[XMA] Fix multiple audio issues in new decoder

- Handle XMA2 frames with headers split across packet boundaries
  (fixes audio cutoff in Capcom games)
- Output buffer minimum space check using actual frame size instead
  of SDC-based formula, allowing tight-buffer contexts to decode
  (fixes PGR4 engine sounds)
- Gate consume-only context clearing with tight-buffer heuristic to
  prevent disrupting playback while still allowing stream completion
  signaling (fixes Borderlands 2 dialog)
- Clamp read offset 0 to packet header size to prevent decode rejection
  loop (fixes Dirt 2 crashes)

src/xenia/apu/xma_context.h

@@ -154,6 +154,11 @@ struct XMA_CONTEXT_DATA {
   const bool IsConsumeOnlyContext() const {
     return (input_buffer_0_packet_count | input_buffer_1_packet_count) == 0;
   }
+  // Whether the SDC-based minimum exceeds the output buffer size.
+  const bool HasTightOutputBuffer() const {
+    return (int32_t)((subframe_decode_count * 2) - 1) >
+           (int32_t)output_buffer_block_count;
+  }
 };
 static_assert_size(XMA_CONTEXT_DATA, 64);
 

src/xenia/apu/xma_context_new.cc

@@ -130,7 +130,16 @@ bool XmaContextNew::Work() {
 
   if (data.IsConsumeOnlyContext()) {
     Consume(&output_rb, &data);
-    if (data.output_buffer_read_offset == data.output_buffer_write_offset) {
+    // Clearing contexts that match TightBufferOutput heuristic can disrupt
+    // playback (e.g. audio noise during races in PGR4), so we only clear
+    // contexts with enough output buffer headroom where empty input reliably
+    // indicates the stream is finished (e.g. needed for dialog completion in
+    // Borderlands 2 startup).
+    // NOTE(has207): this feels wrong to have such a heuristic but it seems to
+    // work for the known cases. May need to be revisited if failing cases are
+    // found.
+    if (!data.HasTightOutputBuffer() &&
+        data.output_buffer_read_offset == data.output_buffer_write_offset) {
       ClearLocked(&data);
     }
     data.Store(context_ptr);
@@ -138,7 +147,7 @@ bool XmaContextNew::Work() {
   }
 
   const int32_t minimum_subframe_decode_count =
-      (data.subframe_decode_count * 2) - 1;
+      (kBytesPerFrameChannel / kOutputBytesPerBlock) << data.is_stereo;
 
   // We don't have enough space to even make one pass
   // Waiting for decoder to return more space.
@@ -337,6 +346,13 @@ void XmaContextNew::Decode(XMA_CONTEXT_DATA* data) {
       static_cast<void*>(current_input_buffer), data->output_buffer_ptr,
       data->output_buffer_block_count);
 
+  // Games like Dirt 2 can kick the decoder with read offset 0 (pointing into
+  // the packet header) before filling in a valid offset. Clamp to the first
+  // valid data position to avoid rejecting the packet entirely.
+  if (data->input_buffer_read_offset < kBitsPerPacketHeader) {
+    data->input_buffer_read_offset = kBitsPerPacketHeader;
+  }
+
   const uint32_t current_input_size = GetCurrentInputBufferSize(data);
   const uint32_t current_input_packet_count =
       current_input_size / kBytesPerPacket;
@@ -360,10 +376,42 @@ void XmaContextNew::Decode(XMA_CONTEXT_DATA* data) {
 
   const uint32_t relative_offset =
       data->input_buffer_read_offset % kBitsPerPacket;
-  const kPacketInfo packet_info = GetPacketInfo(packet, relative_offset);
+  kPacketInfo packet_info = GetPacketInfo(packet, relative_offset);
   const uint32_t packet_to_skip = xma::GetPacketSkipCount(packet) + 1;
   const uint32_t next_packet_index = packet_index + packet_to_skip;
 
+  // Frame header split across packet boundary — combine packets to read
+  // the full 15-bit header and resolve the real frame size.
+  // Only detected for XMA2 packets where the header provides an authoritative
+  // frame count. XMA1 packets lack a frame count field so split headers
+  // cannot be detected — if XMA1 encoders can produce them, those frames
+  // will still be silently lost.
+  if (packet_info.current_frame_size_ == 0) {
+    const uint8_t* next_packet =
+        GetNextPacket(data, next_packet_index, current_input_packet_count);
+    if (!next_packet) {
+      // Matching split-body error handling below; correct error code unknown.
+      data->error_status = 4;
+      return;
+    }
+    std::memcpy(input_buffer_.data(), packet + kBytesPerPacketHeader,
+                kBytesPerPacketData);
+    std::memcpy(input_buffer_.data() + kBytesPerPacketData,
+                next_packet + kBytesPerPacketHeader, kBytesPerPacketData);
+
+    BitStream combined(input_buffer_.data(),
+                       (kBitsPerPacket - kBitsPerPacketHeader) * 2);
+    combined.SetOffset(relative_offset - kBitsPerPacketHeader);
+
+    uint64_t frame_size = combined.Peek(kBitsPerFrameHeader);
+    if (frame_size == xma::kMaxFrameLength) {
+      // Matching split-body error handling below; correct error code unknown.
+      data->error_status = 4;
+      return;
+    }
+    packet_info.current_frame_size_ = (uint32_t)frame_size;
+  }
+
   BitStream stream =
       BitStream(current_input_buffer, (packet_index + 1) * kBitsPerPacket);
   stream.SetOffset(data->input_buffer_read_offset);
@@ -623,7 +671,15 @@ const kPacketInfo XmaContextNew::GetPacketInfo(uint8_t* packet,
 
   if (xma::IsPacketXma2Type(packet)) {
     const uint8_t xma2_frame_count = xma::GetPacketFrameCount(packet);
-    if (xma2_frame_count != packet_info.frame_count_) {
+    if (xma2_frame_count > packet_info.frame_count_) {
+      // Frame header split across packet boundary — scanner couldn't
+      // peek the full 15-bit header. Trust the XMA2 header count.
+      if (packet_info.current_frame_size_ == 0) {
+        // Current frame is the split-header frame
+        packet_info.current_frame_ = packet_info.frame_count_;
+      }
+      packet_info.frame_count_ = xma2_frame_count;
+    } else if (xma2_frame_count != packet_info.frame_count_) {
       XELOGE(
           "XmaContext {}: XMA2 packet header defines different amount of "
           "frames than internally found! (Header: {} Found: {})",