linux-kernel - Re: [PATCH 2/4] watchdog: sama5d4: fix race condition

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Date:   Sat, 4 Mar 2017 07:05:36 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Alexandre Belloni <alexandre.belloni@...e-electrons.com>
Cc:     Wim Van Sebroeck <wim@...ana.be>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Wenyou.Yang@...rochip.com, linux-watchdog@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] watchdog: sama5d4: fix race condition

On 03/02/2017 09:31 AM, Alexandre Belloni wrote:
> WDT_MR and WDT_CR must not updated within three slow clock periods after
> the last ping (write to WDT_CR or WDT_MR). Ensure enough time has elapsed
> before writing those registers.
> wdt_write() waits for 4 periods to ensure at least 3 edges are seen by the
> IP.
>
> Signed-off-by: Alexandre Belloni <alexandre.belloni@...e-electrons.com>

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

> ---
>  drivers/watchdog/sama5d4_wdt.c | 33 +++++++++++++++++++++++++++++----
>  1 file changed, 29 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/watchdog/sama5d4_wdt.c b/drivers/watchdog/sama5d4_wdt.c
> index 5cee20caca78..362fd229786d 100644
> --- a/drivers/watchdog/sama5d4_wdt.c
> +++ b/drivers/watchdog/sama5d4_wdt.c
> @@ -6,6 +6,7 @@
>   * Licensed under GPLv2.
>   */
>
> +#include <linux/delay.h>
>  #include <linux/interrupt.h>
>  #include <linux/io.h>
>  #include <linux/kernel.h>
> @@ -29,6 +30,7 @@ struct sama5d4_wdt {
>  	struct watchdog_device	wdd;
>  	void __iomem		*reg_base;
>  	u32			mr;
> +	unsigned long		last_ping;
>  };
>
>  static int wdt_timeout = WDT_DEFAULT_TIMEOUT;
> @@ -49,8 +51,29 @@ MODULE_PARM_DESC(nowayout,
>  #define wdt_read(wdt, field) \
>  	readl_relaxed((wdt)->reg_base + (field))
>
> -#define wdt_write(wtd, field, val) \
> -	writel_relaxed((val), (wdt)->reg_base + (field))
> +/* 4 slow clock periods is 4/32768 = 122.07µs*/
> +#define WDT_DELAY	usecs_to_jiffies(123)
> +
> +static void wdt_write(struct sama5d4_wdt *wdt, u32 field, u32 val)
> +{
> +	/*
> +	 * WDT_CR and WDT_MR must not be modified within three slow clock
> +	 * periods following a restart of the watchdog performed by a write
> +	 * access in WDT_CR.
> +	 */
> +	while (time_before(jiffies, wdt->last_ping + WDT_DELAY))
> +		usleep_range(30, 125);
> +	writel_relaxed(val, wdt->reg_base + field);
> +	wdt->last_ping = jiffies;
> +}
> +
> +static void wdt_write_nosleep(struct sama5d4_wdt *wdt, u32 field, u32 val)
> +{
> +	if (time_before(jiffies, wdt->last_ping + WDT_DELAY))
> +		udelay(123);
> +	writel_relaxed(val, wdt->reg_base + field);
> +	wdt->last_ping = jiffies;
> +}
>
>  static int sama5d4_wdt_start(struct watchdog_device *wdd)
>  {
> @@ -164,11 +187,12 @@ static int sama5d4_wdt_init(struct sama5d4_wdt *wdt)
>  	 * Else, we have to disable it properly.
>  	 */
>  	if (wdt_enabled) {
> -		wdt_write(wdt, AT91_WDT_MR, wdt->mr);
> +		wdt_write_nosleep(wdt, AT91_WDT_MR, wdt->mr);
>  	} else {
>  		reg = wdt_read(wdt, AT91_WDT_MR);
>  		if (!(reg & AT91_WDT_WDDIS))
> -			wdt_write(wdt, AT91_WDT_MR, reg | AT91_WDT_WDDIS);
> +			wdt_write_nosleep(wdt, AT91_WDT_MR,
> +					  reg | AT91_WDT_WDDIS);
>  	}
>  	return 0;
>  }
> @@ -193,6 +217,7 @@ static int sama5d4_wdt_probe(struct platform_device *pdev)
>  	wdd->ops = &sama5d4_wdt_ops;
>  	wdd->min_timeout = MIN_WDT_TIMEOUT;
>  	wdd->max_timeout = MAX_WDT_TIMEOUT;
> +	wdt->last_ping = jiffies;
>
>  	watchdog_set_drvdata(wdd, wdt);
>
>