Re: [PATCH v2] wifi: mt76: mt7921: fix txpower reporting

[Lucid Duck <lucid_duck@justthetip.ca>]

On Tue, 27 Jan 2026, Felix Fietkau wrote:
> phy->hw->conf.power_level is the user configured power level, not what
> the hardware is capable of transmitting.
>
> To fix it properly, I think you should determine the maximum rate power
> used in the loop within mt76_connac_mcu_rate_txpower_band (updated with
> each call).

Thanks for the feedback, Felix.

I looked into tracking the max rate power in mt76_connac_mcu_rate_txpower_band()
as you suggested. However, I found that this function is only called from
mt7921_set_sar_specs() during SAR configuration changes - it's not in the
normal connection code path when a user associates with an AP.

As an alternative, this patch updates txpower_cur in mt7921_bss_info_changed()
when BSS_CHANGED_TXPOWER is set. It uses info->txpower from bss_conf as the
primary source, with a fallback to chan->max_reg_power for cases where
bss_conf.txpower isn't populated (which can happen on newer kernels).

If I've misunderstood your suggestion or there's a better approach, I'd be
happy to revise.

Background:

I originally started debugging this driver because hcxdumptool was
crashing repeatedly on my new Alfa AWUS036AXML adapter. The crashes
appeared related to the bogus txpower values being reported (3 dBm or
67 dBm). After applying this patch, hcxdumptool runs without crashing
and txpower values are reported correctly.

Test Results:

Tested on Alfa AWUS036AXML (MT7921AU), kernel 6.18.6:

  Test            Expected    Actual      Status
  --------------  ----------  ----------  ------
  2.4GHz ch1      ~33 dBm     33.00 dBm   PASS
  5GHz ch100      ~27 dBm     27.00 dBm   PASS
  6GHz ch5        ~15 dBm     15.00 dBm   PASS
  Band switching  Updates     Correct     PASS
  Consistency     Stable      5/5         PASS
  hcxdumptool     No crash    8927 pkts   PASS

Regulatory baseline (Canada):
- 2.4GHz: 30 dBm limit + 3 dBm path delta = 33 dBm reported
- 5GHz DFS: 24 dBm limit + 3 dBm path delta = 27 dBm reported
- 6GHz: 12 dBm limit + 3 dBm path delta = 15 dBm reported

Before patch:
  $ iw dev wlan0 info | grep txpower
          txpower 3.00 dBm    # bogus value

After patch:
  $ iw dev wlan0 info | grep txpower
          txpower 33.00 dBm   # matches regulatory limit

Lucid