linux-kernel - Re: [PATCH v3 2/2] gnss: ubx: add support for the safeboot gpio

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Message-ID: <aRdDyyIA9Z4e8mBz@hovoldconsulting.com>
Date: Fri, 14 Nov 2025 15:59:23 +0100
From: Johan Hovold <johan@...nel.org>
To: alejandroe1@...tab.com
Cc: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Subject: Re: [PATCH v3 2/2] gnss: ubx: add support for the safeboot gpio

On Thu, Sep 11, 2025 at 02:58:29PM +0200, Alejandro Enrique via B4 Relay wrote:
> From: Alejandro Enrique <alejandroe1@...tab.com>
> 
> U-Blox 8/M8/M9 chip have a pin to start it in safeboot mode, to be
> used to recover from situations where the flash content has become
> corrupted and needs to be restored. If this pin is asserted at power
> up/reset, the receiver starts in safeboot mode and GNSS operation is
> disabled.
> 
> Deassert the safeboot pin when probing this driver.
> 
> Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
> Signed-off-by: Alejandro Enrique <alejandroe1@...tab.com>

Sorry about the late feedback on this. I had to think about how best to
handle this, given that we don't have an interface yet to actually
assert these pins.

I guess we can go ahead and request them at probe as you're doing here
and add an interface for controlling them later if needed (instead of
not claiming them and allowing emergency control through gpiolib).

> @@ -82,6 +83,13 @@ static int ubx_probe(struct serdev_device *serdev)
>  
>  	data = gnss_serial_get_drvdata(gserial);
>  
> +	/* Deassert safeboot */
> +	safeboot = devm_gpiod_get_optional(&serdev->dev, "safeboot", GPIOD_OUT_LOW);
> +	if (IS_ERR(safeboot)) {
> +		ret = PTR_ERR(safeboot);
> +		goto err_free_gserial;
> +	}
> +
>  	data->vcc = devm_regulator_get(&serdev->dev, "vcc");

Note that the driver is not enabling the supply until open() so I moved
the deassert to after the supplies have been requested (but before reset
is deasserted).

Note however that both the RESET_N and SAFEBOOT_N pins should be
declared as open-drain to avoid driving them while the main supply is
off.

I added a comment about this to the commit message about this when
applying and will send a follow-on patch to update the binding example.

>  	if (IS_ERR(data->vcc)) {
>  		ret = PTR_ERR(data->vcc);

Johan