linux-kernel - RE: [PATCH V2 1/2] watchdog: imx7ulp: Strictly follow the sequence for wdog operations

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Message-ID: <DB3PR0402MB3916A03896012BC96A861882F5700@DB3PR0402MB3916.eurprd04.prod.outlook.com>
Date:   Wed, 29 Jul 2020 14:17:26 +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 V2 1/2] watchdog: imx7ulp: Strictly follow the sequence
 for wdog operations

Hi, Guenter


> Subject: Re: [PATCH V2 1/2] watchdog: imx7ulp: Strictly follow the sequence
> for wdog operations
> 
> On 7/28/20 9:50 PM, Anson Huang wrote:
> > Hi, Guenter
> >
> >
> >> Subject: Re: [PATCH V2 1/2] watchdog: imx7ulp: Strictly follow the
> >> sequence for wdog operations
> >>
> >> On 7/28/20 7:20 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>
> >>> ---
> >>> Changes since V1:
> >>> 	- use readl_poll_timeout_atomic() instead of usleep_ranges() since
> >>> IRQ is
> >> disabled.
> >>> ---
> >>>  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..7d2b12e 100644
> >>> --- a/drivers/watchdog/imx7ulp_wdt.c
> >>> +++ b/drivers/watchdog/imx7ulp_wdt.c
> >>> @@ -5,6 +5,7 @@
> >>>
> >>>  #include <linux/clk.h>
> >>>  #include <linux/io.h>
> >>> +#include <linux/iopoll.h>
> >>>  #include <linux/kernel.h>
> >>>  #include <linux/module.h>
> >>>  #include <linux/of.h>
> >>> @@ -36,6 +37,7 @@
> >>>  #define DEFAULT_TIMEOUT	60
> >>>  #define MAX_TIMEOUT	128
> >>>  #define WDOG_CLOCK_RATE	1000
> >>> +#define WDOG_WAIT_TIMEOUT	10000
> >>>
> >>>  static bool nowayout = WATCHDOG_NOWAYOUT;
> >> module_param(nowayout,
> >>> bool, 0000); @@ -48,17 +50,31 @@ struct imx7ulp_wdt_device {
> >>>  	struct clk *clk;
> >>>  };
> >>>
> >>> +static inline void imx7ulp_wdt_wait(void __iomem *base, u32 mask) {
> >>> +	u32 val = readl(base + WDOG_CS);
> >>> +
> >>> +	if (!(val & mask))
> >>> +		WARN_ON(readl_poll_timeout_atomic(base + WDOG_CS, val,
> >>> +						  val & mask, 0,
> >>> +						  WDOG_WAIT_TIMEOUT));
> >>
> >> I am not a friend of WARN_ON, especially in situations like this.
> >> Please explain why this is needed, and why a return of -ETIMEDOUT is
> >> not feasible.
> >
> > OK, I will use return value of -ETIMEOUT and handle it in the caller.
> >
> >>
> >> Also, I do not believe that a 10 milli-second timeout is warranted.
> >> This will need to be backed up by the datasheet.
> >>
> >
> > There is no such info provided in reference manual or datasheet, but I
> > just did an experiment, the unlock window is open in less than 1us
> > after sending unlock command, and ONLY last for ONLY 2~3 us then
> > close, the reconfiguration status bit will be set in less than 1us after register
> write. So what do you recommend for this timeout value? 100mS for safe?
> >
> 
> That would be even worse. You say yourself that the window is only open for a
> few microseconds. Now you are suggesting to hold the entire system hostage
> for up to
> 100 mS if the code misses that window for some reason. Based on what you
> said,
> 100 uS might be barely acceptable. 10-20 uS would be reasonable. But not
> 100 mS.

OK, I will use 20us.

Thanks,
Anson