brymen-dmm: unbreak dBm reference impedance interpretation

The reference impedance for dBm measurements comes in an unexpected
format. Isolate the 4..1200 Ohms value, ignore the (inappropriate?)
"0." and exponent parts of the response. Clearly reflect that Ohms
values are seen in different contexts (dBm reference, continuity,
resistance).

Reword comments in the BM850 response parser's code path for dBm
measurements. The reference value is shown when the function is entered
(verified here) or when the reference value changes (haven't seen this
here but a comment in the previous implementation said so).

src/hardware/brymen-dmm/parser.c

@@ -223,6 +223,7 @@ SR_PRIV int brymen_parse(const uint8_t *buf, float *floatval,
 	uint8_t *bfunc;
 	const char *txt;
 	int txtlen;
+	char *p;
 	char *unit;
 	int ret;
 
@@ -237,6 +238,22 @@ SR_PRIV int brymen_parse(const uint8_t *buf, float *floatval,
 
 	memset(&flags, 0, sizeof(flags));
 	parse_flags(bfunc, &flags);
+	if (flags.is_decibel && flags.is_ohm) {
+		/*
+		 * The reference impedance for the dBm function is in an
+		 * unexpected format. Naive conversion of non-space chars
+		 * gives incorrect results. Isolate the 4..1200 Ohms value
+		 * instead, ignore the "0." and exponent parts of the
+		 * response text.
+		 */
+		if (strncmp(txt, " 0.", strlen(" 0.")) == 0 && strstr(txt, " E")) {
+			txt = &txt[strlen(" 0.")];
+			txtlen -= strlen(" 0.");
+			p = strchr(txt, 'E');
+			if (p)
+				*p = '\0';
+		}
+	}
 	if (flags.is_fahrenheit || flags.is_celsius) {
 		/*
 		 * The value text in temperature mode includes the C/F
@@ -264,7 +281,9 @@ SR_PRIV int brymen_parse(const uint8_t *buf, float *floatval,
 		analog->meaning->unit = SR_UNIT_AMPERE;
 	}
 	if (flags.is_ohm) {
-		if (flags.is_beep)
+		if (flags.is_decibel)
+			analog->meaning->mq = SR_MQ_RESISTANCE;
+		else if (flags.is_beep)
 			analog->meaning->mq = SR_MQ_CONTINUITY;
 		else
 			analog->meaning->mq = SR_MQ_RESISTANCE;
@@ -296,20 +315,25 @@ SR_PRIV int brymen_parse(const uint8_t *buf, float *floatval,
 	}
 
 	/*
-	 * The high-end Brymen models have a configurable reference impedance.
+	 * The high-end Brymen models have a configurable reference
-	 * When the reference impedance is changed, the DMM sends one packet
+	 * impedance for dBm measurements. When the meter's function
-	 * with the value of the new reference impedance. Both decibel and ohm
+	 * is entered, or when the reference impedance is changed, the
-	 * flags are set in this case, so we must be careful to correctly
+	 * meter sends one packet with the value of the new reference.
-	 * identify the value as ohm, not dBmW.
+	 * Both decibel and ohm flags are set in this case, so we must
+	 * be careful to not clobber the resistance value from above,
+	 * and only provide dBm when the measurement is shown and not
+	 * its reference.
+	 *
+	 * The meter's response values also use an unexpected scale
+	 * (always off by factor 1000, as if it was Watts not mW).
+	 *
+	 * Example responses:
+	 * bfunc: 00 00 20 80, text ' 0. 800 E+1' (reference)
+	 * bfunc: 00 00 20 00, text '-0.3702 E-1' (measurement)
 	 */
 	if (flags.is_decibel && !flags.is_ohm) {
 		analog->meaning->mq = SR_MQ_POWER;
 		analog->meaning->unit = SR_UNIT_DECIBEL_MW;
-		/*
-		 * For some reason, dBm measurements are sent by the multimeter
-		 * with a value three orders of magnitude smaller than the
-		 * displayed value.
-		 */
 		*floatval *= 1000;
 	}