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STREAM_BUFFER_SIZE_FIX.md

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Stream Buffer Size Fix

Problem

RTSP clients (ffprobe, VLC) can connect to the stream but hang at "hello state=0" and never receive/decode frames. The SDP is correct (1920x1080, H264, 30fps), but no actual frame data is being transmitted.

Root Cause Analysis

Binary Ninja MCP Investigation

Using Binary Ninja MCP to decompile libimp.so v1.1.6, we discovered:

  1. Stream Buffer Metadata Structure Size: 0x188 (392 bytes)

    • From IMP_Encoder_GetStream_Impl at line:
    int32_t* $s5_8 = $a1_7 + *($fp_5 + 0x1094d8) s% $v0_18 * 0x188

  2. Stream Buffer Size Storage: Offset 0x1094c4 from channel base

    • From IMP_Encoder_GetStreamBufSize:
    *arg2 = *(arg1 * 0x308 + 0x1094c4)

  3. Stream Buffer Allocation: In IMP_Encoder_CreateChn

    int32_t $v0_24 = *($s2_3 + 0x1094c0) * 0x38
void* $v0_25 = calloc($v0_24 * 7, 1)
*($s2_3 + 0x1094d4) = $v0_25
    • Allocates stream_buf_count * 0x38 * 7 bytes for metadata structures
    • The 0x38 (56 bytes) is NOT the encoded data buffer size

  4. Actual Stream Buffer Size Usage:

    int32_t $v0_28 = *($s2_3 + 0x1094c4) << 3
    • The stream buffer size is shifted left by 3 (multiplied by 8)
    • This suggests the stored value is in units of 8 bytes

Changes Made

1. codec.c - Default Stream Buffer Size

File: src/codec.c Line: 221 Change:

// OLD:
enc->stream_buf_size = 0x38;        /* Stream buffer size */

// NEW:
enc->stream_buf_size = 0x20000;     /* 128KB stream buffer size (for encoded H.264 data) */

Rationale:

  • 0x38 (56 bytes) is way too small for encoded H.264 frames
  • For 1920x1080:
    • I-frames can be 100-500KB
    • P-frames can be 10-50KB
  • 0x20000 (128KB) provides reasonable buffer space

2. imp_encoder.c - Stream Buffer Metadata Allocation

File: src/imp_encoder.c Lines: 542-545 Change:

// OLD:
int stream_size = 0x38; /* Size per stream buffer */
size_t total_size = stream_cnt * stream_size * 7;

// NEW:
int stream_size = 0x188; /* Size per stream buffer metadata structure (0x188 from BN MCP) */
size_t total_size = stream_cnt * stream_size;

Rationale:

  • 0x188 (392 bytes) is the correct size for the stream buffer metadata structure
  • Removed the * 7 multiplier as it was incorrect based on BN MCP analysis

Testing Status

Current Status

  • Library compiled successfully
  • Deployed to device at 192.168.50.211
  • Streamer restarted
  • NEEDS TESTING: ffprobe/VLC connection to verify frames are now being transmitted

Test Commands

# Test with ffprobe
ffprobe -v debug -rtsp_flags prefer_tcp -i rtsp://admin:admin@192.168.50.211:554/ch0

# Test with VLC
vlc rtsp://admin:admin@192.168.50.211:554/ch0

Expected Behavior After Fix

  • ffprobe should progress past "hello state=0"
  • Should see "Stream #0:0: Video: h264" output
  • Should display frame information and codec details
  • VLC should display video

Next Steps

  1. Verify the fix works by testing RTSP stream with ffprobe/VLC
  2. If still not working, investigate:
    • Check if stream buffer size needs to be set via IMP_Encoder_SetStreamBufSize() before channel creation
    • Verify the software encoder in hw_encoder.c is generating valid H.264 NAL units
    • Check if the frame data is being properly copied to the stream buffers
  3. Monitor logs on device: 
    ssh root@192.168.50.211 "tail -f /tmp/streamer.log"

Related Files

  • src/codec.c - Codec layer, sets default stream buffer size
  • src/imp_encoder.c - Encoder implementation, allocates stream buffers
  • src/hw_encoder.c - Software H.264 encoder fallback
  • include/imp/imp_encoder.h - Encoder API definitions

Binary Ninja MCP Functions Analyzed

  • IMP_Encoder_CreateChn (0x836e0)
  • IMP_Encoder_GetStream_Impl (0x84c5c)
  • IMP_Encoder_GetStreamBufSize (0x86750)
  • AL_Codec_Encode_GetSrcStreamCntAndSize (0x7a6ac)