iio: adc: ad7124: Fix comparison of channel configs

Checking the binary representation of two structs (of the same type)
for equality doesn't have the same semantic as comparing all members for
equality. The former might find a difference where the latter doesn't in
the presence of padding or when ambiguous types like float or bool are
involved. (Floats typically have different representations for single
values, like -0.0 vs +0.0, or 0.5 * 2² vs 0.25 * 2³. The type bool has
at least 8 bits and the raw values 1 and 2 (probably) both evaluate to
true, but memcmp finds a difference.)

When searching for a channel that already has the configuration we need,
the comparison by member is the one that is needed.

Convert the comparison accordingly to compare the members one after
another. Also add a static_assert guard to (somewhat) ensure that when
struct ad7124_channel_config::config_props is expanded, the comparison
is adapted, too.

This issue is somewhat theoretic, but using memcmp() on a struct is a
bad pattern that is worth fixing.

Fixes: 7b8d045 ("iio: adc: ad7124: allow more than 8 channels")
Signed-off-by: Uwe Kleine-König <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jonathan Cameron <[email protected]>

Author: ukleinek

drivers/iio/adc/ad7124.c

@@ -151,7 +151,11 @@ struct ad7124_chip_info {
 struct ad7124_channel_config {
 	bool live;
 	unsigned int cfg_slot;
-	/* Following fields are used to compare equality. */
+	/*
+	 * Following fields are used to compare for equality. If you
+	 * make adaptations in it, you most likely also have to adapt
+	 * ad7124_find_similar_live_cfg(), too.
+	 */
 	struct_group(config_props,
 		enum ad7124_ref_sel refsel;
 		bool bipolar;
@@ -338,15 +342,38 @@ static struct ad7124_channel_config *ad7124_find_similar_live_cfg(struct ad7124_
 								  struct ad7124_channel_config *cfg)
 {
 	struct ad7124_channel_config *cfg_aux;
-	ptrdiff_t cmp_size;
 	int i;
 
-	cmp_size = sizeof_field(struct ad7124_channel_config, config_props);
+	/*
+	 * This is just to make sure that the comparison is adapted after
+	 * struct ad7124_channel_config was changed.
+	 */
+	static_assert(sizeof_field(struct ad7124_channel_config, config_props) ==
+		      sizeof(struct {
+				     enum ad7124_ref_sel refsel;
+				     bool bipolar;
+				     bool buf_positive;
+				     bool buf_negative;
+				     unsigned int vref_mv;
+				     unsigned int pga_bits;
+				     unsigned int odr;
+				     unsigned int odr_sel_bits;
+				     unsigned int filter_type;
+			     }));
+
 	for (i = 0; i < st->num_channels; i++) {
 		cfg_aux = &st->channels[i].cfg;
 
 		if (cfg_aux->live &&
-		    !memcmp(&cfg->config_props, &cfg_aux->config_props, cmp_size))
+		    cfg->refsel == cfg_aux->refsel &&
+		    cfg->bipolar == cfg_aux->bipolar &&
+		    cfg->buf_positive == cfg_aux->buf_positive &&
+		    cfg->buf_negative == cfg_aux->buf_negative &&
+		    cfg->vref_mv == cfg_aux->vref_mv &&
+		    cfg->pga_bits == cfg_aux->pga_bits &&
+		    cfg->odr == cfg_aux->odr &&
+		    cfg->odr_sel_bits == cfg_aux->odr_sel_bits &&
+		    cfg->filter_type == cfg_aux->filter_type)
 			return cfg_aux;
 	}