linux-kernel - Re: [PATCH] watchdog: gpio_wdt: Support GPO lines with the toggle algorithm

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Message-ID: <8b41a486-92af-2f2d-ba05-205650a90ee2@roeck-us.net>
Date:   Tue, 22 Mar 2022 17:04:53 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Tobias Waldekranz <tobias@...dekranz.com>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>
Cc:     linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] watchdog: gpio_wdt: Support GPO lines with the toggle
 algorithm

On 3/22/22 15:29, Tobias Waldekranz wrote:
> Support using GPO lines (i.e. GPIOs that are output-only) with
> gpio_wdt using the "toggle" algorithm.
> 
> Since its inception, gpio_wdt has configured its GPIO line as an input
> when using the "toggle" algorithm, even though it is used as an output
> when kicking. This needlessly barred hardware with output-only pins
> from using the driver.
> 
> Signed-off-by: Tobias Waldekranz <tobias@...dekranz.com>
> ---
> 
> Hi,
> 
> This patch has been in our downstream tree for a long time. We need it
> because our kick GPIO can't be used as an input.
> 
> What I really can't figure out is why the driver would request the pin
> as in input, when it's always going to end up being used as an output
> anyway.
> 
> So I thought I'd send it upstream in the hopes of either getting it
> merged, or an explanation as to why it is needed.
> 

I _think_ the assumption / idea was that "toggle" implies that the output
is connected to a pull-up resistor and that the pin either floats or is
pulled down to ground, causing the signal to toggle. I don't know if/how
that works in practice, though.

Guenter

>   drivers/watchdog/gpio_wdt.c | 13 +++++--------
>   1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/watchdog/gpio_wdt.c b/drivers/watchdog/gpio_wdt.c
> index 0923201ce874..f7686688e0e2 100644
> --- a/drivers/watchdog/gpio_wdt.c
> +++ b/drivers/watchdog/gpio_wdt.c
> @@ -108,7 +108,6 @@ static int gpio_wdt_probe(struct platform_device *pdev)
>   	struct device *dev = &pdev->dev;
>   	struct device_node *np = dev->of_node;
>   	struct gpio_wdt_priv *priv;
> -	enum gpiod_flags gflags;
>   	unsigned int hw_margin;
>   	const char *algo;
>   	int ret;
> @@ -122,17 +121,15 @@ static int gpio_wdt_probe(struct platform_device *pdev)
>   	ret = of_property_read_string(np, "hw_algo", &algo);
>   	if (ret)
>   		return ret;
> -	if (!strcmp(algo, "toggle")) {
> +
> +	if (!strcmp(algo, "toggle"))
>   		priv->hw_algo = HW_ALGO_TOGGLE;
> -		gflags = GPIOD_IN;
> -	} else if (!strcmp(algo, "level")) {
> +	else if (!strcmp(algo, "level"))
>   		priv->hw_algo = HW_ALGO_LEVEL;
> -		gflags = GPIOD_OUT_LOW;
> -	} else {
> +	else
>   		return -EINVAL;
> -	}
>   
> -	priv->gpiod = devm_gpiod_get(dev, NULL, gflags);
> +	priv->gpiod = devm_gpiod_get(dev, NULL, GPIOD_OUT_LOW);
>   	if (IS_ERR(priv->gpiod))
>   		return PTR_ERR(priv->gpiod);
>