draw: fixup draw_find_shader_output

draw_find_shader_output like most of the code in draw used to
depend on position always being at output slot 0. which meant
that any other attribute being at 0 could signify an error.
unfortunately position can be at any of the output slots, thus
other attributes can occupy slot 0 and we need to mark the ones
which were not found by something else. This commit changes
draw_find_shader_output so that it returns -1 if it can't
find the given attribute and adjust the code that depended
on it returning >0 whenever it correctly found an attrib.

Signed-off-by: Zack Rusin <[email protected]>
Reviewed-by: José Fonseca<[email protected]>
Reviewed-by: Brian Paul <[email protected]>
Reviewed-by: Roland Scheidegger <[email protected]>

1 files changed, 6 insertions, 5 deletions

diff --git a/src/gallium/auxiliary/draw/draw_context.c b/src/gallium/auxiliary/draw/draw_context.c
index 4250f10d13a..63ccf386f29 100644
--- a/src/gallium/auxiliary/draw/draw_context.c
+++ b/src/gallium/auxiliary/draw/draw_context.c
@@ -493,7 +493,7 @@ draw_alloc_extra_vertex_attrib(struct draw_context *draw,
    uint n;
 
    slot = draw_find_shader_output(draw, semantic_name, semantic_index);
-   if (slot > 0) {
+   if (slot >= 0) {
       return slot;
    }
 
@@ -549,9 +549,10 @@ draw_get_shader_info(const struct draw_context *draw)
  * attributes (such as texcoords for AA lines).  The driver can call this
  * function to find those attributes.
  *
- * Zero is returned if the attribute is not found since this is
- * a don't care / undefined situtation.  Returning -1 would be a bit more
- * work for the drivers.
+ * -1 is returned if the attribute is not found since this is
+ * an undefined situtation. Note, that zero is valid and can
+ * be used by any of the attributes, because position is not
+ * required to be attribute 0 or even at all present.
  */
 int
 draw_find_shader_output(const struct draw_context *draw,
@@ -574,7 +575,7 @@ draw_find_shader_output(const struct draw_context *draw,
       }
    }
 
-   return 0;
+   return -1;
 }