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

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Date:   Wed, 1 Sep 2021 12:40:58 -0700
From:   Brian Norris <briannorris@...omium.org>
To:     Jonas Dreßler <verdre@...d.nl>
Cc:     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@...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 Mon, Aug 30, 2021 at 5:37 AM Jonas Dreßler <verdre@...d.nl> wrote:
>
> On the 88W8897 card it's very important the TX ring write pointer is
> updated correctly to its new value before setting the TX ready
> interrupt, otherwise the firmware appears to crash (probably because
> it's trying to DMA-read from the wrong place).
>
> Since PCI uses "posted writes" when writing to a register, it's not
> guaranteed that a write will happen immediately. That means the pointer
> might be outdated when setting the TX ready interrupt, leading to
> firmware crashes especially when ASPM L1 and L1 substates are enabled
> (because of the higher link latency, the write will probably take
> longer).
>
> So fix those firmware crashes by always forcing non-posted writes. We do
> that by simply reading back the register after writing it, just as a lot
> of other drivers do.
>
> There are two reproducers that are fixed with this patch:
>
> 1) During rx/tx traffic and with ASPM L1 substates enabled (the enabled
> substates are platform dependent), the firmware crashes and eventually a
> command timeout appears in the logs. That crash is fixed by using a
> non-posted write in mwifiex_pcie_send_data().
>
> 2) When sending lots of commands to the card, waking it up from sleep in
> very quick intervals, the firmware eventually crashes. That crash
> appears to be fixed by some other non-posted write included here.
>
> Signed-off-by: Jonas Dreßler <verdre@...d.nl>

This might be good for many cases, but please read this commit:

https://git.kernel.org/linus/062e008a6e83e7c4da7df0a9c6aefdbc849e2bb3
mwifiex: pcie: use posted write to wake up firmware

It's very much intentional that this is a posted write in some cases.

Without ensuring this doesn't regress, NAK from me.

Brian