linux-kernel - Re: [PATCH v2 2/7] watchdog: imx7ulp: Add explict memory barrier for unlock sequence

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Date:   Sun, 25 Sep 2022 10:37:46 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     "Alice Guo (OSS)" <alice.guo@....nxp.com>
Cc:     m.felsch@...gutronix.de, wim@...ux-watchdog.org,
        shawnguo@...nel.org, s.hauer@...gutronix.de, festevam@...il.com,
        kernel@...gutronix.de, linux-imx@....com,
        linux-watchdog@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/7] watchdog: imx7ulp: Add explict memory barrier for
 unlock sequence

On Thu, Aug 25, 2022 at 04:32:51PM +0800, Alice Guo (OSS) wrote:
> From: Jacky Bai <ping.bai@....com>
> 
> When reconfiguring the WDOG Timer of i.MX7ULP, there is a certain
> probability causes it to reset. The reason is that the CMD32EN of the
> WDOG Timer of i.MX7ULP is disabled in bootloader. The unlock sequence
> are two 16-bit writes to the CNT register within 16 bus clocks. Adding
> mb() is to guarantee that two 16-bit writes are finished within 16 bus
> clocks. Memory barriers cannot be added between these two 16-bit writes
> so that writel_relaxed is used.
> 
> Suggested-by: Ye Li <ye.li@....com>
> Signed-off-by: Jacky Bai <ping.bai@....com>
> Signed-off-by: Alice Guo <alice.guo@....com>
> Reviewed-by: Ye Li <ye.li@....com>

Reviewed-by: Guenter Roeck <linux@...ck-us.net>

> ---
> 
> Changes for v2:
>  - add the reason why memory barriers are added for unlock sequence in commit log
> 
>  drivers/watchdog/imx7ulp_wdt.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/watchdog/imx7ulp_wdt.c b/drivers/watchdog/imx7ulp_wdt.c
> index 014f497ea0dc..b8ac0cb04d2f 100644
> --- a/drivers/watchdog/imx7ulp_wdt.c
> +++ b/drivers/watchdog/imx7ulp_wdt.c
> @@ -179,9 +179,13 @@ static int imx7ulp_wdt_init(void __iomem *base, unsigned int timeout)
>  	int ret;
>  
>  	local_irq_disable();
> +
> +	mb();
>  	/* unlock the wdog for reconfiguration */
>  	writel_relaxed(UNLOCK_SEQ0, base + WDOG_CNT);
>  	writel_relaxed(UNLOCK_SEQ1, base + WDOG_CNT);
> +	mb();
> +
>  	ret = imx7ulp_wdt_wait(base, WDOG_CS_ULK);
>  	if (ret)
>  		goto init_out;
> -- 
> 2.17.1
>