serial: make failed flush() in open() non-fatal, CP2110 flush() return

Commit cb828f1b3e introduced an unconditional flush() call in the
open() routine's body, and passed its return value in verbatim form to
open() callers. Some of the transports/cables for serial communication
yield the SR_ERR_NA return value. Consider this a non-fatal condition.

Unbreak the CP2110 HID chip's flush() implementation. Don't expect a
return value of 0 from HID write calls, instead expect to see the number
of written bytes for successful calls.

This was tested with ch9325 (UT-D04), cp2110 (UT-D09) and bu86x.

src/serial.c

@@ -128,7 +128,18 @@ SR_PRIV int serial_open(struct sr_serial_dev_inst *serial, int flags)
 			return ret;
 	}
 
-	return serial_flush(serial);
+	/*
+	 * Flush potentially dangling RX data. Availability of the
+	 * flush primitive depends on the transport/cable, absense
+	 * is non-fatal.
+	 */
+	ret = serial_flush(serial);
+	if (ret == SR_ERR_NA)
+		ret = SR_OK;
+	if (ret != SR_OK)
+		return ret;
+
+	return SR_OK;
 }
 
 /**

src/serial_hid_cp2110.c

@@ -280,7 +280,7 @@ static int cp2110_flush(struct sr_serial_dev_inst *serial)
 	buffer[0] = CP2110_FIFO_PURGE;
 	buffer[1] = CP2110_FIFO_PURGE_TX | CP2110_FIFO_PURGE_RX;
 	rc = ser_hid_hidapi_set_data(serial, 0, buffer, sizeof(buffer), 0);
-	if (rc != 0)
+	if (rc != sizeof(buffer))
 		return SR_ERR;
 	return SR_OK;
 }