Re: [PATCH] ath9k: fix per-packet TX-power cap for TPC

[Jonas Jelonek <jelonek.jonas@gmail.com>]

> On 30. Mar 2023, at 14:34, Toke Høiland-Jørgensen <toke@kernel.org> wrote:
> 
> Jonas Jelonek <jelonek.jonas@gmail.com> writes:
> 
>>> On 30. Mar 2023, at 11:31, Toke Høiland-Jørgensen <toke@kernel.org> wrote:
>>> 
>>> Jonas Jelonek <jelonek.jonas@gmail.com> writes:
>>> 
>>>> Fix incorrect usage of plain rate_idx as index into the max (power) per
>>>> rate lookup table.
>>>> 
>>>> For transmit power control (TPC), the ath9k driver maintains internal
>>>> tables (in struct ath_hw) to store the max allowed power level per rate.
>>>> They are used to limit a given TX-power according to regulatory and user
>>>> limits in the TX-path per packet. The tables are filled in a predefined
>>>> order, starting with values for CCK + OFDM rates and followed by the
>>>> values for MCS rates. Thus, the maximum power levels for MCS do not
>>>> start at index 0 in the table but are shifted by a fixed value.
>>>> 
>>>> The TX-power limiting in ath_get_rate_txpower did not apply this shift,
>>>> thus retrieved the incorrect maximum power level. For example, the
>>>> maximum power for OFDM rate 0 was used for MCS rate 0. If STBC was used,
>>>> the power was mostly limited to 0 because the STBC table is zeroed for
>>>> legacy CCK/OFDM rates. This patch fixes this table lookup.
>>>> 
>>>> Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
>>> 
>>> So what effect does this bug have in practice? Also, how did you test
>>> the patch? :)
>>> 
>> 
>> It actually may not have an effect in most of current practical applications. The
>> member ‘tpc_enabled' in struct ath_hw is by default set to false and thus, TPC
>> in ath9k is usually disabled. The code path will be skipped in that case. 
>> But it has an effect as I am still working on TPC per packet in ath9k as part of
>> research. Looking at my traces I saw that the TX-power is capped at 0 for
>> MCS rates 0-7 in case the STBC flag is set in struct ieee80211_tx_info. Thus,
>> transmission was significantly worse as usual, and I also was not able to 
>> manually set a proper TX-power via my testing API. With other rates it is working
>> fine. I also double-checked that it isn’t a problem of how TX-power is set and
>> reported with our upcoming API.
>> 
>> I tested this with my OpenWrt-based AP-STA desk setup using two different
>> ath9k wifi chips (AR9280, AR9580), Kernel 5.15.98. I guess that should be
>> sufficient as ath9k hasn’t changed that significantly since that and several
>> changes are already backported. I compile-tested it with latest 6.3 kernel
>> (+allyesconfig), the patch applied flawlessly and encountered no problems. 
>> After applying my patch, I could see that everything was working as expected.
>> Transmission performs equally as if TPC was disabled, and setting + reporting
>> TX-power was working again. Also verified this with some verbose debugging
>> in ath_get_rate_txpower to see which index + max power is used.
> 
> Alright, cool! Could you please add all this to the commit message and
> resubmit?
> 
> -Toke

Sure, please have a look at it again if I missed something :)

Jonas