rdtech-dps: Retry sr_modbus_read_holding_registers() up to 3 times.

The communication with the rdtech power supplies is not very reliable,
especially during the start. Because of that, the driver tries to read the
modbus registers up to three times.
Nevertheless there is the chance, that the communication fails.

src/hardware/rdtech-dps/protocol.c

@@ -21,6 +21,21 @@
 #include <config.h>
 #include "protocol.h"
 
+SR_PRIV int rdtech_dps_read_holding_registers(struct sr_modbus_dev_inst *modbus,
+		int address, int nb_registers, uint16_t *registers)
+{
+	int i, ret;
+
+	i = 0;
+	do {
+		ret = sr_modbus_read_holding_registers(modbus,
+			address, nb_registers, registers);
+		++i;
+	} while (ret != SR_OK && i < 3);
+
+	return ret;
+}
+
 SR_PRIV int rdtech_dps_get_reg(const struct sr_dev_inst *sdi,
 		uint16_t address, uint16_t *value)
 {
@@ -33,7 +48,7 @@ SR_PRIV int rdtech_dps_get_reg(const struct sr_dev_inst *sdi,
 	modbus = sdi->conn;
 
 	g_mutex_lock(&devc->rw_mutex);
-	ret = sr_modbus_read_holding_registers(modbus, address, 1, registers);
+	ret = rdtech_dps_read_holding_registers(modbus, address, 1, registers);
 	g_mutex_unlock(&devc->rw_mutex);
 	*value = RB16(registers + 0);
 	return ret;
@@ -67,7 +82,7 @@ SR_PRIV int rdtech_dps_get_model_version(struct sr_modbus_dev_inst *modbus,
 	 * No mutex here, because there is no sr_dev_inst when this function
 	 * is called.
 	 */
-	ret = sr_modbus_read_holding_registers(modbus, REG_MODEL, 2, registers);
+	ret = rdtech_dps_read_holding_registers(modbus, REG_MODEL, 2, registers);
 	if (ret == SR_OK) {
 		*model = RB16(registers + 0);
 		*version = RB16(registers + 1);
@@ -119,6 +134,10 @@ SR_PRIV int rdtech_dps_receive_data(int fd, int revents, void *cb_data)
 	devc = sdi->priv;
 
 	g_mutex_lock(&devc->rw_mutex);
+	/*
+	 * Using the libsigrok function here, because it doesn't matter if the
+	 * reading fails. It will be done again in the next acquision cycle anyways.
+	 */
 	ret = sr_modbus_read_holding_registers(modbus, REG_UOUT, 3, registers);
 	g_mutex_unlock(&devc->rw_mutex);
 

src/hardware/rdtech-dps/protocol.h

@@ -85,6 +85,9 @@ enum rdtech_dps_mode {
 	MODE_CC      = 1,
 };
 
+SR_PRIV int rdtech_dps_read_holding_registers(struct sr_modbus_dev_inst *modbus,
+		int address, int nb_registers, uint16_t *registers);
+
 SR_PRIV int rdtech_dps_get_reg(const struct sr_dev_inst *sdi, uint16_t address, uint16_t *value);
 SR_PRIV int rdtech_dps_set_reg(const struct sr_dev_inst *sdi, uint16_t address, uint16_t value);