Re: [PATCH 1/2] mwifiex: Use non-posted PCI register writes

[Andy Shevchenko <andy.shevchenko@gmail.com>]

On Thu, Sep 23, 2021 at 6:28 PM Jonas Dreßler <verdre@v0yd.nl> wrote:
>
> On 9/22/21 2:50 PM, Jonas Dreßler wrote:
> > On 9/20/21 7:48 PM, Brian Norris wrote:
> >> On Sat, Sep 18, 2021 at 12:37 AM Jonas Dreßler <verdre@v0yd.nl> wrote:
> >>> Thanks for the pointer to that commit Brian, it turns out this is
> >>> actually the change that causes the "Firmware wakeup failed" issues that
> >>> I'm trying to fix with the second patch here.
> >>
> >> Huh. That's interesting, although I guess it makes some sense given
> >> your theory of "dropped writes". FWIW, this strategy (post a single
> >> write, then wait for wakeup) is the same used by some other
> >> chips/drivers too (e.g., ath10k/pci), although in those cases card
> >> wakeup is much much faster. But if the bus was dropping writes
> >> somehow, those strategies would fail too.
> >>
> >>> Also my approach is a lot messier than just reverting
> >>> 062e008a6e83e7c4da7df0a9c6aefdbc849e2bb3 and also appears to be blocking
> >>> even longer...
> >>
> >> For the record, in case you're talking about my data ("blocking even
> >> longer"): I was only testing patch 1. Patch 2 isn't really relevant to
> >> my particular systems (Rockchip RK3399 + Marvell 8997/PCIe), because
> >> (a) I'm pretty sure my system isn't "dropping" any reads or writes
> >> (b) all my delay is in the read-back; the Rockchip PCIe bus is waiting
> >> indefinitely for the card to wake up, instead of timing out and
> >> reporting all-1's like many x86 systems appear to do (I've tested
> >> this).
> >>
> >> So, the 6ms delay is entirely sitting in the ioread32(), not a delay
> >> loop.
> >>
> >> I haven't yet tried your version 2 (which avoids the blocking read to
> >> wake up; good!), but it sounds like in theory it could solve your
> >> problem while avoiding 6ms delays for me. I intend to test your v2
> >> this week.
> >>
> >
> > With "blocking even longer" I meant that (on my system) the delay-loop
> > blocks even longer than waking up the card via mwifiex_read_reg() (both
> > are in the orders of milliseconds). And given that in certain cases the
> > card wakeup (or a write getting through to the card, I have no idea) can
> > take extremely long, I'd feel more confident going with the
> > mwifiex_read_reg() method to wake up the card.
> >
> > Anyway, you know what's even weirder with all this: I've been testing
> > the first commit of patch v2 (so just the single read-back instead of
> > the big hammer) together with 062e008a6e83e7c4da7df0a9c6aefdbc849e2bb3
> > reverted for a good week now and haven't seen any wakeup failure yet.
> > Otoh I'm fairly sure the big hammer with reading back every write wasn't
> > enough to fix the wakeup failures, otherwise I wouldn't even have
> > started working on the second commit.
> >
> > So that would mean there's a difference between writing and then reading
> > back vs only reading to wake up the card: Only the latter fixes the
> > wakeup failures.
> >
> >>> Does anyone have an idea what could be the reason for the posted write
> >>> not going through, or could that also be a potential firmware bug in the
> >>> chip?
> >>
> >> I have no clue about that. That does sound downright horrible, but so
> >> are many things when dealing with this family of hardware/firmware.
> >> I'm not sure how to prove out whether this is a host bus problem, or
> >> an endpoint/firmware problem, other than perhaps trying the same
> >> module/firmware on another system, if that's possible.
> >>
> >> Anyway, to reiterate: I'm not fundamentally opposed to v2 (pending a
> >> test run here), even if it is a bit ugly and perhaps not 100%
> >> understood.
> >>
> >
> > I'm not 100% sure about all this yet, I think I'm gonna try to confirm
> > my older findings once again now and then we'll see. FTR, would you be
> > fine with using the mwifiex_read_reg() method to wake up the card and
> > somehow quirking your system to use write_reg()?
> >
> >> Brian
> >>
> >
>
> Okay, so I finally managed to find my exact reproducer for the bug again:
>
> 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
>
> Using this reproducer I was able to clear things up a bit:
>
> - There still are wakeup failures when using (only) mwifiex_read_reg()
> to wake the card, so there's no weird difference between waking up using
> read vs write+read-back
>
> - Just calling mwifiex_write_reg() once and then blocking until the card
> wakes up using my delay-loop doesn't fix the issue, it's actually
> writing multiple times that fixes the issue
>
> These observations sound a lot like writes (and even reads) are actually
> being dropped, don't they?



-- 
With Best Regards,
Andy Shevchenko