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

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Date:   Tue, 23 Aug 2022 11:10:27 +0200
From:   Marco Felsch <m.felsch@...gutronix.de>
To:     "Alice Guo (OSS)" <alice.guo@....nxp.com>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        "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 22-08-23, Alice Guo (OSS) wrote:
> 
> 
> > -----Original Message-----
> > From: Guenter Roeck <groeck7@...il.com> On Behalf Of Guenter Roeck
> > Sent: Monday, August 22, 2022 10:04 PM
> > To: Marco Felsch <m.felsch@...gutronix.de>
> > Cc: Alice Guo (OSS) <alice.guo@....nxp.com>; wim@...ux-watchdog.org;
> > 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 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
> > >
> > > >
> 
> 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.

Also this is a explanation worth to be added to the commit message ;)

> 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