hal: more implementation comments

wgpu-hal/src/metal/mod.rs

@@ -1,3 +1,18 @@
+/*!
+# Metal API internals.
+
+## Pipeline Layout
+
+In Metal, push constants, vertex buffers, and resources in the bind groups
+are all placed together in the native resource bindings, which work similarly to D3D11:
+there are tables of textures, buffers, and samplers.
+
+We put push constants first (if any) in the table, followed by bind group 0
+resources, followed by other bind groups. The vertex buffers are bound at the very
+end of the VS buffer table.
+
+!*/
+
 mod adapter;
 mod command;
 mod conv;

wgpu-hal/src/vulkan/mod.rs

@@ -1,3 +1,25 @@
+/*!
+# Vulkan API internals.
+
+## Stack memory
+
+Ash expects slices, which we don't generally have available.
+We cope with this requirement by the combination of the following ways:
+  - temporarily allocating `Vec` on heap, where overhead is permitted
+  - growing temporary local storage
+  - using `implace_it` on iterators
+
+## Framebuffers and Render passes
+
+Render passes are cached on the device and kept forever.
+
+Framebuffers are also cached on the device, but they are removed when
+any of the image views (they have) gets removed.
+If Vulkan supports image-less framebuffers,
+then the actual views are excluded from the framebuffer key.
+
+!*/
+
 mod adapter;
 mod command;
 mod conv;