Subject: [PATCH 2/2] watchdog: sama5d4: write the mode register in two steps

From: [email-masked] (Alexandre Belloni)
Date: Fri, 14 Sep 2018 12:27:32 +0200

On 14/09/2018 12:13:39+0200, Romain Izard wrote:
> The specification for SAMA5D2 and SAMA5D4 chips, that use this IP for
> their watchdog timer, has the following advice regarding the Mode Register:
> 
> "When setting the WDDIS bit, and while it is set, the fields WDV and WDD
> must not be modified."
> 
> I have observed on a board based on a SAMA5D2 chip that using any other
> timeout duration than the default 16s in the device tree will reset the
> board when the watchdog device is opened; this is probably due to ignoring
> the aforementioned constraint.
> 
> To fix this, read the content of the Mode Register before writing it,
> and split the access into two parts if WDV or WDD need to be changed.
> 

Hum, that is really weird because when I developed
015b528644a84b0018d3286ecd6ea5f82dce0180, I tested with a program doing:

	flags = WDIOS_DISABLECARD;
	ioctl(fd, WDIOC_SETOPTIONS, &flags);
	for (i = 16; i > 2; i--) {
		ioctl(fd, WDIOC_SETTIMEOUT, &i);
	}

	ioctl(fd, WDIOC_KEEPALIVE, &dummy);

	flags = WDIOS_ENABLECARD;
	ioctl(fd, WDIOC_SETOPTIONS, &flags);

	for (i = 16; i > 2; i--) {
		ioctl(fd, WDIOC_SETTIMEOUT, &i);
	}

This would immediately reproduce the reset when changing WDV/WDD with
WDDIS set.

I'll test again.

> Signed-off-by: Romain Izard <[email-masked]>
> ---
>  drivers/watchdog/sama5d4_wdt.c | 27 ++++++++++++++++++++-------
>  1 file changed, 20 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/watchdog/sama5d4_wdt.c b/drivers/watchdog/sama5d4_wdt.c
> index 1e93c1b0e3cf..1e05268ad94b 100644
> --- a/drivers/watchdog/sama5d4_wdt.c
> +++ b/drivers/watchdog/sama5d4_wdt.c
> @@ -46,7 +46,10 @@ MODULE_PARM_DESC(nowayout,
>  	"Watchdog cannot be stopped once started (default="
>  	__MODULE_STRING(WATCHDOG_NOWAYOUT) ")");
>  
> -#define wdt_enabled (!(wdt->mr & AT91_WDT_WDDIS))
> +#define wdt_enabled(reg) (!((reg) & AT91_WDT_WDDIS))
> +
> +#define wdt_different_counters(reg_a, reg_b) \
> +	(((reg_a) ^ (reg_b)) & (AT91_WDT_WDV | AT91_WDT_WDD))
>  
>  #define wdt_read(wdt, field) \
>  	readl_relaxed((wdt)->reg_base + (field))
> @@ -78,8 +81,11 @@ static void wdt_write_nosleep(struct sama5d4_wdt *wdt, u32 field, u32 val)
>  static int sama5d4_wdt_start(struct watchdog_device *wdd)
>  {
>  	struct sama5d4_wdt *wdt = watchdog_get_drvdata(wdd);
> +	u32 reg = wdt_read(wdt, AT91_WDT_MR);
>  
>  	wdt->mr &= ~AT91_WDT_WDDIS;
> +	if (!wdt_enabled(reg) && wdt_different_counters(reg, wdt->mr))
> +		wdt_write(wdt, AT91_WDT_MR, reg & ~AT91_WDT_WDDIS);
>  	wdt_write(wdt, AT91_WDT_MR, wdt->mr);
>  
>  	return 0;
> @@ -88,8 +94,11 @@ static int sama5d4_wdt_start(struct watchdog_device *wdd)
>  static int sama5d4_wdt_stop(struct watchdog_device *wdd)
>  {
>  	struct sama5d4_wdt *wdt = watchdog_get_drvdata(wdd);
> +	u32 reg = wdt_read(wdt, AT91_WDT_MR);
>  
>  	wdt->mr |= AT91_WDT_WDDIS;
> +	if (wdt_enabled(reg) && wdt_different_counters(reg, wdt->mr))
> +		wdt_write(wdt, AT91_WDT_MR, wdt->mr & ~AT91_WDT_WDDIS);
>  	wdt_write(wdt, AT91_WDT_MR, wdt->mr);
>  
>  	return 0;
> @@ -122,7 +131,7 @@ static int sama5d4_wdt_set_timeout(struct watchdog_device *wdd,
>  	 * If the watchdog is enabled, then the timeout can be updated. Else,
>  	 * wait that the user enables it.
>  	 */
> -	if (wdt_enabled)
> +	if (wdt_enabled(wdt->mr))
>  		wdt_write(wdt, AT91_WDT_MR, wdt->mr & ~AT91_WDT_WDDIS);
>  
>  	wdd->timeout = timeout;
> @@ -186,13 +195,17 @@ static int sama5d4_wdt_init(struct sama5d4_wdt *wdt)
>  	 * If the watchdog is already running, we can safely update it.
>  	 * Else, we have to disable it properly.
>  	 */
> -	if (wdt_enabled) {
> +	reg = wdt_read(wdt, AT91_WDT_MR);
> +	if (wdt_enabled(reg)) {
> +		if (!wdt_enabled(wdt->mr))
> +			wdt_write_nosleep(wdt, AT91_WDT_MR,
> +					  wdt->mr & ~AT91_WDT_WDDIS);
>  		wdt_write_nosleep(wdt, AT91_WDT_MR, wdt->mr);
> -	} else {
> -		reg = wdt_read(wdt, AT91_WDT_MR);
> -		if (!(reg & AT91_WDT_WDDIS))
> +	} else if (wdt_enabled(wdt->mr)) {
> +		if (wdt_different_counters(reg, wdt->mr))
>  			wdt_write_nosleep(wdt, AT91_WDT_MR,
> -					  reg | AT91_WDT_WDDIS);
> +					  reg & ~AT91_WDT_WDDIS);
> +		wdt_write_nosleep(wdt, AT91_WDT_MR, wdt->mr);
>  	}
>  	return 0;
>  }
> -- 
> 2.17.1
> 

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com