fix: Pi 4 Bluetooth - hold RTS deasserted across BT_ON power-on, require Read Local Version to sync, mini UART no flow control first

kernel/src/Drivers/Bluetooth/A2dp.cpp

@@ -1837,6 +1837,36 @@ namespace Drivers::Bluetooth::A2dp {
     // ConfigureStream
     // =========================================================================
 
+    // A UART transport has a hard byte budget: at 460800 baud it carries
+    // 46 KB/s, and SBC at bitpool 53 / 48 kHz needs ~46 KB/s with its
+    // headers, so the stream fell behind whenever anything else crossed the
+    // link and the sink played concealment. Keep the stream within three
+    // quarters of the line rate by lowering the bitpool (any value in the
+    // negotiated range is valid per frame). Over USB nothing changes.
+    static void FitBitpoolToTransport(uint32_t sampleRate) {
+        uint32_t budget = Hci::TransportBytesPerSecond() / 4 * 3;
+        if (budget == 0) return;
+        uint32_t rate = sampleRate ? sampleRate : 48000;
+        uint8_t minBitpool = g_cfgSbc[2] < 2 ? 2 : g_cfgSbc[2];
+        uint8_t original = g_sbcEncoder.Bitpool;
+        auto bytesPerSecond = [&]() {
+            // H4 + ACL + L2CAP + RTP + SCMS-T + SBC headers (23 bytes) per
+            // packet, spread over the ~4-5 frames bundled into each.
+            uint32_t frameCost = Sbc::GetFrameSize(&g_sbcEncoder) + 6;
+            return (uint64_t)frameCost * rate / Sbc::GetSamplesPerFrame(&g_sbcEncoder);
+        };
+        while (g_sbcEncoder.Bitpool > minBitpool && bytesPerSecond() > budget) {
+            Sbc::Configure(&g_sbcEncoder, g_cfgSbc[0], g_cfgSbc[1],
+                           (uint8_t)(g_sbcEncoder.Bitpool - 1));
+        }
+        if (g_sbcEncoder.Bitpool != original) {
+            KernelLogStream(INFO, "BT-A2DP") << "Bitpool " << (uint64_t)original << " -> "
+                << (uint64_t)g_sbcEncoder.Bitpool << " to fit the transport ("
+                << bytesPerSecond() << " of " << (uint64_t)Hci::TransportBytesPerSecond()
+                << " bytes/s)";
+        }
+    }
+
     bool ConfigureStream(uint32_t sampleRate, uint8_t channels, uint8_t bitsPerSample) {
         // Encoder configuration and PumpMedia both mutate the SBC encoder.
         // Device switching normally configures an Open stream, but explicitly
@@ -1851,6 +1881,7 @@ namespace Drivers::Bluetooth::A2dp {
         // agreed to (Init's defaults may differ from the negotiated subset).
         if (g_cfgSbc[0] || g_cfgSbc[1]) {
             Sbc::Configure(&g_sbcEncoder, g_cfgSbc[0], g_cfgSbc[1], g_cfgSbc[3]);
+            FitBitpoolToTransport(sampleRate);
         }
         g_sbcInitialized = true;
         g_seqNum = 0;