analog: use SI prefix only with units that accept SI prefixes

include/libsigrok/proto.h

@@ -31,6 +31,7 @@
 SR_API int sr_analog_to_float(const struct sr_datafeed_analog *analog,
 		float *buf);
 SR_API const char *sr_analog_si_prefix(float *value, int *digits);
+SR_API gboolean sr_analog_si_prefix_friendly(enum sr_unit unit);
 SR_API int sr_analog_unit_to_string(const struct sr_datafeed_analog *analog,
 		char **result);
 SR_API void sr_rational_set(struct sr_rational *r, int64_t p, uint64_t q);

src/analog.c

@@ -329,6 +329,48 @@ SR_API const char *sr_analog_si_prefix(float *value, int *digits)
 	return prefixes[prefix + NEG_PREFIX_COUNT];
 }
 
+/**
+ * Check if a unit "accepts" an SI prefix.
+ *
+ * E.g. SR_UNIT_VOLT is SI prefix friendly while SR_UNIT_DECIBEL_MW or
+ * SR_UNIT_PERCENTAGE are not.
+ *
+ * @param[in] unit The unit to check for SI prefix "friendliness".
+ *
+ * @return TRUE if the unit "accept" an SI prefix.
+ *
+ * @since 0.5.0
+ */
+SR_API gboolean sr_analog_si_prefix_friendly(enum sr_unit unit)
+{
+	static const enum sr_unit prefix_friendly_units[] = {
+		SR_UNIT_VOLT,
+		SR_UNIT_AMPERE,
+		SR_UNIT_OHM,
+		SR_UNIT_FARAD,
+		SR_UNIT_KELVIN,
+		SR_UNIT_HERTZ,
+		SR_UNIT_SECOND,
+		SR_UNIT_SIEMENS,
+		SR_UNIT_VOLT_AMPERE,
+		SR_UNIT_WATT,
+		SR_UNIT_WATT_HOUR,
+		SR_UNIT_METER_SECOND,
+		SR_UNIT_HENRY,
+		SR_UNIT_GRAM
+	};
+	unsigned int i;
+
+	for (i = 0; i < ARRAY_SIZE(prefix_friendly_units); i++)
+		if (unit == prefix_friendly_units[i])
+			break;
+
+	if (unit != prefix_friendly_units[i])
+		return FALSE;
+
+	return TRUE;
+}
+
 /**
  * Convert the unit/MQ/MQ flags in the analog struct to a string.
  *

src/output/analog.c

@@ -113,11 +113,14 @@ static int receive(const struct sr_output *o, const struct sr_datafeed_packet *p
 			/* TODO we don't know how to print by number of bits yet. */
 			digits = 6;
 		}
+		gboolean si_friendly = sr_analog_si_prefix_friendly(analog->meaning->unit);
 		sr_analog_unit_to_string(analog, &suffix);
 		for (i = 0; i < analog->num_samples; i++) {
 			for (l = analog->meaning->channels, c = 0; l; l = l->next, c++) {
 				float value = fdata[i * num_channels + c];
-				const char *prefix = sr_analog_si_prefix(&value, &digits);
+				const char *prefix = "";
+				if (si_friendly)
+					prefix = sr_analog_si_prefix(&value, &digits);
 				ch = l->data;
 				g_string_append_printf(*out, "%s: ", ch->name);
 				number = g_strdup_printf("%.*f", MAX(digits, 0), value);