GCS_MAVLink: Cleanup and Reduce chance of GCS FTP timeout

Delete unneeded orphan comment
replace get_last_txbuf() with a predicate
Make txbuf flow control threashold consistent between Parameter download and FTP and keep it in range where we are also slowing down normal streams
Delay sending text banner until after first FTP response to reduce latency on slow links
Don't let flow control delay setting ftp.last_send_ms so as to slow down normal streams as soon as possible to improve FTP response time

libraries/GCS_MAVLink/GCS.h

@@ -319,7 +319,7 @@ class GCS_MAVLINK
         return last_radio_status.remrssi_ms;
     }
     static float telemetry_radio_rssi(); // 0==no signal, 1==full signal
-    static uint8_t get_last_txbuf();
+    static bool last_txbuf_is_greater(uint8_t txbuf_limit);
 
     // mission item index to be sent on queued msg, delayed or not
     uint16_t mission_item_reached_index = AP_MISSION_CMD_INDEX_NONE;
@@ -814,7 +814,6 @@ class GCS_MAVLINK
 
     // number of extra ms to add to slow things down for the radio
     uint16_t         stream_slowdown_ms;
-    // last reported radio buffer percent available
 
     // outbound ("deferred message") queue.
 

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -833,13 +833,13 @@ float GCS_MAVLINK::telemetry_radio_rssi()
     return last_radio_status.rssi/254.0f;
 }
 
-uint8_t GCS_MAVLINK::get_last_txbuf()
+bool GCS_MAVLINK::last_txbuf_is_greater(uint8_t txbuf_limit)
 {
     if (AP_HAL::millis() - last_radio_status.received_ms > 5000) {
         // stale report
-        return 100;
+        return true;
     }
-    return last_radio_status.txbuf;
+    return last_radio_status.txbuf > txbuf_limit;
 }
 
 void GCS_MAVLINK::handle_radio_status(const mavlink_message_t &msg)
@@ -1173,7 +1173,7 @@ uint16_t GCS_MAVLINK::get_reschedule_interval_ms(const deferred_message_bucket_t
         interval_ms *= 4;
     }
 #if AP_MAVLINK_FTP_ENABLED
-    if (AP_HAL::millis() - ftp.last_send_ms < 500) {
+    if (AP_HAL::millis() - ftp.last_send_ms < 1000) {
         // we are sending ftp replies
         interval_ms *= 4;
     }

libraries/GCS_MAVLink/GCS_FTP.cpp

@@ -97,15 +97,11 @@ void GCS_MAVLINK::handle_file_transfer_protocol(const mavlink_message_t &msg) {
 
 bool GCS_MAVLINK::send_ftp_reply(const pending_ftp &reply)
 {
-    /*
-      provide same banner we would give with old param download
-    */
-    if (ftp.need_banner_send_mask & (1U<<reply.chan)) {
-        ftp.need_banner_send_mask &= ~(1U<<reply.chan);
-        send_banner();
+    if (!last_txbuf_is_greater(33)) { // It helps avoid GCS timeout if this is less than the threshold where we slow down normal streams (<=49)
+        return false;
     }
     WITH_SEMAPHORE(comm_chan_lock(reply.chan));
-    if (!HAVE_PAYLOAD_SPACE(chan, FILE_TRANSFER_PROTOCOL) || get_last_txbuf() < 33) {
+    if (!HAVE_PAYLOAD_SPACE(chan, FILE_TRANSFER_PROTOCOL)) {
         return false;
     }
     uint8_t payload[251] = {};
@@ -121,11 +117,6 @@ bool GCS_MAVLINK::send_ftp_reply(const pending_ftp &reply)
         reply.chan,
         0, reply.sysid, reply.compid,
         payload);
-    if (reply.req_opcode == FTP_OP::TerminateSession) {
-        ftp.last_send_ms = 0;
-    } else {
-        ftp.last_send_ms = AP_HAL::millis();
-    }
     return true;
 }
 
@@ -155,9 +146,25 @@ void GCS_MAVLINK::ftp_error(struct pending_ftp &response, FTP_ERROR error) {
 // send our response back out to the system
 void GCS_MAVLINK::ftp_push_replies(pending_ftp &reply)
 {
+    ftp.last_send_ms = AP_HAL::millis(); // Used to detect active FTP session
+
     while (!send_ftp_reply(reply)) {
         hal.scheduler->delay(2);
     }
+
+    if (reply.req_opcode == FTP_OP::TerminateSession) {
+        ftp.last_send_ms = 0;
+    }
+    /*
+      provide same banner we would give with old param download
+      Do this after send_ftp_reply() to get the first FTP response out sooner
+      on slow links to avoid GCS timeout.  The slowdown of normal streams in
+      get_reschedule_interval_ms() should help for subsequent responses.
+    */
+    if (ftp.need_banner_send_mask & (1U<<reply.chan)) {
+        ftp.need_banner_send_mask &= ~(1U<<reply.chan);
+        send_banner();
+    }
 }
 
 void GCS_MAVLINK::ftp_worker(void) {

libraries/GCS_MAVLink/GCS_Param.cpp

@@ -75,7 +75,7 @@ GCS_MAVLINK::queued_param_send()
     }
     count -= async_replies_sent_count;
 
-    while (count && _queued_parameter != nullptr && get_last_txbuf() > 50) {
+    while (count && _queued_parameter != nullptr && last_txbuf_is_greater(33)) {
         char param_name[AP_MAX_NAME_SIZE];
         _queued_parameter->copy_name_token(_queued_parameter_token, param_name, sizeof(param_name), true);