linux-kernel - RE: [PATCH 1/2] watchdog: imx7ulp: Strictly follow the sequence for wdog operations

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Message-ID: <DB3PR0402MB3916744794CDCCE7BC332147F5730@DB3PR0402MB3916.eurprd04.prod.outlook.com>
Date:   Tue, 28 Jul 2020 23:36:03 +0000
From:   Anson Huang <anson.huang@....com>
To:     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>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "festevam@...il.com" <festevam@...il.com>,
        "linux-watchdog@...r.kernel.org" <linux-watchdog@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     dl-linux-imx <linux-imx@....com>
Subject: RE: [PATCH 1/2] watchdog: imx7ulp: Strictly follow the sequence for
 wdog operations

Hi, Guenter


> Subject: Re: [PATCH 1/2] watchdog: imx7ulp: Strictly follow the sequence for
> wdog operations
> 
> On 7/27/20 11:42 PM, Anson Huang wrote:
> > According to reference manual, the i.MX7ULP WDOG's operations should
> > follow below sequence:
> >
> > 1. disable global interrupts;
> > 2. unlock the wdog and wait unlock bit set; 3. reconfigure the wdog
> > and wait for reconfiguration bit set; 4. enabel global interrupts.
> >
> > Strictly follow the recommended sequence can make it more robust.
> >
> > Signed-off-by: Anson Huang <Anson.Huang@....com>
> > ---
> >  drivers/watchdog/imx7ulp_wdt.c | 29 +++++++++++++++++++++++++++++
> >  1 file changed, 29 insertions(+)
> >
> > diff --git a/drivers/watchdog/imx7ulp_wdt.c
> > b/drivers/watchdog/imx7ulp_wdt.c index 7993c8c..b414ecf 100644
> > --- a/drivers/watchdog/imx7ulp_wdt.c
> > +++ b/drivers/watchdog/imx7ulp_wdt.c
> > @@ -4,6 +4,7 @@
> >   */
> >
> >  #include <linux/clk.h>
> > +#include <linux/delay.h>
> >  #include <linux/io.h>
> >  #include <linux/kernel.h>
> >  #include <linux/module.h>
> > @@ -48,17 +49,32 @@ struct imx7ulp_wdt_device {
> >  	struct clk *clk;
> >  };
> >
> > +static inline void imx7ulp_wdt_wait(void __iomem *base, u32 mask) {
> > +	int retries = 100;
> > +
> > +	do {
> > +		if (readl_relaxed(base + WDOG_CS) & mask)
> > +			return;
> > +		usleep_range(200, 1000);
> > +	} while (retries--);
> 
> Sleep with interrupts disabled ? I can not imagine that this works well in a
> single CPU system. On top of that, it seems quite pointless.
> Either you don't want to be interrupted or you do, but sleeping with interrupts
> disabled really doesn't make sense. And does it really take 200-1000 uS for the
> watchdog subsystem to react, and sometimes up to 200 * 100 = 20 mS ? That
> seems highly unlikely. If such a delay loop is indeed needed, it should be
> limited by a time, not by number of repetitions.
> 
> Unless there is evidence that there is a problem that needs to be solved, I am
> not going to accept this code.
> 

Oops, this is a mistake of using sleep with interrupt disabled, sorry for that.
The best option is to use readl_relaxed_poll_timeout_atomic() to poll the status
bit, however, the i.MX7ULP watchdog is very special that the unlock window ONLY
open for several cycles, that means the unlock status bit will be set and then clear
automatically after those cycles, using readl_relaxed_poll_timeout_atomic() will
fail since there are many timeout handle code in it and the unlock window is open
and close during this timeout handle interval, so it fail to catch the unlock bit.

The ideal option is using atomic polling without any other timeout check to make
sure the unlock window is NOT missed, but I think Linux kernel will NOT accept
a while loop without timeout, and that is why I tried to use usleep_ranges(), but
obviously I made a mistake of using it with IRQ disabled.

Do you have any suggestion of how to handle such case? If the hardware ONLY
unlock the register for a small window, how to poll the status bit with timeout
handle and also make sure the timeout handle code as quick as possible to NOT
miss the window?

Thanks,
Anson