linux-kernel - Re: [PATCH 3/3] watchdog: sprd: check busy bit before kick watchdog

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Date:   Mon, 26 Oct 2020 07:44:22 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Chunyan Zhang <zhang.lyra@...il.com>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>
Cc:     linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org,
        Orson Zhai <orsonzhai@...il.com>,
        Baolin Wang <baolin.wang7@...il.com>,
        Chunyan Zhang <chunyan.zhang@...soc.com>
Subject: Re: [PATCH 3/3] watchdog: sprd: check busy bit before kick watchdog

On 10/26/20 1:09 AM, Chunyan Zhang wrote:
> From: Lingling Xu <ling_ling.xu@...soc.com>
> 
> As the specification described, checking busy bit must be done before kick
> watchdog.
> 

That is a key functional change: So far the code checked if a value
was accepted after loading it. That is no longer the case. Effectively,
with this change, the _next_ operation will now check if the previous
operation was accepted. Is this intentional ?

Also, does this really solve a problem, or is it just an optimization ?
By checking for busy prior to an operation instead of after it the only
real difference is that the busy check will most likely succeed immediately
because enough time has passed since the last write.

Ultimately it is your call how you want to handle this, but I think the
impact should be spelled out.

Guenter

> Fixes: 477603467009 ("watchdog: Add Spreadtrum watchdog driver")
> Signed-off-by: Lingling Xu <ling_ling.xu@...soc.com>
> Signed-off-by: Chunyan Zhang <chunyan.zhang@...soc.com>
> ---
>  drivers/watchdog/sprd_wdt.c | 27 ++++++++++++++-------------
>  1 file changed, 14 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/watchdog/sprd_wdt.c b/drivers/watchdog/sprd_wdt.c
> index 4f2a8c6d6485..14071c66ff49 100644
> --- a/drivers/watchdog/sprd_wdt.c
> +++ b/drivers/watchdog/sprd_wdt.c
> @@ -108,20 +108,8 @@ static int sprd_wdt_load_value(struct sprd_wdt *wdt, u32 timeout,
>  	u32 tmr_step = timeout * SPRD_WDT_CNT_STEP;
>  	u32 prtmr_step = pretimeout * SPRD_WDT_CNT_STEP;
>  
> -	sprd_wdt_unlock(wdt->base);
> -	writel_relaxed((tmr_step >> SPRD_WDT_CNT_HIGH_SHIFT) &
> -		      SPRD_WDT_LOW_VALUE_MASK, wdt->base + SPRD_WDT_LOAD_HIGH);
> -	writel_relaxed((tmr_step & SPRD_WDT_LOW_VALUE_MASK),
> -		       wdt->base + SPRD_WDT_LOAD_LOW);
> -	writel_relaxed((prtmr_step >> SPRD_WDT_CNT_HIGH_SHIFT) &
> -			SPRD_WDT_LOW_VALUE_MASK,
> -		       wdt->base + SPRD_WDT_IRQ_LOAD_HIGH);
> -	writel_relaxed(prtmr_step & SPRD_WDT_LOW_VALUE_MASK,
> -		       wdt->base + SPRD_WDT_IRQ_LOAD_LOW);
> -	sprd_wdt_lock(wdt->base);
> -
>  	/*
> -	 * Waiting the load value operation done,
> +	 * Waiting the last load value operation done,
>  	 * it needs two or three RTC clock cycles.
>  	 */
>  	do {
> @@ -134,6 +122,19 @@ static int sprd_wdt_load_value(struct sprd_wdt *wdt, u32 timeout,
>  
>  	if (delay_cnt >= SPRD_WDT_LOAD_TIMEOUT)
>  		return -EBUSY;
> +
> +	sprd_wdt_unlock(wdt->base);
> +	writel_relaxed((tmr_step >> SPRD_WDT_CNT_HIGH_SHIFT) &
> +		      SPRD_WDT_LOW_VALUE_MASK, wdt->base + SPRD_WDT_LOAD_HIGH);
> +	writel_relaxed((tmr_step & SPRD_WDT_LOW_VALUE_MASK),
> +		       wdt->base + SPRD_WDT_LOAD_LOW);
> +	writel_relaxed((prtmr_step >> SPRD_WDT_CNT_HIGH_SHIFT) &
> +			SPRD_WDT_LOW_VALUE_MASK,
> +		       wdt->base + SPRD_WDT_IRQ_LOAD_HIGH);
> +	writel_relaxed(prtmr_step & SPRD_WDT_LOW_VALUE_MASK,
> +		       wdt->base + SPRD_WDT_IRQ_LOAD_LOW);
> +	sprd_wdt_lock(wdt->base);
> +
>  	return 0;
>  }
>  
>