BVH Debug

1. INTEL_DEBUG=bvh_tlas,bvh_blas will generate tlas_{id}.txt or blas_{id}.txt in bvh_dump/BVH_ANV directory.
2. INTEL_DEBUG=bvh_tlas_ir_hdr,bvh_blas_ir_hdr will generate tlas_{id}.txt or blas_{id}.txt in bvh_dump/BVH_IR_HDR directory.
3. INTEL_DEBUG=bvh_tlas_ir_as,bvh_blas_ir_as will generate tlas_{id}.txt or blas_{id}.txt in bvh_dump/BVH_IR_AS directory.
4. INTEL_DEBUG=bvh_no_build will skip the intel-specific-encoding part. This will make the bvh NULL and the HW traversal will see this as a miss.

The dumped text file contains memory dump, byte-by-byte in hex. The contents are contiguous memory of a certain region.

1. The dump in BVH_ANV starts from the beginning of anv_accel_struct_header to the end of the bvh.
2. The dump in BVH_IR_HDR records vk_ir_header, which sits at the beginning of ir bvh.
3. The dump in BVH_IR_AS records all vk_ir_{leaf_type}_node and vk_ir_box_node in ir bvh. The region starts from where leaves are encoded to the end of ir bvh.

Decode the dump

We have a way to decode the dump in BVH_ANV.

• To decode this memory dump, use a python script to parse the file and generate a human-readable json.
• To further visualize the tree, there is a html that parses the json and draws the tree topology and 3D views of bounding boxes.

# Using blas_0 as an example
xxd -r -p bvh_dump/BVH_ANV/blas_0.txt > input.bin

# Use a python script to generate a human-readable json file
cd mesa/src/intel/vulkan/bvh/
python3 interpret.py <path/to/input.bin>

# To further visualize the tree, the html parses the json and draws it in 3D.
cd mesa/src/intel/vulkan/bvh/
python3 -m http.server 8000
# go to localhost:8000/visualize_json.html

Note and Limitations:

1. The python script use ctypes to interpret the memory dump, so the structure defined in the script should match the structure defined in the driver.
2. The memory dump is a snapshot of a VkBuffer captured at the end of CmdBuildAccelerationStructure call. It won't capture any bvh obtained from CmdCopy.
3. CPU will wait for the the command buffer to be completed, and saves the recorded bvh into files on disk.