Require indirect draw calls use a firstInstance of 0 (#1928)

toji · web-flow · commit 69d9780ff8fd · 2021-07-13T10:39:56.000-07:00
diff --git a/spec/index.bs b/spec/index.bs
@@ -7225,9 +7225,12 @@ enum GPUStoreOp {
             drawIndirectParameters[0] = vertexCount;
             drawIndirectParameters[1] = instanceCount;
             drawIndirectParameters[2] = firstVertex;
-            drawIndirectParameters[3] = firstInstance;
+            drawIndirectParameters[3] = 0; // firstInstance. Must be 0.
         </pre>
 
+        The value corresponding to `firstInstance` is reserved for future use and must be zero. If
+        it is not zero the {{GPURenderEncoderBase/drawIndirect()}} call will be treated as a no-op.
+
         <div algorithm="GPURenderEncoderBase.drawIndirect">
             **Called on:** {{GPURenderEncoderBase}} this.
 
@@ -7269,9 +7272,13 @@ enum GPUStoreOp {
             drawIndexedIndirectParameters[1] = instanceCount;
             drawIndexedIndirectParameters[2] = firstIndex;
             drawIndexedIndirectParameters[3] = baseVertex;
-            drawIndexedIndirectParameters[4] = firstInstance;
+            drawIndexedIndirectParameters[4] = 0; // firstInstance. Must be 0.
         </pre>
 
+        The value corresponding to `firstInstance` is reserved for future use and must be zero. If
+        it is not zero the {{GPURenderEncoderBase/drawIndexedIndirect()}} call will be treated as a
+        no-op.
+
         <div algorithm="GPURenderEncoderBase.drawIndexedIndirect">
             **Called on:** {{GPURenderEncoderBase}} this.
 
@@ -7299,6 +7306,10 @@ enum GPUStoreOp {
         </div>
 </dl>
 
+Note: The ability to set a `firstInstance` for indirect draw calls is initially disabled to expand
+the hardware that supports WebGPU. It is likely to be added as a feature in the future along with
+other new indirect drawing features.
+
 <div algorithm>
     To determine if it's <dfn abstract-op>valid to draw</dfn> with {{GPURenderEncoderBase}} |encoder|
     run the following steps:
