq=2447 qual=0 noise=-89~ level=-49 snr=40* 
> > flags=0xb age=0 est=65000
> > a0:55:4f:66:d5:10 freq=5240 qual=0 noise=-92~ level=-67 snr=25 
> > flags=0xb age=0 est=390001
> > 
> > perhaps the scoring can be tweaked to more aggressively favor 5.2 in case of close good SNR. 
> > 
> SNR of 25 is not above the current GREAT_SNR (30) definition.
> Because of that, this happens to be just one dB too large a difference to allow estimated
> throughput to be considered since abs(snr_b - snr_a) == 5 in this case (that snr=40 gets
> dropped to GREAT_SNR = 30) and < 5 would have resulted in using the throughput estimate
> (of which the 5 GHz AP here has significantly higher due to possibility of using VHT80).
> 
> Taken into account the throughput estimates (which are much newer than the GREAT_SNR definition)
> are actually assuming the highest rate to be available with SNR of 24, it would sound reasonable
> to drop the GREAT_SNR value to something like 25. The 30 is already documented as being "a conservative value"..
> 
> If you (John) want to test the impact of such a change, modify the following line in
> wpa_supplicant/scan.c to use value 25 instead of 30:
> 
> #define GREAT_SNR 30
> 
> and rebuild wpa_supplicant.
> 
Using:

--- wpa_supplicant/scan.c.orig	2017-02-16 13:39:16.809997441 +0000
+++ wpa_supplicant/scan.c	2017-02-16 13:39:53.536663769 +0000
@@ -1738,7 +1738,7 @@
  * recommends 25 as a minimum SNR for 54 Mbps data rate. 30 is chosen here as a
  * conservative value.
  */
-#define GREAT_SNR 30
+#define GREAT_SNR 25
 
 #define IS_5GHZ(n) (n > 4000)

I now get:

Sorted scan results
a0:55:4f:66:d5:10 freq=5240 qual=0 noise=-92~ level=-67 snr=25* flags=0xb age=0 est=390001IEs
a0:55:4f:66:d5:18 freq=2447 qual=0 noise=-89~ level=-53 snr=36* flags=0xb age=0 est=65000IEs

..and connect to the 5GHz channel.

Thanks :)