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

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

> -----Original Message-----
> From: Marco Felsch <m.felsch@pengutronix.de>
> Sent: Thursday, August 25, 2022 3:50 PM
> To: Alice Guo (OSS) <alice.guo@oss.nxp.com>
> Cc: Guenter Roeck <linux@roeck-us.net>; 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 22-08-24, Alice Guo (OSS) wrote:
> >
> > > -----Original Message-----
> > > From: Marco Felsch <m.felsch@pengutronix.de>
> > > Sent: Wednesday, August 24, 2022 5:06 PM
> > > To: Alice Guo (OSS) <alice.guo@oss.nxp.com>
> > > Cc: Guenter Roeck <linux@roeck-us.net>; 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
> > >
> > > Hi Alice,
> > >
> > > On 22-08-24, Alice Guo (OSS) wrote:
> > > > > -----Original Message-----
> > > > > From: Marco Felsch <m.felsch@pengutronix.de>
> > > > > Sent: Wednesday, August 24, 2022 4:04 PM
> > > > > To: Alice Guo (OSS) <alice.guo@oss.nxp.com>
> > > > > Cc: Guenter Roeck <linux@roeck-us.net>; 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
> > > > >
> > > > > 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.
> > >
> > > You don't have to according the RM:
> > > 8<------------------------------------------------------------------
> > > ----
> > > 59.5.2 Disable Watchdog after Reset
> > >
> > > All of watchdog registers are unlocked by reset. Therefore, unlock
> > > sequence is unnecessary, but it needs to write all of watchdog
> > > registers to make the new configuration take effect. The code
> > > snippet below shows an example of disabling watchdog after reset.
> > > 8<------------------------------------------------------------------
> > > ----
> > >
> > > also the RM tells us:
> > > 8<------------------------------------------------------------------
> > > ----
> > > 59.4.3.1 Configuring the Watchdog Once
> > >
> > > The new configuration takes effect only after all registers except
> > > CNT are written after reset. Otherwise, the WDOG uses the reset
> > > values by default. If window mode is not used (CS[WIN] is 0),
> > > writing to WIN is not required to make the new configuration take effect.
> > > 8<------------------------------------------------------------------
> > > ----
> > >
> > > > Support for 32-bit unlock command write words in enabled in
> > > > imx7ulp_wdt_init now.
> > >
> > > So.. after reading the IMX7ULP RM, which was not my intention, I
> > > found out that most of the WDOG_CS regiter bits are write-once bits.
> > > This means if you didn't set it within the bootloader you still in case
> "59.4.3.1".
> > >
> > > So the imx7ulp_wdt_init() function just needs to check if the
> > > WDOG_CS_UPDATE bit was set. If it is not the case, then you need to
> > > write the WDOG_CS register as currently done. If the bit is set,
> > > than you need know that the bootloader did the job for you and you
> > > can exit
> > > imx7ulp_wdt_init() early. In both cases the unlock is not required.
> > >
> > > Can you please check/test if this is working for you?
> > >
> > > Regards,
> > >   Marco
> > >
> >
> > Hi Marco,
> >
> > Rom code has already configured the WDOG once, so we cannot use "
> > Configuring the Watchdog Once".
> 
> What? How does the ROM code configure the WDOG? Also this would be
> worth a comment within the code. Also still assume that this "16bit unlock"
> seq.
> is useless since you writing 32bit anyway.
> 
> Regards,
>   Marco

Hi Marco,

The ROM code of i.MX7ULP configures the WDOG to support 16-bit unlock command. I plan to add a comment to explain it in code, and keep "mb(); writel_relaxed; writel_relaxed; mb()" unchanged.

Best Regards,
Alice Guo