RE: [PATCH 2/7] watchdog: imx7ulp: Add explict memory barrier for unlock sequence

["Alice Guo (OSS)" <alice.guo@oss.nxp.com>]

> -----Original Message-----
> From: Guenter Roeck <groeck7@gmail.com> On Behalf Of Guenter Roeck
> Sent: Tuesday, August 23, 2022 8:02 PM
> To: Marco Felsch <m.felsch@pengutronix.de>
> Cc: Alice Guo (OSS) <alice.guo@oss.nxp.com>; wim@linux-watchdog.org;
> shawnguo@kernel.org; s.hauer@pengutronix.de; festevam@gmail.com;
> linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org;
> dl-linux-imx <linux-imx@nxp.com>; kernel@pengutronix.de;
> linux-watchdog@vger.kernel.org
> Subject: Re: [PATCH 2/7] watchdog: imx7ulp: Add explict memory barrier for
> unlock sequence
> 
> On Tue, Aug 23, 2022 at 11:10:27AM +0200, Marco Felsch wrote:
> > On 22-08-23, Alice Guo (OSS) wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: Guenter Roeck <groeck7@gmail.com> On Behalf Of Guenter Roeck
> > > > Sent: Monday, August 22, 2022 10:04 PM
> > > > To: Marco Felsch <m.felsch@pengutronix.de>
> > > > Cc: Alice Guo (OSS) <alice.guo@oss.nxp.com>;
> > > > wim@linux-watchdog.org; shawnguo@kernel.org;
> > > > s.hauer@pengutronix.de; festevam@gmail.com;
> > > > linux-arm-kernel@lists.infradead.org;
> > > > linux-kernel@vger.kernel.org; dl-linux-imx <linux-imx@nxp.com>;
> > > > kernel@pengutronix.de; linux-watchdog@vger.kernel.org
> > > > Subject: Re: [PATCH 2/7] watchdog: imx7ulp: Add explict memory
> > > > barrier for unlock sequence
> > > >
> > > > On Mon, Aug 22, 2022 at 10:00:10AM +0200, Marco Felsch wrote:
> > > > > On 22-08-22, Alice Guo (OSS) wrote:
> > > > > > > -----Original Message-----
> > > > > > > From: Marco Felsch <m.felsch@pengutronix.de>
> > > > > > > Sent: Tuesday, August 16, 2022 2:24 PM
> > > > > > > To: Alice Guo (OSS) <alice.guo@oss.nxp.com>
> > > > > > > Cc: wim@linux-watchdog.org; linux@roeck-us.net;
> > > > > > > shawnguo@kernel.org; s.hauer@pengutronix.de;
> > > > > > > festevam@gmail.com; linux-arm-kernel@lists.infradead.org;
> > > > > > > linux-kernel@vger.kernel.org; dl-linux-imx
> > > > > > > <linux-imx@nxp.com>; kernel@pengutronix.de;
> > > > > > > linux-watchdog@vger.kernel.org
> > > > > > > Subject: Re: [PATCH 2/7] watchdog: imx7ulp: Add explict
> > > > > > > memory barrier for unlock sequence
> > > > > > >
> > > > > > > On 22-08-16, Alice Guo (OSS) wrote:
> > > > > > > > From: Jacky Bai <ping.bai@nxp.com>
> > > > > > > >
> > > > > > > > Add explict memory barrier for the wdog unlock sequence.
> > > > > > >
> > > > > > > Did you inspected any failures? It's not enough to say what
> > > > > > > you did, you need to specify the why as well.
> > > > > > >
> > > > > > > Regards,
> > > > > > >   Marco
> > > > > >
> > > > > > Hi,
> > > > > >
> > > > > > Two 16-bit writes of unlocking the Watchdog should be
> > > > > > completed within a
> > > > certain time. The first mb() is used to ensure that previous
> > > > instructions are completed.
> > > > > > The second mb() is used to ensure that the unlock sequence
> > > > > > cannot be
> > > > affected by subsequent instructions. The reason will be added in
> > > > the commit log of v2.
> > > > >
> > > > > Hi,
> > > > >
> > > > > I know what memory barriers are. My question was, did you see
> > > > > any issues? Since the driver is used mainline and no one reported
> issues.
> > > > >
> > > > > Also just don't use the *_relaxed() versions is more common,
> > > > > than adding
> > > > > mb() calls around *_relaxed() versions.
> > > > >
> > > >
> > > > Agreed with both. The series is a bit short in explaining _why_
> > > > the changes are made.
> > > >
> > > > Guenter
> > > >
> > > > > Regards,
> > > > >   Marco
> > > > >
> > > > > >
> > >
> > > Hi Guenter and Marco,
> > >
> > > 1. did you see any issues?
> > > This WDOG Timer first appeared in i.MX7ULP, no one report issues
> > > probably because few people use i.MX7ULP. This issue was found when
> > > we did a stress test on it. When we reconfigure the WDOG Timer,
> > > there is a certain probability that it reset. The reason for the
> > > error is that when WDOG_CS[CMD32EN] is 0, the unlock sequence is two
> > > 16-bit writes (0xC520, 0xD928) to the CNT register within 16 bus
> > > clocks, and improper unlock sequence causes the WDOG to reset.
> > > Adding mb() is to guarantee that two 16-bit writes are finished within 16
> bus clocks.
> >
> > After this explanation the whole imx7ulp_wdt_init() seems a bit buggy
> > because writel_relaxed() as well as writel() are 32bit access functions.
> > So the very first thing to do is to enable the 32-bit mode.
> >
> Agreed. This is much better than having extra code to deal with both 16-bit
> and 32-bit access.
> 
> > Also this is a explanation worth to be added to the commit message ;)
> >
> 
> Definitely. Also, the use of mb(), if it should indeed be needed, would have to
> be explained in a code comment.
> 
> Thanks,
> Guenter

Hi Marco and Guenter,

Thank you for your comments. I plan to enable support for 32-bit unlock command write words in bootloader. In this way, there is no need to distinguish whether the unlock command is a 32-bit command or a 16-bit command in driver.

Best Regards,
Alice Guo

> > > 2. Also just don't use the *_relaxed() versions is more common, than
> > > adding mb() calls around *_relaxed() versions.  Memory barriers
> > > cannot be added between two 16-bit writes. I do not know the reason.
> >
> > As written above, writel() as well as writel_relaxed() are not 16-bit
> > access functions.
> >
> > So to me it looks as you need first to ensure that 32-bit access mode
> > is enabled. After that you can write drop the to writel_relaxed()
> > functions and instead just write:
> >
> >    writel(UNLOCK, wdt->base + WDOG_CNT);
> >
> > Also why do we need to unlock the watchdog during imx7ulp_wdt_init()?
> > This is handled just fine during imx7ulp_wdt_enable() and during
> > imx7ulp_wdt_set_timeout(). So just drop those relaxed writes and
> > everything should be fine.
> >
> > Regards,
> >   Marco