[bug report] wifi: mt76: mt7925: iw set txpower fixed accepted but ignored

[Bradley Pizzimenti]

Hi there maintainers,

`iw dev <iface> set txpower fixed N` returns success on mt7925 for any
N tested, but the reported txpower never changes from a stuck value of
3.00 dBm. The kernel accepts and ignores the call silently in both
directions (above and below the displayed value), and well below the
regulatory ceiling.

I'm aware there's prior art on the cosmetic 3.00 dBm display issue
(Razvan Grigore's v2 series, Feb 2025; Ming Yen Hsieh's txpower init
refactor, Sept 2025). What seems potentially distinct here is that the
user-issued `iw set txpower fixed N` itself is silently no-op'd,
separate from the reported-value question. Reporting as breadcrumbs
in case the second observation is a separate bug rather than the same
one.

Hardware
--------
MEDIATEK MT7925 [Filogic 360], 802.11be 2x2, PCI 14c3:7925
ASIC revision 0x79250000
Driver in use: mt7925e (in-tree)

Firmware (from dmesg at probe)
------------------------------
mt7925e 0000:01:00.0: HW/SW Version: 0x8a108a10,
                     Build Time: 20260106153007a
mt7925e 0000:01:00.0: WM Firmware Version: ____000000,
                     Build Time: 20260106153120
Files: mediatek/mt7925/WIFI_MT7925_PATCH_MCU_1_1_hdr.bin
       mediatek/mt7925/WIFI_RAM_CODE_MT7925_1_1.bin

Kernel
------
6.18.18-1-MANJARO (close to vanilla 6.18 stable; not yet tested on
wireless-next or nbd168/wireless HEAD -- happy to retest if needed,
but flagging the data point in case it helps as-is).

Tools: iw version 6.17

Regulatory
----------
$ iw reg get
country US: DFS-FCC
   ...
   (5730 - 5850 @ 80), (N/A, 30), (N/A), AUTO-BW
   ...

Connection context: 5GHz channel 161 (5805 MHz), 80 MHz, VHT-MCS,
NSS 1. So we are on a band with a 30 dBm regulatory cap.

Observed
--------
$ iw dev wlp1s0 info | grep txpower
        txpower 3.00 dBm

$ sudo iw dev wlp1s0 set txpower fixed 100   # 1 dBm
$ iw dev wlp1s0 info | grep txpower
        txpower 3.00 dBm

$ sudo iw dev wlp1s0 set txpower fixed 1500  # 15 dBm
$ iw dev wlp1s0 info | grep txpower
        txpower 3.00 dBm

$ sudo iw dev wlp1s0 set txpower auto
$ iw dev wlp1s0 info | grep txpower
        txpower 3.00 dBm

All four `set` invocations return exit code 0. The reported value
never moves.

Expected
--------
Either:
  - The reported txpower follows the requested value (or, where
    capped, the actual applied value with extack indicating the
    cap reason), or
  - The set call returns an error rather than silently ignoring the
    request.

Caveats
-------
- Not yet tested on wireless-next or nbd168/wireless HEAD. If a
  reproduction on a current dev tree would be useful, I can do that.
- I have not verified whether the actual radiated TX power changes
  in response to `set txpower fixed`; I am reporting only the
  user-visible behavior.

Thanks,
Bradley