iio: adc: ad7124: fix config comparison

The ad7124_find_similar_live_cfg() computes the compare size by
substracting the address of the cfg struct from the address of the live
field. Because the live field is the first field in the struct, the
result is 0.

Also, the memcmp() call is made from the start of the cfg struct, which
includes the live and cfg_slot fields, which are not relevant for the
comparison.

Fix by grouping the relevant fields with struct_group() and use the
size of the group to compute the compare size; make the memcmp() call
from the address of the group.

Fixes: 7b8d045e ("iio: adc: ad7124: allow more than 8 channels")
Signed-off-by: Dumitru Ceclan <dumitru.ceclan@analog.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240731-ad7124-fix-v1-2-46a76aa4b9be@analog.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ad7124.c

@@ -147,6 +147,8 @@ struct ad7124_chip_info {
 struct ad7124_channel_config {
 	bool live;
 	unsigned int cfg_slot;
+	/* Following fields are used to compare equality. */
+	struct_group(config_props,
 		enum ad7124_ref_sel refsel;
 		bool bipolar;
 		bool buf_positive;
@@ -156,6 +158,7 @@ struct ad7124_channel_config {
 		unsigned int odr;
 		unsigned int odr_sel_bits;
 		unsigned int filter_type;
+	);
 };
 
 struct ad7124_channel {
@@ -334,11 +337,12 @@ static struct ad7124_channel_config *ad7124_find_similar_live_cfg(struct ad7124_
 	ptrdiff_t cmp_size;
 	int i;
 
-	cmp_size = (u8 *)&cfg->live - (u8 *)cfg;
+	cmp_size = sizeof_field(struct ad7124_channel_config, config_props);
 	for (i = 0; i < st->num_channels; i++) {
 		cfg_aux = &st->channels[i].cfg;
 
-		if (cfg_aux->live && !memcmp(cfg, cfg_aux, cmp_size))
+		if (cfg_aux->live &&
+		    !memcmp(&cfg->config_props, &cfg_aux->config_props, cmp_size))
 			return cfg_aux;
 	}