Re: [PATCH] iio: temperature: ltc2983: avoid -Wstring-compare warning

[David Laight]

On Fri,  2 Oct 2026 14:48:51 +0200
Arnd Bergmann <arnd@xxxxxxxxxx> wrote:

> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> When __ltc2983_custom_sensor_new() is inlined into a caller, gcc notices
> that the string compare can never succeed and warns about this being a
> possible mistake:

That function looks like it should be marked __noinline_for_bloat :-)

Although doing a strcmp() like that is just silly.

David

> 
> In function '__ltc2983_custom_sensor_new',
>     inlined from 'ltc2983_thermistor_new' at drivers/iio/temperature/ltc2983.c:1079:24:
> drivers/iio/temperature/ltc2983.c:559:29: error: 'strcmp' of a string of length 24 and an array of size 22 evaluates to nonzero [-Werror=string-compare]
>   559 |    if ((index % 2) != 0 && !strcmp(propname, "adi,custom-leak-detector"))
>       |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> This is a false positive warning as the code always works correctly,
> though the strcmp() is a somewhat elaborate way of identifying the caller.
> 
> Rework this by replacing both the strcmp() and the is_steinhart flag with
> the sensor->type that is always available anyway and identifies with
> code path gets taken.
> 
> Fixes: 3dd0c048409e ("iio: temperature: ltc2983: Add support for ADT7604")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  drivers/iio/temperature/ltc2983.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/iio/temperature/ltc2983.c b/drivers/iio/temperature/ltc2983.c
> index 6655453c9abf..efad747c9e8d 100644
> --- a/drivers/iio/temperature/ltc2983.c
> +++ b/drivers/iio/temperature/ltc2983.c
> @@ -482,11 +482,12 @@ static int __ltc2983_chan_custom_sensor_assign(struct ltc2983_data *st,
>  
>  static struct ltc2983_custom_sensor *
>  __ltc2983_custom_sensor_new(struct ltc2983_data *st, const struct fwnode_handle *fn,
> -			    const char *propname, const bool is_steinhart,
> +			    const char *propname, int type,
>  			    const u32 resolution, const bool has_signed)
>  {
>  	struct ltc2983_custom_sensor *new_custom;
>  	struct device *dev = &st->spi->dev;
> +	bool is_steinhart = type == LTC2983_SENSOR_THERMISTOR_STEINHART;
>  	/*
>  	 * For custom steinhart, the full u32 is taken. For all the others
>  	 * the MSB is discarded.
> @@ -556,7 +557,7 @@ __ltc2983_custom_sensor_new(struct ltc2983_data *st, const struct fwnode_handle
>  			 * to µK so __convert_to_raw() produces the correct hardware
>  			 * encoding: P + 273.15 K.
>  			 */
> -			if ((index % 2) != 0 && !strcmp(propname, "adi,custom-leak-detector"))
> +			if ((index % 2) != 0 && type == LTC2983_SENSOR_LEAK_DETECTOR)
>  				temp = temp * 1000000 + 273150000;
>  
>  			if ((index % 2) != 0)
> @@ -846,7 +847,8 @@ ltc2983_thermocouple_new(const struct fwnode_handle *child, struct ltc2983_data
>  		const char *propname = "adi,custom-thermocouple";
>  
>  		thermo->custom = __ltc2983_custom_sensor_new(st, child,
> -							     propname, false,
> +							     propname,
> +							     sensor->type,
>  							     16384, true);
>  		if (IS_ERR(thermo->custom))
>  			return ERR_CAST(thermo->custom);
> @@ -961,7 +963,8 @@ ltc2983_rtd_new(const struct fwnode_handle *child, struct ltc2983_data *st,
>  	if (sensor->type == LTC2983_SENSOR_RTD_CUSTOM) {
>  		rtd->custom = __ltc2983_custom_sensor_new(st, child,
>  							  "adi,custom-rtd",
> -							  false, 2048, false);
> +							  sensor->type,
> +							  2048, false);
>  		if (IS_ERR(rtd->custom))
>  			return ERR_CAST(rtd->custom);
>  	}
> @@ -1066,11 +1069,9 @@ ltc2983_thermistor_new(const struct fwnode_handle *child, struct ltc2983_data *s
>  
>  	/* check custom sensor */
>  	if (sensor->type >= LTC2983_SENSOR_THERMISTOR_STEINHART) {
> -		bool steinhart = false;
>  		const char *propname;
>  
>  		if (sensor->type == LTC2983_SENSOR_THERMISTOR_STEINHART) {
> -			steinhart = true;
>  			propname = "adi,custom-steinhart";
>  		} else {
>  			propname = "adi,custom-thermistor";
> @@ -1078,7 +1079,7 @@ ltc2983_thermistor_new(const struct fwnode_handle *child, struct ltc2983_data *s
>  
>  		thermistor->custom = __ltc2983_custom_sensor_new(st, child,
>  								 propname,
> -								 steinhart,
> +								 sensor->type,
>  								 64, false);
>  		if (IS_ERR(thermistor->custom))
>  			return ERR_CAST(thermistor->custom);
> @@ -1195,7 +1196,7 @@ ltc2983_copper_trace_new(const struct fwnode_handle *child, struct ltc2983_data
>  						 "adi,custom-copper-trace is required for >1 ohm copper trace\n");
>  
>  		ct->custom = __ltc2983_custom_sensor_new(st, child, "adi,custom-copper-trace",
> -							 false, 2048, false);
> +							 sensor->type, 2048, false);
>  		if (IS_ERR(ct->custom))
>  			return ERR_CAST(ct->custom);
>  
> @@ -1286,7 +1287,7 @@ ltc2983_leak_detector_new(const struct fwnode_handle *child, struct ltc2983_data
>  					 "adi,custom-leak-detector is required for leak detectors\n");
>  
>  	ld->custom = __ltc2983_custom_sensor_new(st, child, "adi,custom-leak-detector",
> -						 false, 16, false);
> +						 sensor->type, 16, false);
>  	if (IS_ERR(ld->custom))
>  		return ERR_CAST(ld->custom);
>  
> @@ -1499,7 +1500,8 @@ static struct ltc2983_sensor *ltc2983_temp_new(struct fwnode_handle *child,
>  					 sensor->chan);
>  
>  	temp->custom = __ltc2983_custom_sensor_new(st, child, "adi,custom-temp",
> -						   false, 4096, true);
> +						   sensor->type,
> +						   4096, true);
>  	if (IS_ERR(temp->custom))
>  		return ERR_CAST(temp->custom);
>