Tint: add doc for chromium_experimental_input_attachments.
Bug: 341117913
Change-Id: I6df64eaa327bb10d88de79c1273a73ae9511b7ad
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/188680
Commit-Queue: Quyen Le <lehoangquyen@chromium.org>
Reviewed-by: James Price <jrprice@google.com>
diff --git a/docs/tint/extensions/chromium_experimental_input_attachments.md b/docs/tint/extensions/chromium_experimental_input_attachments.md
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+# Chromium Experimental Input Attachments
+
+The `chromium_experimental_input_attachments` extension adds support for input attachment global variables to WGSL.
+This is similar to Vulkan's `subpassInput` variable.
+
+## Status
+
+Input attachments support in Tint is highly experimental and only meant to be used in internal transforms at this stage.
+This extension is only relevant to SPIRV backend atm.
+Specification work in the WebGPU group hasn't started.
+
+## Pseudo-specification
+
+This extension adds:
+- A new `input_attachment<T>` type in the `handle` address space, where `T` is `u32`, `i32` or `f32`. It's only allowed on module-scoped variable declarations.
+ - This will be mapped to `OpTypeImage` with Dim=`SubpassData` in SPIRV.
+- A new `input_attachment_index(n)` attribute where `n` is a `u32` or `i32` with a positive value. Unlike `binding` which specifies the binding point in a BindGroup, this attribute specifies the index of the input attachment in the render pass descriptor. This is required for `input_attachment<T>` type.
+ - This is equivalent to `InputAttachmentIndex` decoration in SPIRV.
+- A new `inputAttachmentLoad` builtin function with signature:
+ `inputAttachmentLoad(input_attachment<T>) -> vec4<T>`
+ - This will emit `OpImageRead` function call in SPIRV.
+- All of the above are only available in fragment state.
+
+## Example usage
+
+```
+@group(0) @binding(0) @input_attachment_index(0)
+ var input_tex : input_attachment<f32>;
+
+@fragment fn main() -> @location(0) vec4f {
+ return inputAttachmentLoad(input_tex);
+}
+```
