# GPU Documentation

## The Xenos Chip

The [Xenos](https://en.wikipedia.org/wiki/Xenos_\(graphics_chip\)) is a graphics
chip designed by AMD based off of the R5xx architecture.

### Command Processing

The Xenos runs commands supplied to it directly by the DirectX bare-bones driver
via a ringbuffer located in system memory.

The bulk of the command processing code is located at
[src/xenia/gpu/command_processor.cc](../src/xenia/gpu/command_processor.cc)

### EDRAM

The Xenos uses special high-speed memory located on the same die as the chip to 
store framebuffers/render targets.

TODO: More documentation

## Options

### General

See the top of [src/xenia/gpu/gpu_flags.cc](../src/xenia/gpu/gpu_flags.cc).

`--vsync=false` will attempt to render the game as fast as possible instead of
waiting for a fixed 60hz timer.

### Vulkan

See the top of [src/xenia/gpu/vulkan/vulkan_gpu_flags.cc](../src/xenia/vulkan/vulkan_gpu_flags.cc).

`vulkan_dump_disasm=true` "Dump shader disassembly. NVIDIA only supported."

## Tools

### Shaders

#### Shader Dumps

Adding `--dump_shaders=path/` will write all translated shaders to the given
path with names based on input hash (so they'll be stable across runs).
Binaries containing the original microcode will be placed side-by-side with
the dumped output to make it easy to pipe to `xe-gpu-shader-compiler`.

#### xe-gpu-shader-compiler

A standalone shader compiler exists to allow for quick shader translation
testing. You can pass a binary ucode shader in and get either disassembled
ucode or translated source out. This is best used through the Shader
Playground tool.

```
  xe-gpu-shader-compiler \
      --shader_input=input_file.bin.vs (or .fs)
      --shader_output=output_file.txt
      --shader_output_type=ucode (or spirvtext)
```

#### Shader Playground

Built separately (for now) under [tools/shader-playground/](../tools/shader-playground/)
is a GUI for interactive shader assembly, disassembly, validation, and
translation.

![Shader Playground Screenshot](images/shader_playground.png?raw=true)

Entering shader microcode on the left will invoke the XNA Game Studio
D3D compiler to translate the ucode to binary. The D3D compiler is then
used to disassemble the binary and display the optimized form. If
`xe-gpu-shader-compiler` has been built the ucode will be passed to that
for disassembly and that will then be passed through D3D compiler. If
the output of D3D compiler on the xenia disassembly doesn't match the
original D3D compiler output the box will turn red, indicating that the
disassembly is broken. Finally, the right most box will show the
translated shader in the desired format.

For more information and setup instructions see
[tools/shader-playground/README.md](../tools/shader-playground/README.md).

### xe-gpu-trace-viewer

To quickly iterate on graphical issues, xenia can dump frames (or sequences of
frames) while running that can be opened and inspected in a separate app.

The basic workflow is:

1. Capture the frame in game (using F4) or a stream of frames.
2. Add the file path to the xe-gpu-trace-viewer Debugging command line in
Visual Studio.
3. Launch xe-gpu-trace-viewer.
4. Poke around, find issues, etc.
5. Modify code.
6. Build and relaunch.
7. Goto 4.

#### Capturing Frames

First, specify a path to capture traces to with
`--trace_gpu_prefix=path/file_prefix_`. All files will have a randomish name
based on that.

When running xenia.exe you can hit F4 at any time to capture the next frame the
game tries to draw (up until a VdSwap call). The file can be used immediately.

#### Capturing Sequences

Passing `--trace_gpu_stream` will write all frames rendered to a file, allowing
you to seek through them in the trace viewer. These files will get large.

## References

### Command Buffer/Registers

Registers documented at [src/xenia/gpu/register_table.inc](../src/xenia/gpu/register_table.inc).

PM4 commands documented at [src/xenia/gpu/xenos.h](../src/xenia/gpu/xenos.h#L521).

### Shaders

* [LLVM R600 Tables](https://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/AMDGPU/R600Instructions.td)
** The opcode formats don't match, but the name->psuedo code is correct.
* [xemit](https://github.com/gligli/libxemit/blob/master/xemitops.c)