On Thu, Feb 15, 2018 at 08:52:53AM +0000, Jean Pierre TOSONI wrote: > > -----Message d'origine----- > > De : quozl@xxxxxxxxxx [mailto:quozl@xxxxxxxxxx] > > Envoyé : jeudi 15 février 2018 08:21 > > À : Kalle Valo > > Cc : Jean Pierre TOSONI; linux-wireless@xxxxxxxxxxxxxxx; ath9k- > > devel@xxxxxxxxxxxxxxxx > > Objet : Re: [PATCH v2] ath9k: mark RSSI as invalid if frame received > > during channel setup > > > > On Thu, Feb 15, 2018 at 07:51:28AM +0200, Kalle Valo wrote: > > > James Cameron <quozl@xxxxxxxxxx> writes: > > > > > >> On Wed, Feb 14, 2018 at 04:26:42PM +0000, Jean Pierre TOSONI > > wrote: > > >>> ath9k returns a wrong RSSI value for frames received > > >>> in a 30ms time window after a channel change. The > > >>> correct value is typically 10dB below the returned value. > > >> > > >> How was your correct value determined? > > >> > > 1) test setup: > Connecting the AP through coax and attenuators, then making 500 passive scans off-channel, then drawing an histogram of the beacon signals found by the chip. The off-channel period is 108 ms. The probability of being in the 30 ms window is 28%. The histogram shows 2 spikes, one large with the expected value, one small at around +10dB above. > > 2) value determination > Adjust the delay (CONFIG_HZ=250) by trial and error. 25ms was not enough to completely absorb the +10dB spike in the histogram, while 30ms was enough. > > Do you think of a better approach? No, I think your approach is fine. I was curious. Thanks for explaining. > Maybe the guys at Qualcomm know the correct value? Yes, that seems likely. > > >>> This was found with a Atheros AR9300 Rev:3 chip (WLE350NX / > > >>> JWX6083 cards), during offchannel scans. > > >>> > > >>> Mark the signal value as invalid in this case. > > >> > > >> Why not adjust by 10dB? > > I considered that also. But, > 1) during how much time should I do this adjustment? Around 30 ms after channel switch? Yes. If RSSI is so critical for your application, you'll do what you can to get a real RSSI rather than drop it. > 2) The histogram shows a scattering of the measures in a +/- 3dB range around the mean value. Perhaps a sampling error by the device. > So I could not decide for sure if it needed -9dB, -10dB or -11dB? > > > >> > > >> Speculating: in a typical card, RSSI is calculated by firmware > > from > > >> readings of ADCs attached to the receiver. Firmware may average > > >> several readings. Firmware may apply other offsets or > > calibrations, > > >> based on frequency and temperature. This sounds like a firmware > > >> problem. > > > > > > ath9k does not have firmware, only ath9k_htc has it. > > > > Heh. s/firmware/silicon implementation/g > > Oh well, if it's silicon problem, then it's a hardware problem, and > I am right to correct it that way, since there is no other way :-) Yes, if it can be reproduced by every ath9k. -- James Cameron http://quozl.netrek.org/
