From b223f95bdf2557136d72bfacadeedf89b158ce99 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Do=C4=9Fukan=20Korkmazt=C3=BCrk?= <d.korkmazturk@gmail.com>
Date: Wed, 27 Sep 2023 21:23:18 -0400
Subject: [PATCH] Update the packet submission logic

Packet submission logic is updated to provide a lower channel update
latency.
---
 src/src/rx-serial/SerialSatellite.cpp | 44 +++++++++++++++------------
 src/src/rx-serial/SerialSatellite.h   |  9 +++---
 2 files changed, 30 insertions(+), 23 deletions(-)

diff --git a/src/src/rx-serial/SerialSatellite.cpp b/src/src/rx-serial/SerialSatellite.cpp
index 294fa0e9a8..2976e4e1f6 100644
--- a/src/src/rx-serial/SerialSatellite.cpp
+++ b/src/src/rx-serial/SerialSatellite.cpp
@@ -65,29 +65,35 @@ 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];
+
+    // 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.
+    for (uint8_t channelCount = 0, packetDataCount = 0;
+         (channelCount < SATELLITE_MAX_CHANNELS) && (packetDataCount < SATELLITE_CHANNELS_PER_PACKET);
+         ++channelCount)
     {
-        // These channels are sent in every packet
-        if (ii < SATELLITE_FIRST_RR_CHANNEL)
+        // If the channel data is updated, add it to the packet. Or, if the
+        // space remaining in the packet is equal to the number of remaining
+        // channels to be checked for updates, just fill the packet with the
+        // remaining channels no matter if they have updates or not because we
+        // need to send exactly 7 channel data in every packet.
+        if ((prevChannelData[roundRobinIndex] != channelData[roundRobinIndex]) ||
+            ((SATELLITE_CHANNELS_PER_PACKET - packetDataCount) == (SATELLITE_MAX_CHANNELS - channelCount)))
         {
-            outgoingPacket[ii] = crsfToSatellite(channelData[ii],
-                                                 CRSF_TO_SATELLITE_CH_MAP[ii],
-                                                 satelliteSystem);
+            prevChannelData[roundRobinIndex] = channelData[roundRobinIndex];
+            outgoingPacket[packetDataCount++] =
+                crsfToSatellite(channelData[roundRobinIndex],
+                                CRSF_TO_SATELLITE_CH_MAP[roundRobinIndex],
+                                satelliteSystem);
         }
-        // Round-robin channels
-        else
+
+        ++roundRobinIndex;
+        if (roundRobinIndex >= SATELLITE_MAX_CHANNELS)
         {
-            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;
-            }
+            roundRobinIndex = 0;
         }
     }
 
diff --git a/src/src/rx-serial/SerialSatellite.h b/src/src/rx-serial/SerialSatellite.h
index 6e63d62c4e..46171a836d 100644
--- a/src/src/rx-serial/SerialSatellite.h
+++ b/src/src/rx-serial/SerialSatellite.h
@@ -13,13 +13,14 @@ 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] = {0};
+
     uint8_t fadeCount{0};
-    uint8_t rr{SATELLITE_FIRST_RR_CHANNEL};
+    uint8_t roundRobinIndex{0};
     bool sendPackets{false};
 };
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