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

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Message-ID: <20210901155110.xgje2qrtq65loawh@pali>
Date:   Wed, 1 Sep 2021 17:51:10 +0200
From:   Pali Rohár <pali@...nel.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>,
        "open list:TI WILINK WIRELES..." <linux-wireless@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <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>
Subject: Re: [PATCH 1/2] mwifiex: Use non-posted PCI register writes

On Wednesday 01 September 2021 16:01:54 Jonas Dreßler wrote:
> On 8/30/21 2:49 PM, Andy Shevchenko wrote:
> > On Mon, Aug 30, 2021 at 3:38 PM 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 might be outdated when setting the TX ready interrupt, leading to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.
> >
> > Thanks for all this work!
> >
> > Nevertheless, do we have any commits that may be a good candidate to
> > be in the Fixes tag here?
> >
> 
> I don't think there's any commit we could point to, given that the bug is
> probably somewhere in the firmware code.

Then please add Cc: stable@...r.kernel.org tag into commit message. Such
bugfix is a good candidate for backporting into stable releases.

> >> Signed-off-by: Jonas Dreßler <verdre@...d.nl>