linux-kernel - Re: [PATCH v3 3/3] gnss: ubx: add support for the reset gpio

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Date:   Mon, 16 Oct 2023 15:55:17 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc:     linux-renesas-soc@...r.kernel.org,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] gnss: ubx: add support for the reset gpio

On Thu, Sep 21, 2023 at 03:32:01PM +0200, Wolfram Sang wrote:
> Tested with a Renesas KingFisher board. Because my GNSS device is hooked
> up via UART and I2C simultaneously, I could verify functionality by
> opening/closing the GNSS device using UART and see if the corresponding
> I2C device was visible on the bus.

Again, please try to make the commit message self-contained (e.g.
without implicitly referring to Subject).

Also say something about which u-blox module this is needed for since
the modules I've seen do not have a reset line.

>  static int ubx_set_active(struct gnss_serial *gserial)
> @@ -29,6 +31,8 @@ static int ubx_set_active(struct gnss_serial *gserial)
>  	if (ret)
>  		return ret;
>  
> +	gpiod_set_value_cansleep(data->reset_gpio, 0);
> +
>  	return 0;
>  }
>  
> @@ -37,6 +41,8 @@ static int ubx_set_standby(struct gnss_serial *gserial)
>  	struct ubx_data *data = gnss_serial_get_drvdata(gserial);
>  	int ret;
>  
> +	gpiod_set_value_cansleep(data->reset_gpio, 1);
> +
>  	ret = regulator_disable(data->vcc);
>  	if (ret)
>  		return ret;
> @@ -90,6 +96,13 @@ static int ubx_probe(struct serdev_device *serdev)
>  	if (ret < 0 && ret != -ENODEV)
>  		goto err_free_gserial;
>  
> +	/* Start with reset asserted */
> +	data->reset_gpio = devm_gpiod_get_optional(&serdev->dev, "reset", GPIOD_OUT_HIGH);
> +	if (IS_ERR(data->reset_gpio)) {
> +		ret = PTR_ERR(data->reset_gpio);
> +		goto err_free_gserial;
> +	}

So this means that the reset line will be asserted indefinitely in case
the GNSS receiver is not used. Are you sure that's a good idea?

I don't know yet which module this is for, or how this signal is
supposed to be used, but the above makes this look more like an
active-high (regulator) enable signal. Perhaps that's what it is or
should be modelled as (i.e. as a fixed regulator).

> +
>  	ret = gnss_serial_register(gserial);
>  	if (ret)
>  		goto err_free_gserial;

Johan