Re: [PATCH 3/7] watchdog: imx7ulp_wdt: Check CMD32EN in wdog init

[Guenter Roeck <linux@roeck-us.net>]

On Tue, Aug 23, 2022 at 05:46:55AM +0000, Alice Guo (OSS) wrote:
> 
> 
> > -----Original Message-----
> > From: Guenter Roeck <groeck7@gmail.com> On Behalf Of Guenter Roeck
> > Sent: Monday, August 22, 2022 10:06 PM
> > To: Alice Guo (OSS) <alice.guo@oss.nxp.com>
> > Cc: wim@linux-watchdog.org; shawnguo@kernel.org;
> > s.hauer@pengutronix.de; festevam@gmail.com; kernel@pengutronix.de;
> > dl-linux-imx <linux-imx@nxp.com>; linux-watchdog@vger.kernel.org;
> > linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org
> > Subject: Re: [PATCH 3/7] watchdog: imx7ulp_wdt: Check CMD32EN in wdog
> > init
> > 
> > On Tue, Aug 16, 2022 at 12:36:39PM +0800, Alice Guo (OSS) wrote:
> > > From: Ye Li <ye.li@nxp.com>
> > >
> > > When bootloader has enabled the CMD32EN bit, switch to use 32bits
> > > unlock command to unlock the CS register. Using 32bits command will
> > > help on avoiding 16 bus cycle window violation for two 16 bits
> > > commands.
> > >
> > > Signed-off-by: Ye Li <ye.li@nxp.com>
> > > Signed-off-by: Alice Guo <alice.guo@nxp.com>
> > > Reviewed-by: Jacky Bai <ping.bai@nxp.com>
> > > Acked-by: Jason Liu <jason.hui.liu@nxp.com>
> > > ---
> > >  drivers/watchdog/imx7ulp_wdt.c | 15 ++++++++++-----
> > >  1 file changed, 10 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/drivers/watchdog/imx7ulp_wdt.c
> > > b/drivers/watchdog/imx7ulp_wdt.c index b8ac0cb04d2f..a0f6b8cea78f
> > > 100644
> > > --- a/drivers/watchdog/imx7ulp_wdt.c
> > > +++ b/drivers/watchdog/imx7ulp_wdt.c
> > > @@ -180,11 +180,16 @@ static int imx7ulp_wdt_init(void __iomem *base,
> > > unsigned int timeout)
> > >
> > >  	local_irq_disable();
> > >
> > > -	mb();
> > > -	/* unlock the wdog for reconfiguration */
> > > -	writel_relaxed(UNLOCK_SEQ0, base + WDOG_CNT);
> > > -	writel_relaxed(UNLOCK_SEQ1, base + WDOG_CNT);
> > > -	mb();
> > > +	val = readl(base + WDOG_CS);
> > > +	if (val & WDOG_CS_CMD32EN) {
> > > +		writel(UNLOCK, base + WDOG_CNT);
> > > +	} else {
> > > +		mb();
> > > +		/* unlock the wdog for reconfiguration */
> > > +		writel_relaxed(UNLOCK_SEQ0, base + WDOG_CNT);
> > > +		writel_relaxed(UNLOCK_SEQ1, base + WDOG_CNT);
> > > +		mb();
> > 
> > Now this is intermixing writel() with writel_relaxed(), making the code all but
> > impossible to understand.
> > 
> > Guenter
> 
> Hi Guenter,
> 
> Intermixing writel() with writel_relaxed() is unavoidable here. Because there cannot be a memory barrier between writing UNLOCK_SEQ0 and writing UNLOCK_SEQ1. This may be determined by hardware design.
> 

If it is indeed impossible to configure the watchdog for 32-bit
access mode, that needs to be explained in the code and backed up,
for example with a reference to the documentation. Similar,
it needs to be documented in the code why writel() does not work
here and why mb() is needed.

Thanks,
Guenter

> Best Regards,
> Alice Guo
>  
> > > +	}
> > >
> > >  	ret = imx7ulp_wdt_wait(base, WDOG_CS_ULK);
> > >  	if (ret)
> > > --
> > > 2.17.1
> > >