Comment # 9 on bug 1213757 from Takashi Iwai

Once after confirming that the problem persists with the latest SP4 kernel,
another test is to swap the modules.  Namely, copy the kernel module file from
the working kernel to the broken kernel.  e.g. for replacing cfg80211 module,

  mkdir /lib/modules/5.14.21-$BROKEN/updates
  cp /lib/modules/5.14.21-$WORKING/kernel/net/wireless/cfg80211.ko.zst \
    /lib/modules/5.14.21-$BROKEN/updates/
  depmod -a 5.14.21-$BROKEN

and reboot/retest.  Thanks to kABI compatibility, most of modules can be
swapped among different SP4 kernels.  By checking piece by piece, you can
figure out which module became broken.  As far as I see, there was no code
change in the broadcom WiFi driver, hence something else in the network stack
might be the culprit.