linux-kernel - Re: [PATCH v2] watchdog: da9062: make restart handler atomic safe

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Date:   Wed, 15 Jan 2020 08:54:47 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Marco Felsch <m.felsch@...gutronix.de>
Cc:     support.opensource@...semi.com, contact@...fanchrist.eu,
        Adam.Thomson.Opensource@...semi.com,
        linux-watchdog@...r.kernel.org, kernel@...gutronix.de,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] watchdog: da9062: make restart handler atomic safe

On Wed, Jan 15, 2020 at 05:23:07PM +0100, Marco Felsch wrote:
> The restart handler is executed during the shutdown phase which is
> atomic/irq-less. The i2c framework supports atomic transfers since
> commit 63b96983a5dd ("i2c: core: introduce callbacks for atomic
> transfers") to address this use case. Using regmap within an atomic
> context is allowed only if the regmap type is MMIO and the cache type
> 'flat' or no cache is used. Using the i2c_smbus_write_byte_data()
> function can be done without additional tests because:
>  1) the DA9062 is an i2c-only device and
>  2) the i2c framework emulates the smbus protocol if the host adapter
>     does not support smbus_xfer by using the master_xfer.
> 
> Signed-off-by: Marco Felsch <m.felsch@...gutronix.de>

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

> ---
> Hi,
> 
> This patch is based on Stefan Lengfeld's RFC Patch [1].
> 
> [1] https://patchwork.ozlabs.org/patch/1085942/
> 
> Changes:
> 
> v2:
> - adapt commit message
> - add comment
> - make use of i2c_smbus_write_byte_data()
> ---
>  drivers/watchdog/da9062_wdt.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/watchdog/da9062_wdt.c b/drivers/watchdog/da9062_wdt.c
> index c9b9d6394525..77b6b5336067 100644
> --- a/drivers/watchdog/da9062_wdt.c
> +++ b/drivers/watchdog/da9062_wdt.c
> @@ -11,6 +11,7 @@
>  #include <linux/platform_device.h>
>  #include <linux/uaccess.h>
>  #include <linux/slab.h>
> +#include <linux/i2c.h>
>  #include <linux/delay.h>
>  #include <linux/jiffies.h>
>  #include <linux/mfd/da9062/registers.h>
> @@ -149,12 +150,13 @@ static int da9062_wdt_restart(struct watchdog_device *wdd, unsigned long action,
>  			      void *data)
>  {
>  	struct da9062_watchdog *wdt = watchdog_get_drvdata(wdd);
> +	struct i2c_client *client = to_i2c_client(wdt->hw->dev);
>  	int ret;
>  
> -	ret = regmap_write(wdt->hw->regmap,
> -			   DA9062AA_CONTROL_F,
> -			   DA9062AA_SHUTDOWN_MASK);
> -	if (ret)
> +	/* Don't use regmap because it is not atomic safe */
> +	ret = i2c_smbus_write_byte_data(client, DA9062AA_CONTROL_F,
> +					DA9062AA_SHUTDOWN_MASK);
> +	if (ret < 0)
>  		dev_alert(wdt->hw->dev, "Failed to shutdown (err = %d)\n",
>  			  ret);
>  
> -- 
> 2.20.1
>