vc4.ko causes another misterious error

[Ryutaroh Matsumoto]

Hi Stefan, thanks again for your response!

> In order to narrow this down (more a general 5 GHz or a DFS specific
> issue), are you able to change temporary the channel of your AP to one
> of the following:
(deleted frequencies here)
> and retest with enabled vc4? Does the situation improve?

I should be able to do that. But before doing that, I'd like to confirm the following situation.
The reported problem between 5GHz WiFi and vc4.ko is observed with
plain Debian kernel 5.10.24 and Debian firmware-brcm80211 20201218-3.

The combination of plain Debian kernel 5.10.24 and Debian firmware-brcm80211 20201218-3
has another problem WITH and WITHOUT vc4. That is Bluetooth and 2.4GHz WiFi
cannot be used simultaneously ("rfkill"ing bluetooth let 2.4GHz WiFi works).

On the other hand, upgrading Debian firmware-brcm80211 to 20210208-4
(in testing) or 20210315-1 (in unstable) makes 5GHz WiFi completely unusable,
in the sense that no 5GHz SSID are shown by iw wlan0 scan.

Since versions 20210208-1 to 20210208-3 made wlan0 completely disappear
by not loading firmware for RPi4B WiFi, I noticed this situation last night...

In light of the above ugly situation, should I (and you) continue investigation
with Debian firmware-brcm80211 20201218-3?

Or should I install some firmware files by hand? If so, could you provide
some pointers to the firmware.

The above situation has been reported to the Debian BTS as:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=984844
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=984489
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=985632

Best regards, Ryutaroh