Re: [PATCH 2/3] drm/sitronix/st7571-i2c: Make the reset GPIO to be optional

[Javier Martinez Canillas <javierm@redhat.com>]

Marcus Folkesson <marcus.folkesson@gmail.com> writes:

Hello Marcus,

Thanks for your feedback.

> On Thu, Jul 10, 2025 at 12:24:34PM +0200, Javier Martinez Canillas wrote:
>> Some Sitronix LCD controllers (such as the ST7567) don't have a reset pin,
>> so lets relax this in the driver and make the reset GPIO to be optional.
>> 
>> The devm_gpiod_get_optional() helper is similar to devm_gpiod_get(), but
>> returns NULL when there isn't a reset-gpios property defined in a DT node.
>> 
>> The DT binding schema for "sitronix,st7571" that require a reset GPIO will
>> enforce the "reset-gpios" to be present, due being a required DT property.
>> But in the driver itself the property can be made optional if not defined.
>> 
>> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
>> ---
>> 
>>  drivers/gpu/drm/sitronix/st7571-i2c.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>> 
>> diff --git a/drivers/gpu/drm/sitronix/st7571-i2c.c b/drivers/gpu/drm/sitronix/st7571-i2c.c
>> index eec846892962..73e8db25f895 100644
>> --- a/drivers/gpu/drm/sitronix/st7571-i2c.c
>> +++ b/drivers/gpu/drm/sitronix/st7571-i2c.c
>> @@ -802,15 +802,19 @@ static int st7571_parse_dt(struct st7571_device *st7571)
>>  	st7571->nlines = dt.vactive.typ;
>>  	st7571->ncols = dt.hactive.typ;
>>  
>> -	st7571->reset = devm_gpiod_get(dev, "reset", GPIOD_OUT_HIGH);
>> +	st7571->reset = devm_gpiod_get_optional(dev, "reset", GPIOD_OUT_HIGH);
>>  	if (IS_ERR(st7571->reset))
>> -		return PTR_ERR(st7571->reset);
>> +		return dev_err_probe(dev, PTR_ERR(st7571->reset),
>> +				     "Failed to get reset gpio\n");
>
> devm_gpiod_get_optional() returns -ENOENT when the GPIO is not found,
> and that is no error we want to propagage upwards.
>
> Maybe something like this instead:
> if (IS_ERR(st7571->reset) && IS_ERR(st7571->reset) != -ENOENT)
>

Are you sure about that? As far as I know, that is exactly the
difference between gpiod_get() and gpiod_get_optional() variants.

From the gpiod_get_optional() function helper kernel-doc [0]:

/**
 * gpiod_get_optional - obtain an optional GPIO for a given GPIO function
 * @dev: GPIO consumer, can be NULL for system-global GPIOs
 * @con_id: function within the GPIO consumer
 * @flags: optional GPIO initialization flags
 *
 * This is equivalent to gpiod_get(), except that when no GPIO was assigned to
 * the requested function it will return NULL. This is convenient for drivers
 * that need to handle optional GPIOs.
 *
 * Returns:
 * The GPIO descriptor corresponding to the function @con_id of device
 * dev, NULL if no GPIO has been assigned to the requested function, or
 * another IS_ERR() code if an error occurred while trying to acquire the GPIO.
 */

while the gpiod_get() kernel-doc says the following:

/**
 * gpiod_get - obtain a GPIO for a given GPIO function
 * @dev:	GPIO consumer, can be NULL for system-global GPIOs
 * @con_id:	function within the GPIO consumer
 * @flags:	optional GPIO initialization flags
 *
 * Returns:
 * The GPIO descriptor corresponding to the function @con_id of device
 * dev, -ENOENT if no GPIO has been assigned to the requested function, or
 * another IS_ERR() code if an error occurred while trying to acquire the GPIO.
 */

[0]: https://elixir.bootlin.com/linux/v6.16-rc5/source/drivers/gpio/gpiolib.c#L4755

>
> Best regards,
> Marcus Folkesson

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat