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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 currently does not apply
this shift, thus retrieves the incorrect maximum power level for a given
rate. In particular for MCS rates, the maximum power levels for CCK/OFDM
rates were used, e.g. maximum power for OFDM 0 was used for MCS 0. If
STBC is used, the power is mostly limited to 0 because the STBC table
is zeroed for legacy CCK/OFDM rates. Encountered this during testing of
our work-in-progress TPC per packet for ath9k.
This only has an effect when TPC is enabled in ath9k (tpc_enabled in
struct ath_hw) which defaults to false. In this case it has a
significant impact on the used TX-power, throughput + RSSI. Otherwise
the affected code is just skipped and TX-power is limited with the
hardware registers only. This patch fixes this table lookup.

Tested on OpenWrt (kernel 5.15.98, but backported ath9k driver) with
small desk setup using ath9k chips AR9280 and AR9580. Cap of TX-power is
working properly for all rates now, throughput and RSSI as expected,
equal to as if TPC was disabled.
Compile-tested with latest 6.3 kernel + allyesconfig.
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230330132159.758088-1-jelonek.jonas@gmail.com