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

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Date:   Mon, 20 Sep 2021 10:48:21 -0700
From:   Brian Norris <briannorris@...omium.org>
To:     Jonas Dreßler <verdre@...d.nl>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Amitkumar Karwar <amitkarwar@...il.com>,
        Ganapathi Bhat <ganapathi017@...il.com>,
        Xinming Hu <huxinming820@...il.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Tsuchiya Yuto <kitakar@...il.com>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        "<netdev@...r.kernel.org>" <netdev@...r.kernel.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Maximilian Luz <luzmaximilian@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Pali Rohár <pali@...nel.org>
Subject: Re: [PATCH 1/2] mwifiex: Use non-posted PCI register writes

On Sat, Sep 18, 2021 at 12:37 AM Jonas Dreßler <verdre@...d.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.

> 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.

Brian