Vulkan: phase 05

[midwan]

Phase 5: Multi-threading and Synchronization

Objective

Take full advantage of Vulkan by separating the emulation (CPU) thread from the rendering submission (GPU) thread, removing CPU render stalls.

Target Files to Modify

• src/osdep/vulkan_renderer.cpp
• src/osdep/amiberry_gfx.cpp

Context and Instructions

1. The Rendering Thread Problem

Currently, the emulation loop calls SDL2_renderframe. This function locks the emulation thread until SDL_RenderCopy processes the texture. If Vsync is forced, it blocks the entire VM execution (causing the black screen or audio stutter issues seen previously).

2. Double-Buffering the Render State

• Instead of the emulation thread mapping memory and submitting commands linearly, implement a ring buffer (size 2 or 3) indicating Frames In Flight.
• We need:
  • 2 x amiga_surface equivalent CPU-side backing buffers.
  • 2 x Staging Buffers.
  • 2 x Command Buffers.
• The emulation thread should write to buffer[N], signal a condition variable, and immediately begin working on buffer[N+1] (which shouldn't block unless the queue is entirely full).

3. Vulkan Synchronization Primitives

• Create a dedicated Render Thread for the VulkanRenderer if one doesn't exist.
• Use VkSemaphore to synchronize operations on the GPU:
  • imageAvailableSemaphore: Sent to vkAcquireNextImageKHR. Let the Command Buffer wait on it at the VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT stage.
  • renderFinishedSemaphore: Signaled by the Command Buffer completion. Give this to vkQueuePresentKHR.
• Use VkFence to synchronize GPU to CPU:
  • Wait on an inFlightFence at the start of your render loop before overwriting staging buffers.
  • Reset the fence vkResetFences and pass it to vkQueueSubmit.

4. Zero-Copy Picasso96 Optimization

If currprefs.rtg_zerocopy is enabled, Amiga VRAM changes can be streamed directly into mapped device memory (if the Vulkan memory allocator supports VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT | VK_MEMORY_PROPERTY_HOST_COHERENT_BIT). This avoids the staging buffer -> device local copy overhead entirely for RTG screens.

Acceptance Criteria

• Stuttering previously caused by vertical sync or heavy emulation CPU usage should disappear.
• Enabling Vulkan validation layers continues to report zero synchronization/lifetime hazard errors.
• Amiberry remains stable during fast-forward mode and normal 50/60Hz modes.