scpi: Add a function to read and wait on a *OPC? reply.

The SCPI standard specifies the "*OPC?" command (Operation complete query) which
queries the instrument for its operative state. When all pending operations are
complete, the instrument responds with a "1".

Some manufacturers block before completing all operations and don't respond
with anything and some of them respond with a "0". This function handles both
cases uniformly.

hardware/common/scpi.c

@@ -307,6 +307,31 @@ SR_PRIV int sr_scpi_get_double(struct sr_serial_dev_inst *serial,
 	return ret;
 }
 
+/**
+ * Send a SCPI *OPC? command, read the reply and return the result of the
+ * command.
+ *
+ * @param serial Previously initialized serial port structure.
+ *
+ * @return SR_OK on success, SR_ERR on failure.
+ */
+SR_PRIV int sr_scpi_get_opc(struct sr_serial_dev_inst *serial)
+{
+	unsigned int i;
+	gboolean opc;
+
+	for (i = 0; i < SCPI_READ_RETRIES; ++i) {
+		sr_scpi_get_bool(serial, SCPI_CMD_OPC, &opc);
+
+		if (opc)
+			return SR_OK;
+
+		g_usleep(SCPI_READ_RETRY_TIMEOUT);
+	}
+
+	return SR_ERR;
+}
+
 /**
  * Send the *IDN? SCPI command, receive the reply, parse it and store the
  * reply as a sr_scpi_hw_info structure in the supplied scpi_response pointer.

libsigrok-internal.h

@@ -288,6 +288,7 @@ SR_PRIV int sr_scpi_get_float(struct sr_serial_dev_inst *serial,
 			      const char *command, float *scpi_response);
 SR_PRIV int sr_scpi_get_double(struct sr_serial_dev_inst *serial,
 			      const char *command, double *scpi_response);
+SR_PRIV int sr_scpi_get_opc(struct sr_serial_dev_inst *serial);
 SR_PRIV int sr_scpi_get_hw_id(struct sr_serial_dev_inst *serial,
 			      struct sr_scpi_hw_info **scpi_reponse);
 SR_PRIV void sr_scpi_hw_info_free(struct sr_scpi_hw_info *hw_info);