gallium/docs: document TGSI_SEMANTIC_EDGEFLAG

diff --git a/src/gallium/docs/source/tgsi.rst b/src/gallium/docs/source/tgsi.rst
index f5678c744b5b4671dd849e15401bc35d3020404e..205e7b8539df965004afdb4d80cde70c9c1674fd 100644 (file)
--- a/src/gallium/docs/source/tgsi.rst
+++ b/src/gallium/docs/source/tgsi.rst
@@ -1397,7 +1397,15 @@ back-facing.
 TGSI_SEMANTIC_EDGEFLAG
 """"""""""""""""""""""
 
-XXX no clue
+For vertex shaders, this sematic label indicates that an input or
+output is a boolean edge flag.  The register layout is [F, x, x, x]
+where F is 0.0 or 1.0 and x = don't care.  Normally, the vertex shader
+simply copies the edge flag input to the edgeflag output.
+
+Edge flags are used to control which lines or points are actually
+drawn when the polygon mode converts triangles/quads/polygons into
+points or lines.
+
 
 
 Properties