[GPU] Reasons to keep non-Vulkan backends [ci skip]

src/xenia/app/xenia_main.cc

@@ -259,6 +259,77 @@ std::unique_ptr<apu::AudioSystem> EmulatorApp::CreateAudioSystem(
 }
 
 std::unique_ptr<gpu::GraphicsSystem> EmulatorApp::CreateGraphicsSystem() {
+  // While Vulkan is supported by a large variety of operating systems (Windows,
+  // GNU/Linux, Android, also via the MoltenVK translation layer on top of Metal
+  // on macOS and iOS), please don't remove platform-specific GPU backends from
+  // Xenia.
+  //
+  // Regardless of the operating system, having multiple options provides more
+  // stability to users. In case of driver issues, users may try switching
+  // between the available backends. For example, in June 2022, on Nvidia Ampere
+  // (RTX 30xx), Xenia had synchronization issues that resulted in flickering,
+  // most prominently in 4D5307E6, on Direct3D 12 - but the same issue was not
+  // reproducible in the Vulkan backend, however, it used ImageSampleExplicitLod
+  // with explicit gradients for cubemaps, which triggered a different driver
+  // bug on Nvidia (every 1 out of 2x2 pixels receiving junk).
+  //
+  // Specifically on Microsoft platforms, there are a few reasons why supporting
+  // Direct3D 12 is desirable rather than limiting Xenia to Vulkan only:
+  // - Wider hardware support for Direct3D 12 on x86 Windows desktops.
+  //   Direct3D 12 requires the minimum of Nvidia Fermi, or, with a pre-2021
+  //   driver version, Intel HD Graphics 4200. Vulkan, however, is supported
+  //   only starting with Nvidia Kepler and a much more recent Intel UHD
+  //   Graphics generation.
+  // - Wider hardware support on other kinds of Microsoft devices. The Xbox One
+  //   and the Xbox Series X|S only support Direct3D as the GPU API in their UWP
+  //   runtime, and only version 12 can be granted expanded resource access.
+  //   Qualcomm, as of June 2022, also doesn't provide a Vulkan implementation
+  //   for their Arm-based Windows devices, while Direct3D 12 is available.
+  //   - Both older Intel GPUs and the Xbox One apparently, as well as earlier
+  //     Windows 10 versions, also require Shader Model 5.1 DXBC shaders rather
+  //     than Shader Model 6 DXIL ones, so a DXBC shader translator should be
+  //     available in Xenia too, a DXIL one doesn't fully replace it.
+  // - As of June 2022, AMD also refuses to implement the
+  //   VK_EXT_fragment_shader_interlock Vulkan extension in their drivers, as
+  //   well as its OpenGL counterpart, which is heavily utilized for accurate
+  //   support of Xenos render target formats that don't have PC equivalents
+  //   (8_8_8_8_GAMMA, 2_10_10_10_FLOAT, 16_16 and 16_16_16_16 with -32 to 32
+  //   range, D24FS8) with correct blending. Direct3D 12, however, requires
+  //   support for similar functionality (rasterizer-ordered views) on the
+  //   feature level 12_1, and the AMD driver implements it on Direct3D, as well
+  //   as raster order groups in their Metal driver.
+  //
+  // Additionally, different host GPU APIs receive feature support at different
+  // paces. VK_EXT_fragment_shader_interlock first appeared in 2019, for
+  // instance, while Xenia had been taking advantage of rasterizer-ordered views
+  // on Direct3D 12 for over half a year at that point (they have existed in
+  // Direct3D 12 since the first version).
+  //
+  // MoltenVK on top Metal also has its flaws and limitations. Metal, for
+  // instance, as of June 2022, doesn't provide a switch for primitive restart,
+  // while Vulkan does - so MoltenVK is not completely transparent to Xenia,
+  // many of its issues that may be not very obvious (unlike when the Metal API
+  // is used directly) should be taken into account in Xenia. Also, as of June
+  // 2022, MoltenVK translates SPIR-V shaders into the C++-based Metal Shading
+  // Language rather than AIR directly, which likely massively increases
+  // pipeline object creation time - and Xenia translates shaders and creates
+  // pipelines when they're first actually used for a draw command by the game,
+  // thus it can't precompile anything that hasn't ever been encountered before
+  // there's already no time to waste.
+  //
+  // Very old hardware (Direct3D 10 level) is also not supported by most Vulkan
+  // drivers. However, in the future, Xenia may be ported to it using the
+  // Direct3D 11 API with the feature level 10_1 or 10_0. OpenGL, however, had
+  // been lagging behind Direct3D prior to versions 4.x, and didn't receive
+  // compute shaders until a 4.2 extension (while 4.2 already corresponds
+  // roughly to Direct3D 11 features) - and replacing Xenia compute shaders with
+  // transform feedback / stream output is not always trivial (in particular,
+  // will need to rely on GL_ARB_transform_feedback3 for skipping over memory
+  // locations that shouldn't be overwritten).
+  //
+  // For maintainability, as much implementation code as possible should be
+  // placed in `xe::gpu` and shared between the backends rather than duplicated
+  // between them.
   Factory<gpu::GraphicsSystem> factory;
 #if XE_PLATFORM_WIN32
   factory.Add<gpu::d3d12::D3D12GraphicsSystem>("d3d12");