Re: [PATCH 2/2] mwifiex: Try waking the firmware until we get an interrupt

On Mon, Aug 30, 2021 at 3:39 PM Jonas Dreßler [off-list ref] wrote:

It seems that the firmware of the 88W8897 card sometimes ignores or
misses when we try to wake it up by reading 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 (unfortunately that's not unlikely with this card).

Turns out that most of the time the firmware actually didn't hang up,
but simply "missed" our wakeup request and doesn't send us an AWAKE

didn't

event.

Trying again to read the firmware status register after a short timeout
usually makes the firmware wake we up as expected, so add a small retry

wake up

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.

A good reproducer for this issue is letting the firmware sleep and wake
up in very short intervals, for example by pinging an device on the

a device

network every 0.1 seconds.

...

+       /* Access the fw_status register to wake up the device.
+        * Since the 88W8897 firmware sometimes appears to ignore or miss
+        * that wakeup request, we continue trying until we receive an
+        * interrupt from the card.
+        */
+       do {
+               if (mwifiex_write_reg(adapter, reg->fw_status, FIRMWARE_READY_PCIE)) {
+                       mwifiex_dbg(adapter, ERROR,
+                                   "Writing fw_status register failed\n");
+                       return -1;
+               }
+
+               n_tries++;
+
+               if (n_tries <= 15)
+                       usleep_range(400, 700);
+               else
+                       msleep(10);
+       } while (n_tries <= 50 && READ_ONCE(adapter->int_status) == 0);

NIH read_poll_timeout() from iopoll.h.

-- 
With Best Regards,
Andy Shevchenko