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

[Alice Guo (OSS)]

> -----Original Message-----
> From: Marco Felsch <m.felsch at pengutronix.de>
> Sent: Wednesday, August 24, 2022 4:04 PM
> To: Alice Guo (OSS) <alice.guo at oss.nxp.com>
> Cc: Guenter Roeck <linux at roeck-us.net>; wim at linux-watchdog.org;
> shawnguo at kernel.org; s.hauer at pengutronix.de; festevam at gmail.com;
> linux-arm-kernel at lists.infradead.org; linux-kernel at vger.kernel.org;
> dl-linux-imx <linux-imx at nxp.com>; kernel at pengutronix.de;
> linux-watchdog at vger.kernel.org
> Subject: Re: [PATCH 2/7] watchdog: imx7ulp: Add explict memory barrier for
> unlock sequence
> 
> Hi Alice,
> 
> On 22-08-24, Alice Guo (OSS) wrote:
> 
> ...
> 
> > > > > 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.
> 
> Please don't move this into the bootloader, enabling it within the init seq. is
> just fine. If you move it into the bootloader then you can't ensure that the bit is
> set since there are plenty of bootloaders out there.
> 
> As I said, just drop the "16bit" unlock sequence from the init function because
> the unlock is handled just fine in all the watchdog_ops.
> 
> Regards,
>   Marco

Hi Marco,

Sorry, I did not tell you that all watchdog control bits, timeout value, and window value cannot be set until the watchdog is unlocked. Support for 32-bit unlock command write words in enabled in imx7ulp_wdt_init now.

Best Regards,
Alice Guo