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

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Message-ID: <566adde7-c397-72f1-145d-fbca9de77cd7@roeck-us.net>
Date:   Tue, 28 Jul 2020 07:34:07 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Anson Huang <Anson.Huang@....com>, wim@...ux-watchdog.org,
        shawnguo@...nel.org, s.hauer@...gutronix.de, kernel@...gutronix.de,
        festevam@...il.com, linux-watchdog@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Linux-imx@....com
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.

Thanks,
Guenter

> +}
> +
>  static void imx7ulp_wdt_enable(struct watchdog_device *wdog, bool enable)
>  {
>  	struct imx7ulp_wdt_device *wdt = watchdog_get_drvdata(wdog);
>  
>  	u32 val = readl(wdt->base + WDOG_CS);
>  
> +	local_irq_disable();
>  	writel(UNLOCK, wdt->base + WDOG_CNT);
> +	imx7ulp_wdt_wait(wdt->base, WDOG_CS_ULK);
>  	if (enable)
>  		writel(val | WDOG_CS_EN, wdt->base + WDOG_CS);
>  	else
>  		writel(val & ~WDOG_CS_EN, wdt->base + WDOG_CS);
> +	imx7ulp_wdt_wait(wdt->base, WDOG_CS_RCS);
> +	local_irq_enable();
>  }
>  
>  static bool imx7ulp_wdt_is_enabled(void __iomem *base)
> @@ -72,7 +88,12 @@ static int imx7ulp_wdt_ping(struct watchdog_device *wdog)
>  {
>  	struct imx7ulp_wdt_device *wdt = watchdog_get_drvdata(wdog);
>  
> +	local_irq_disable();
> +	writel(UNLOCK, wdt->base + WDOG_CNT);
> +	imx7ulp_wdt_wait(wdt->base, WDOG_CS_ULK);
>  	writel(REFRESH, wdt->base + WDOG_CNT);
> +	imx7ulp_wdt_wait(wdt->base, WDOG_CS_RCS);
> +	local_irq_enable();
>  
>  	return 0;
>  }
> @@ -98,8 +119,12 @@ static int imx7ulp_wdt_set_timeout(struct watchdog_device *wdog,
>  	struct imx7ulp_wdt_device *wdt = watchdog_get_drvdata(wdog);
>  	u32 val = WDOG_CLOCK_RATE * timeout;
>  
> +	local_irq_disable();
>  	writel(UNLOCK, wdt->base + WDOG_CNT);
> +	imx7ulp_wdt_wait(wdt->base, WDOG_CS_ULK);
>  	writel(val, wdt->base + WDOG_TOVAL);
> +	imx7ulp_wdt_wait(wdt->base, WDOG_CS_RCS);
> +	local_irq_enable();
>  
>  	wdog->timeout = timeout;
>  
> @@ -140,15 +165,19 @@ static void imx7ulp_wdt_init(void __iomem *base, unsigned int timeout)
>  {
>  	u32 val;
>  
> +	local_irq_disable();
>  	/* unlock the wdog for reconfiguration */
>  	writel_relaxed(UNLOCK_SEQ0, base + WDOG_CNT);
>  	writel_relaxed(UNLOCK_SEQ1, base + WDOG_CNT);
> +	imx7ulp_wdt_wait(base, WDOG_CS_ULK);
>  
>  	/* set an initial timeout value in TOVAL */
>  	writel(timeout, base + WDOG_TOVAL);
>  	/* enable 32bit command sequence and reconfigure */
>  	val = WDOG_CS_CMD32EN | WDOG_CS_CLK | WDOG_CS_UPDATE;
>  	writel(val, base + WDOG_CS);
> +	imx7ulp_wdt_wait(base, WDOG_CS_RCS);
> +	local_irq_enable();
>  }
>  
>  static void imx7ulp_wdt_action(void *data)
>