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

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Message-ID: <20220822140347.GA4087281@roeck-us.net>
Date:   Mon, 22 Aug 2022 07:03:47 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Marco Felsch <m.felsch@...gutronix.de>
Cc:     "Alice Guo (OSS)" <alice.guo@....nxp.com>,
        "wim@...ux-watchdog.org" <wim@...ux-watchdog.org>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "festevam@...il.com" <festevam@...il.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "linux-watchdog@...r.kernel.org" <linux-watchdog@...r.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@...gutronix.de>
> > > Sent: Tuesday, August 16, 2022 2:24 PM
> > > To: Alice Guo (OSS) <alice.guo@....nxp.com>
> > > Cc: wim@...ux-watchdog.org; linux@...ck-us.net; shawnguo@...nel.org;
> > > s.hauer@...gutronix.de; festevam@...il.com;
> > > linux-arm-kernel@...ts.infradead.org; linux-kernel@...r.kernel.org;
> > > dl-linux-imx <linux-imx@....com>; kernel@...gutronix.de;
> > > linux-watchdog@...r.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@....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
> 
> > 
> > Best Regards,
> > Alice Guo
> > 
> > > 
> > > >
> > > > Suggested-by: Ye Li <ye.li@....com>
> > > > Signed-off-by: Jacky Bai <ping.bai@....com>
> > > > Signed-off-by: Alice Guo <alice.guo@....com>
> > > > Reviewed-by: Ye Li <ye.li@....com>
> > > > ---
> > > >  drivers/watchdog/imx7ulp_wdt.c | 4 ++++
> > > >  1 file changed, 4 insertions(+)
> > > >
> > > > diff --git a/drivers/watchdog/imx7ulp_wdt.c
> > > > b/drivers/watchdog/imx7ulp_wdt.c index 014f497ea0dc..b8ac0cb04d2f
> > > > 100644
> > > > --- a/drivers/watchdog/imx7ulp_wdt.c
> > > > +++ b/drivers/watchdog/imx7ulp_wdt.c
> > > > @@ -179,9 +179,13 @@ static int imx7ulp_wdt_init(void __iomem *base,
> > > unsigned int timeout)
> > > >  	int ret;
> > > >
> > > >  	local_irq_disable();
> > > > +
> > > > +	mb();
> > > >  	/* unlock the wdog for reconfiguration */
> > > >  	writel_relaxed(UNLOCK_SEQ0, base + WDOG_CNT);
> > > >  	writel_relaxed(UNLOCK_SEQ1, base + WDOG_CNT);
> > > > +	mb();
> > > > +
> > > >  	ret = imx7ulp_wdt_wait(base, WDOG_CS_ULK);
> > > >  	if (ret)
> > > >  		goto init_out;
> > > > --
> > > > 2.17.1
> > > >
> > > >
> > > >
> >