MAVLink: RC_CHANNELS_OVERRIDE generate correct RC channel count (PX4#11219)

* mavlink_receiver: generate rc channel count * mavlink_receiver: zero out ignored rc channels, add comments

Author: MaEtUgR

src/modules/mavlink/mavlink_receiver.cpp

@@ -1834,7 +1834,6 @@ MavlinkReceiver::handle_message_rc_channels_override(mavlink_message_t *msg)
 // metadata
 rc.timestamp = hrt_absolute_time();
 rc.timestamp_last_signal = rc.timestamp;
-rc.channel_count = 18;
 rc.rssi = RC_INPUT_RSSI_MAX;
 rc.rc_failsafe = false;
 rc.rc_lost = false;
@@ -1862,6 +1861,22 @@ MavlinkReceiver::handle_message_rc_channels_override(mavlink_message_t *msg)
 rc.values[16] = man.chan17_raw;
 rc.values[17] = man.chan18_raw;
 
+// check how many channels are valid
+for (int i = 17; i >= 0; i--) {
+const bool ignore_max = rc.values[i] == UINT16_MAX; // ignore any channel with value UINT16_MAX
+const bool ignore_zero = (i > 7) && (rc.values[i] == 0); // ignore channel 8-18 if value is 0
+
+if (ignore_max || ignore_zero) {
+// set all ignored values to zero
+rc.values[i] = 0;
+
+} else {
+// first channel to not ignore -> set count considering zero-based index
+rc.channel_count = i + 1;
+break;
+}
+}
+
 // publish uORB message
 int instance; // provides the instance ID or the publication
 ORB_PRIO priority = ORB_PRIO_HIGH; // since it is an override, set priority high