linux-kernel - Re: [PATCH v2 1/2] mwifiex: Use non-posted PCI write when setting TX ring write pointer

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Date:   Wed, 22 Sep 2021 14:08:39 +0200
From:   Jonas Dreßler <verdre@...d.nl>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
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@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        Maximilian Luz <luzmaximilian@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Pali Rohár <pali@...nel.org>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Johannes Berg <johannes@...solutions.net>,
        Brian Norris <briannorris@...omium.org>, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mwifiex: Use non-posted PCI write when setting TX
 ring write pointer

On 9/22/21 1:17 PM, Andy Shevchenko wrote:
> On Tue, Sep 14, 2021 at 01:48:12PM +0200, Jonas Dreßler 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). The issue is present in
>> the latest firmware version 15.68.19.p21 of the pcie+usb card.
> 
> Please, be consistent in the commit message(s) and the code (esp. if the term
> comes from a specification).
> 
> Here, PCIe (same in the code, at least that I have noticed, but should be done
> everywhere).
> 
>> 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 might be outdated when setting the TX ready interrupt, leading to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 using a non-posted write for
>> this specific register write. We do that by simply reading back the
>> register after writing it, just as a few other PCI drivers do.
>>
>> This fixes a bug where during rx/tx traffic and with ASPM L1 substates
> 
> Ditto. TX/RX.
> 
>> enabled (the enabled substates are platform dependent), the firmware
>> crashes and eventually a command timeout appears in the logs.
> 
> Should it have a Fixes tag?
> 

Don't think so, there's the infamous 
(https://bugzilla.kernel.org/show_bug.cgi?id=109681) Bugzilla bug it 
fixes though, I'll mention that in v3.

>> Cc: stable@...r.kernel.org
>> Signed-off-by: Jonas Dreßler <verdre@...d.nl>
> 
> ...
> 
>> -		/* Write the TX ring write pointer in to reg->tx_wrptr */
>> -		if (mwifiex_write_reg(adapter, reg->tx_wrptr,
>> -				      card->txbd_wrptr | rx_val)) {
>> +		/* Write the TX ring write pointer in to reg->tx_wrptr.
>> +		 * The firmware (latest version 15.68.19.p21) of the 88W8897
>> +		 * pcie+usb card seems to crash when getting the TX ready
>> +		 * interrupt but the TX ring write pointer points to an outdated
>> +		 * address, so it's important we do a non-posted write here to
>> +		 * force the completion of the write.
>> +		 */
>> +		if (mwifiex_write_reg_np(adapter, reg->tx_wrptr,
>> +				        card->txbd_wrptr | rx_val)) {
> 
>>   			mwifiex_dbg(adapter, ERROR,
>>   				    "SEND DATA: failed to write reg->tx_wrptr\n");
>>   			ret = -1;
> 
> I'm not sure how this is not a dead code.
> 
> On top of that, I would rather to call old function and explicitly put the
> dummy read after it
> 
> 		/* Write the TX ring write pointer in to reg->tx_wrptr */
> 		if (mwifiex_write_reg(adapter, reg->tx_wrptr,
> 				      card->txbd_wrptr | rx_val)) {
> 			...eliminate dead code in the following patch(es)...
> 		}
> 
> +		/* The firmware (latest version 15.68.19.p21) of the 88W8897
> +		 * pcie+usb card seems to crash when getting the TX ready
> +		 * interrupt but the TX ring write pointer points to an outdated
> +		 * address, so it's important we do a non-posted write here to
> +		 * force the completion of the write.
> +		 */
> 		mwifiex_read_reg(...);
> 
> Now, since I found the dummy read function to be present, perhaps you need to
> dive more into the code and understand why it exists.
> 

Interesting, I haven't noticed that mwifiex_write_reg() always returns 
0. So are you suggesting to remove that return value and get rid of all 
the "if (mwifiex_write_reg()) {}" checks in a separate commit?

As for why the dummy read/write functions exist, I have no idea. Looking 
at git history it seems they were always there (only change is that 
mwifiex_read_reg() started to handle read errors with commit 
af05148392f50490c662dccee6c502d9fcba33e2). My bet would be that they 
were created to be consistent with sdio.c which is the oldest supported 
bus type in mwifiex.