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

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Message-ID: <f293c619399ba8bd60240879a20ee34db1248255.camel@sipsolutions.net>
Date:   Wed, 01 Sep 2021 19:07:58 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Heiner Kallweit <hkallweit1@...il.com>,
        Pali Rohár <pali@...nel.org>,
        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 Wed, 2021-09-01 at 18:51 +0200, Heiner Kallweit wrote:
> On 01.09.2021 17:51, Pali Rohár wrote:
> > 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).
> > > > > 
> 
> This sounds somehow like the typical case where you write DMA descriptors
> and then ring the doorbell. This normally requires a dma_wmb().
> Maybe something like that is missing here?

But it looks like this "TX ring write pointer" is actually the register?

However, I would agree that doing it in mwifiex_write_reg() is possibly
too big a hammer - could be done only for reg->tx_wrptr, not all the
registers?

Actually, can two writes actually cross on PCI?

johannes