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

[Jonas Dreßler]

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 might be outdated when setting the TX ready interrupt, leading to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@xxxxxxxxxxxxxxx
> > Signed-off-by: Jonas Dreßler <verdre@xxxxxxx>
>
> ...
>
> > -		/* 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 +		 * interrupt but the TX ring write pointer points to an outdated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.