linux-kernel - Re: [PATCH 1/2] watchdog: dw: RMW the control register

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Date:   Fri, 9 Mar 2018 19:20:38 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Brian Norris <briannorris@...omium.org>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>
Cc:     linux-kernel@...r.kernel.org, linux-watchdog@...r.kernel.org,
        Doug Anderson <dianders@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>
Subject: Re: [PATCH 1/2] watchdog: dw: RMW the control register

Hi Brian,

On 03/09/2018 06:44 PM, Brian Norris wrote:
> RK3399 has rst_pulse_length in CONTROL_REG[4:2], determining the length
> of pulse to issue for system reset. We shouldn't clobber this value,
> because that might make the system reset ineffective. On RK3399, we're
> seeing that a value of 000b (meaning 2 cycles) yields an unreliable
> (partial?) reset, and so we only fully reset after the watchdog fires a
> second time. If we retain the system default (010b, or 8 clock cycles),
> then the watchdog reset is much more reliable.
> 
> Read-modify-write retains the system value and improves reset
> reliability.
> 
> Signed-off-by: Brian Norris <briannorris@...omium.org>
> ---
>   drivers/watchdog/dw_wdt.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/watchdog/dw_wdt.c b/drivers/watchdog/dw_wdt.c
> index c2f4ff516230..6925d3e6c6b3 100644
> --- a/drivers/watchdog/dw_wdt.c
> +++ b/drivers/watchdog/dw_wdt.c
> @@ -34,6 +34,7 @@
>   
>   #define WDOG_CONTROL_REG_OFFSET		    0x00
>   #define WDOG_CONTROL_REG_WDT_EN_MASK	    0x01
> +#define WDOG_CONTROL_REG_RESP_MODE_MASK	    0x02
>   #define WDOG_TIMEOUT_RANGE_REG_OFFSET	    0x04
>   #define WDOG_TIMEOUT_RANGE_TOPINIT_SHIFT    4
>   #define WDOG_CURRENT_COUNT_REG_OFFSET	    0x08
> @@ -124,11 +125,16 @@ static int dw_wdt_set_timeout(struct watchdog_device *wdd, unsigned int top_s)
>   static int dw_wdt_start(struct watchdog_device *wdd)
>   {
>   	struct dw_wdt *dw_wdt = to_dw_wdt(wdd);
> +	u32 val;
>   
>   	dw_wdt_set_timeout(wdd, wdd->timeout);
>   
> -	writel(WDOG_CONTROL_REG_WDT_EN_MASK,
> -	       dw_wdt->regs + WDOG_CONTROL_REG_OFFSET);
> +	val = readl(dw_wdt->regs + WDOG_CONTROL_REG_OFFSET);
> +	/* Disable interrupt mode; always perform system reset. */
> +	val &= ~WDOG_CONTROL_REG_RESP_MODE_MASK;

You don't talk about this change in the description.

> +	/* Enable watchdog. */
> +	val |= WDOG_CONTROL_REG_WDT_EN_MASK;
> +	writel(val, dw_wdt->regs + WDOG_CONTROL_REG_OFFSET);
>   
>   	return 0;
>   }
> 

Similar code is in dw_wdt_restart(), where it may be equally or even
more important. Granted, only if the watchdog isn't running, but still...

Guenter