add simple doc

docs/markdown/_sidebar.md

@@ -19,3 +19,6 @@
   - [Assembly Emitter](/asm_emitter.md)
   - [Porting to x86](/porting_to_x86.md)
   - [Register Handling](/registers.md)
+
+- PC Port Documentation
+  - [Graphics](/graphics.md)

docs/markdown/graphics.md

@@ -0,0 +1,54 @@
+# Graphics
+
+There are three frames in flight at a time:
+- One frame being processed by the CPU, building DMA lists. This is called the "calc frame"
+- One frame being processed by the GPU, going through the DMA lists and drawing triangles to one frame buffer. I will call this the "draw" frame.
+- One frame being displayed on the screen, stored in the other frame buffer.  This is called the "on-screen" frame.
+
+
+
+## Synchronization
+
+The PC Port only synchronizes on vsync. This waits for the "draw" frame to finish rendering, and for the buffers to be swapped.
+
+The game's code for this is kind of messy and confusing. On the PS2, you make sure rendering is done with `sync-path`, which waits for the DMA chain to finish.  But they call this earlier than I think they need to, and I don't really understand why.  I don't see any place where they read back from the finished frame or depth buffer.  Or where they would overwrite the memory. There's a second call to `sync-path` right where you would expected, right before the `syncv`.  After `syncv`, they call some Sony library function to actually display the correct framebuffer, then immediately start sending the next DMA chain.
+
+The stuff between `sync-path` and `syncv` is:
+- depth cue "calc" (seems fast)
+- screen filter "calc" (very fast, just DMA for a single quad)
+- calc for letterbox bars and blackout
+- debug draw
+- draw profiler (look at all the stuff that happens after!)
+- deci count draw
+- file info draw
+- stdcon text draw
+- iop/memcard info
+- tie buffer init
+- merc buffer init
+- post draw hook (does nothing)
+- bucket patching
+- cache flush
+- a second `sync-path`
+
+I'm really not sure why they have the first `sync-path` there. One theory is that they didn't want debug code and non-debug rendering running at the same time - the debug code will compete with the rendering to use the main bus, and will make the rendering slower.
+
+For now, the PC Port doesn't do anything on `sync-path`. I think there's two ways this could be an issue in the future:
+- If they reuse the DMA buffer after `sync-path`, and our renderer is still reading from it. Currently not an issue because of dma copy, described later.
+- If they need to read the completed depth buffer or frame buffer after `sync-path`. I don't see any example of this.
+
+
+
+## DMA Copy
+
+
+
+## Buckets
+
+## Textures
+
+## Shaders
+
+## Direct Renderer
+
+## Mysteries
+