Re: [PATCH v2 2/2] mwifiex: Try waking the firmware until we get an interrupt

[Jonas Dreßler]

On 9/22/21 1:19 PM, Andy Shevchenko wrote:
> On Tue, Sep 14, 2021 at 01:48:13PM +0200, Jonas Dreßler wrote:
> > It seems that the firmware 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 (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 didn't send us 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.
> >
> > A good reproducer for this issue is letting the firmware sleep and wake
> > up in very short intervals, for example by pinging a device on the
> > network every 0.1 seconds.
>
> ...
>
> > +	do {
> > +		if (mwifiex_write_reg(adapter, reg->fw_status, FIRMWARE_READY_PCIE)) {
> > +			mwifiex_dbg(adapter, ERROR,
> > +				    "Writing fw_status register failed\n");
> > +			return -EIO;
> > +		}
> > +
> > +		n_tries++;
> > +
> > +		if (n_tries <= N_WAKEUP_TRIES_SHORT_INTERVAL)
> > +			usleep_range(400, 700);
> > +		else
> > +			msleep(10);
> > +	} while (n_tries <= N_WAKEUP_TRIES_SHORT_INTERVAL + N_WAKEUP_TRIES_LONG_INTERVAL &&
> > +		 READ_ONCE(adapter->int_status) == 0);
>
> Can't you use read_poll_timeout() twice instead of this custom approach?

I've tried this now, but read_poll_timeout() is not ideal for our 
use-case. What we'd need would be read->sleep->poll->repeat instead of 
read->poll->sleep->repeat. With read_poll_timeout() we always end up 
doing one more (unnecessary) write.

> > +	mwifiex_dbg(adapter, EVENT,
> > +		    "event: Tried %d times until firmware woke up\n", n_tries);