It seems that the PCIe+USB firmware (latest version 15.68.19.p21) of the
88W8897 card sometimes ignores or misses when we try to wake it up by
writing to the firmware status register. This leads to the firmware
wakeup timeout expiring and the driver resetting the card because we
assume the firmware has hung up or crashed.

Turns out that the firmware actually didn't hang up, but simply "missed"
our wakeup request and didn't send us an interrupt with an AWAKE event.

Trying again to read the firmware status register after a short timeout
usually makes the firmware wake up as expected, so add a small retry
loop to mwifiex_pm_wakeup_card() that looks at the interrupt status to
check whether the card woke up.

The number of tries and timeout lengths for this were determined
experimentally: The firmware usually takes about 500 us to wake up
after we attempt to read the status register. In some cases where the
firmware is very busy (for example while doing a bluetooth scan) it
might even miss our requests for multiple milliseconds, which is why
after 15 tries the waiting time gets increased to 10 ms. The maximum
number of tries it took to wake the firmware when testing this was
around 20, so a maximum number of 50 tries should give us plenty of
safety margin.

Here's a reproducer for those firmware wakeup failures I've found:

1) Make sure wifi powersaving is enabled (iw dev wlp1s0 set power_save on)
2) Connect to any wifi network (makes firmware go into wifi powersaving
mode, not deep sleep)
3) Make sure bluetooth is turned off (to ensure the firmware actually
enters powersave mode and doesn't keep the radio active doing bluetooth
stuff)
4) To confirm that wifi powersaving is entered ping a device on the LAN,
pings should be a few ms higher than without powersaving
5) Run "while true; do iwconfig; sleep 0.0001; done", this wakes and
suspends the firmware extremely often
6) Wait until things explode, for me it consistently takes <5 minutes

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=109681
Cc: stable@vger.kernel.org
Signed-off-by: Jonas Dreßler <verdre@v0yd.nl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211011133224.15561-3-verdre@v0yd.nl