Re: [RFT] ath9k: multi-rate-retry fails at HW level

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Zefir Kurtisi <zefir.kurtisi@westermo.com> writes:

> CC += adrian
>
> On 24.11.20 15:45, Toke Høiland-Jørgensen wrote:
>> Zefir Kurtisi <zefku@westermo.com> 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