Firmware regression: BCM43362 WiFi stops working when Bluetooth is connected

Firmware regression: BCM43362 WiFi stops working when Bluetooth is connected

[Cyan Dro]

Hi,

I have a AMPAK AP6476 with BCM43362 (WiFi) and BCM2076 (Bluetooth) in
an Intel (Bay Trail) tablet. Ever since upgrading linux-firmware a few
weeks ago, I've been having seemingly random WiFi issues. Sometimes,
WiFi stops responding while being connected, or fails to authenticate
to WPA2-PSK networks. Today I realized why it happens seemingly
random: Once connected to a Bluetooth device, WiFi stops working. As
soon as I disconnect the Bluetooth device, WiFi starts working again.

Before upgrading, I've been using firmware 5.90.195.89 (2013-04-22)
added in af2a88b901af ("brcm: add firmware for the BCM43362 WLAN
device"). With that firmware, everything (except P2P) works fine, even
when connected to Bluetooth device(s).

The problem occurs with all publicly available newer firmware
versions. I have tested the following:
  - 5.90.240.0 (2017-10-23): be1c535efe7e ("brcm: update firmware for
bcm43362 sdio")
  - 5.90.244 (2018-05-16): 253a573936ee ("brcm: update firmware for
bcm43362 sdio")
  - 5.90.247 (2018-09-19): https://community.cypress.com/docs/DOC-15932

As a reproduction example, I connect to a WiFi network and then start
pinging some host. Once I connect to a Bluetooth device, pings are
sent only very sporadically, with long delays. Few seconds after I
disconnect from the Bluetooth device, they start working correctly
again.

"Broken" firmware (here 5.90.244):
  - Ping log: https://paste.ubuntu.com/p/qM6RKNPHV5/ (note the ping
time and "Destination Host Unreachable" after connecting to a
Bluetooth device)
  - Kernel log*: https://paste.ubuntu.com/p/CPYJB9R9Qj/

"Working" firmware (5.90.195.89):
  - Ping log: https://paste.ubuntu.com/p/v2BCHtGNrP/ (pings are
transmitted continuously)
  - Kernel log*: https://paste.ubuntu.com/p/wHZw6hYshZ/

NVRAM: https://paste.ubuntu.com/p/wCkQGFZGTS/ (extracted from the stock system)

It seems like Bluetooth and WiFi are somehow interfering with each
other. Looking at the kernel logs I can see that brcmfmac seems to
wake up every second, but doesn't always receive a packet...
Since it works with the old firmware, something in it must have
changed that broke this. Any ideas?

Aside from the KRACK fixes, I would like to use the newer firmware
because it finally has working P2P that has been broken for many years
now with the old firmware. :(

Thanks in advance!
Cyan

* Note: For privacy reasons, I have omitted a part of the log that
contains WiFi scan results. It is from before the problem happens, so
I hope you don't mind. :)