Update the packet submission logic

dkorkmazturk · dkorkmazturk · commit fe544655e08f · 2023-09-27T21:24:12.000-04:00
Packet submission logic is updated to provide a lower channel update
latency.
diff --git a/src/src/rx-serial/SerialSatellite.cpp b/src/src/rx-serial/SerialSatellite.cpp
@@ -65,29 +65,52 @@ uint32_t SerialSatellite::sendRCFrame(bool frameAvailable, uint32_t *channelData
     }
     sendPackets = true;
 
-    uint16_t outgoingPacket[SATELLITE_CHANNEL_DATA_LENGTH];
-    for (uint8_t ii = 0; ii < SATELLITE_CHANNEL_DATA_LENGTH; ++ii)
+    uint16_t outgoingPacket[SATELLITE_CHANNELS_PER_PACKET];
+    uint8_t outgoingPacketIndex = 0;
+    const auto roundRobinIndexStartValue = roundRobinIndex;
+
+    // Our bandwidth is limited, so try to send updates only for the channels
+    // with new values to reduce the channel update latency. Updates are
+    // checked in round-robin fashion to make sure that the channels do not
+    // starve.
+    do
     {
-        // These channels are sent in every packet
-        if (ii < SATELLITE_FIRST_RR_CHANNEL)
+        if (prevChannelData[roundRobinIndex] != channelData[roundRobinIndex])
         {
-            outgoingPacket[ii] = crsfToSatellite(channelData[ii],
-                                                 CRSF_TO_SATELLITE_CH_MAP[ii],
-                                                 satelliteSystem);
+            prevChannelData[roundRobinIndex] = channelData[roundRobinIndex];
+            outgoingPacket[outgoingPacketIndex++] =
+                crsfToSatellite(channelData[roundRobinIndex],
+                                CRSF_TO_SATELLITE_CH_MAP[roundRobinIndex],
+                                satelliteSystem);
+            // Mark the channel as "sent", so we wouldn't submit it more than
+            // once in a packet.
+            channelData[roundRobinIndex] = UINT32_MAX;
         }
-        // Round-robin channels
-        else
+
+        ++roundRobinIndex;
+        if (roundRobinIndex >= SATELLITE_MAX_CHANNELS)
+        {
+            roundRobinIndex = 0;
+        }
+        if (outgoingPacketIndex >= SATELLITE_CHANNELS_PER_PACKET)
+        {
+            break;
+        }
+    } while (roundRobinIndex != roundRobinIndexStartValue);
+
+    // If less than 7 channel values have been changed since the last update,
+    // fill the rest of the packet with data from arbitrary channels.
+    for (uint8_t index = 0;
+         (outgoingPacketIndex < SATELLITE_CHANNELS_PER_PACKET) && (index < SATELLITE_MAX_CHANNELS);
+         ++index)
+    {
+        // If the channel is not marked as "sent", add it to the packet.
+        if (channelData[index] != UINT32_MAX)
         {
-            outgoingPacket[ii] = crsfToSatellite(channelData[rr],
-                                                 CRSF_TO_SATELLITE_CH_MAP[rr],
-                                                 satelliteSystem);
-
-            // Update the round-robin index
-            ++rr;
-            if (rr >= SATELLITE_NUM_CHANNELS)
-            {
-                rr = SATELLITE_FIRST_RR_CHANNEL;
-            }
+            outgoingPacket[outgoingPacketIndex++] =
+                crsfToSatellite(channelData[index],
+                                CRSF_TO_SATELLITE_CH_MAP[index],
+                                satelliteSystem);
         }
     }
 
diff --git a/src/src/rx-serial/SerialSatellite.h b/src/src/rx-serial/SerialSatellite.h
@@ -4,7 +4,9 @@ class SerialSatellite : public SerialIO
 {
 public:
     SerialSatellite(Stream &out, Stream &in)
-        : SerialIO(&out, &in) {}
+        : SerialIO(&out, &in) {
+            memset(prevChannelData, UINT32_MAX, sizeof(prevChannelData));
+        }
 
     virtual ~SerialSatellite() = default;
 
@@ -13,13 +15,14 @@ class SerialSatellite : public SerialIO
     uint32_t sendRCFrame(bool frameAvailable, uint32_t *channelData) override;
 
 private:
-    static constexpr uint16_t SATELLITE_NUM_CHANNELS = 12;
-    static constexpr uint8_t SATELLITE_FIRST_RR_CHANNEL = 4;
-    static constexpr uint8_t SATELLITE_CHANNEL_DATA_LENGTH = 7;
+    static constexpr uint16_t SATELLITE_MAX_CHANNELS = 12;
+    static constexpr uint8_t SATELLITE_CHANNELS_PER_PACKET = 7;
 
     void processBytes(uint8_t *bytes, uint16_t size) override{};
 
+    uint32_t prevChannelData[SATELLITE_CHANNELS_PER_PACKET];
+
     uint8_t fadeCount{0};
-    uint8_t rr{SATELLITE_FIRST_RR_CHANNEL};
+    uint8_t roundRobinIndex{0};
     bool sendPackets{false};
 };
