Zefir Kurtisi <zefir.kurtisi@xxxxxxxxxxxx> writes: > CC += adrian > > On 24.11.20 15:45, Toke Høiland-Jørgensen wrote: >> Zefir Kurtisi <zefku@xxxxxxxxxxxx> writes: >> >>> Hi, >>> >>> I am running into a strange issue with the ath9k operating a 9590 >>> device which to me seems like a HW issue, but since work on rate >>> controllers is already going for decades, I hardly can imagine this >>> never showed up. >>> >>> The issue observed is this: the TX status descriptors never report >>> rateindex 1, it is always 0, 2, or 3, but never 1. >>> >>> I noticed this by overwriting the rate configuration provided by >>> minstrel to a static setup, e.g. (7,3)(5,3)(3,3)(1,3), all MCS. The >>> device operates as iperf client to a connected AP and continuously >>> transmits data. While at that, the attenuation between the endpoints >>> is gradually increased, expecting to see a gradual shift in the >>> reported TX status rateindex from 0 to 3. But nada, the values >>> reported are 0,2, and 3 - never 1. >>> >>> I double checked that the TX descriptors are correctly set with the >>> rates and retry counts - all looking sane. >>> >>> More obvious, after changing the rate configuration to >>> (7,3)(1,3)(5,3)(3,3) the expectation would be to have either 0 or 1 >>> reported as rateidx, since the transmission ought to be successful >>> with the lowest rate or never. Again all rates are reported but 1. >>> >>> Now the question for me is: what is the HW exactly doing with such a >>> configuration? Is it skipping the second rate, or is it just reporting >>> wrong? >> >> You should be able to see this by looking at the rates the frames are >> being sent at, shouldn't you? >> > Yes, did that and from there it points to that the second rate is just skipped. > > Here are some use cases and their sniffing results. Setup is a 11ng STA connected > to AP with the attenuation adjusted such that MCS 7 fails, while MCS 5 and below > succeed. Monitor is sniffing while sending a single ping from AP to STA. > > With a rate configuration of (7/2)(3/2)(1/2) we get: > 14:02:42.923880 9481489761us tsft 2412 MHz 11n -68dBm signal 65.0 Mb/s MCS 7 20 > MHz long GI RX-STBC0 -68dBm signal antenna 0 Data IV: e Pad 20 KeyID 0 > 14:02:42.923909 9481490037us tsft 2412 MHz 11n -69dBm signal 65.0 Mb/s MCS 7 20 > MHz long GI RX-STBC0 -69dBm signal antenna 0 Data IV: e Pad 20 KeyID 0 > 14:02:42.925244 9481491044us tsft 2412 MHz 11n -68dBm signal 13.0 Mb/s MCS 1 20 > MHz long GI RX-STBC0 -68dBm signal antenna 0 Data IV: e Pad 20 KeyID 0 > > > with (7/2)(1/2)(3/2): > 13:59:37.073147 9295637087us tsft 2412 MHz 11n -69dBm signal 65.0 Mb/s MCS 7 20 > MHz long GI RX-STBC0 -69dBm signal antenna 0 Data IV: c Pad 20 KeyID 0 > 13:59:37.073467 9295637438us tsft 2412 MHz 11n -69dBm signal 65.0 Mb/s MCS 7 20 > MHz long GI RX-STBC0 -69dBm signal antenna 0 Data IV: c Pad 20 KeyID 0 > 13:59:37.074591 9295638498us tsft 2412 MHz 11n -68dBm signal 26.0 Mb/s MCS 3 20 > MHz long GI RX-STBC0 -68dBm signal antenna 0 Data IV: c Pad 20 KeyID 0 > > and with (7/2)(3/2): > 14:04:27.269806 9585836783us tsft 2412 MHz 11n -69dBm signal 65.0 Mb/s MCS 7 20 > MHz long GI RX-STBC0 -69dBm signal antenna 0 Data IV: 10 Pad 20 KeyID 0 > 14:04:27.270342 9585837344us tsft 2412 MHz 11n -68dBm signal 65.0 Mb/s MCS 7 20 > MHz long GI RX-STBC0 -68dBm signal antenna 0 Data IV: 10 Pad 20 KeyID 0 > 14:04:27.271368 9585838370us tsft 2412 MHz 11n -68dBm signal 65.0 Mb/s MCS 7 20 > MHz long GI RX-STBC0 -68dBm signal antenna 0 Data IV: 10 Pad 20 KeyID 0 > [..] > > a total of 14 attempts at MCS 7 with the ping finally failing. > >>> Both possibilities have great impact, since upper layers (like >>> airtime) use the returned rateidx to calculate and configure operating >>> parameters at runtime. >> >> Have you actually observed any issues from this? If it's just skipping a >> rate, minstrel should still be able to make decisions based on the >> actual values returned, no? >> > The issues arise from the fact that the driver reports a > (tx-rateindex/tx-attemp-index) per TX descriptor, leaving the driver to calculate > what was put on air based on these two values. If one had rates set to > (7/2)(3/7)(1/2) and the TX status reports (tx-rateindex=2/tx-attempt-index=0), > driver assumes there were 10 attempts in total while in fact they were 3 when the > second rate is skipped. What direct effect this has on RC I can't grasp, but it > definitively falsifies statistics. > > Same goes for airtime: check how this falsifies its calculation in > ath_tx_count_airtime(). Ah, right, I was assuming that rates[1].count would be reset to zero somehow. Have you confirmed that the attempts actually go up on in the Minstrel stats for the skipped rate? > Also, the above mentioned is an immediate visible issue: if RC > provides two rates e.g. (7/3)(5/3) of which the first is too high and > the second is not even attempted, frames don't make it through. Yeah, rate control would likely take longer to converge to the right rate. I suppose if this is a hardware model-specific issue that a quirks bit could be added to instruct Minstrel to disregard the second index. But it does sound a bit odd; have you verified that it's consistent on different units of the same model (and not just a busted device)? -Toke
