[Mesa-dev] [PATCH v2] mesa: Fix glDrawBuffer/glDrawBuffers logic in _mesa_drawbuffer.

[Pavel Popov]

Piglit test 'gl30basic' fails on Debug Mesa with the assert:
  'main/buffers.c:520: _mesa_drawbuffers: 
  Assertion `__builtin_popcount(destMask[buf]) == 1' failed.'.

According to spec (OpenGL 4.0 specification, pages 254-255) we have a
  different bits set for one buffer and for multiple buffers. For
  glDrawBuffer we may have up to four bits set but for glDrawBuffers we
  can only have one bit set.

The _mesa_drawbuffers is called with ctx->Const.MaxDrawBuffers and NULL
  arguments when _mesa_update_framebuffer or _mesa_update_draw_buffers
  is called. In this case glDrawBuffers is always used if MaxDrawBuffers > 1.
  But glDrawBuffer has to be used instead of glDrawBuffers if only
  destMask[0] is set.

v2 (Brian Paul): Only 0th entry requires special validation for (m == 1).

Signed-off-by: Pavel Popov <pavel.e.popov at intel.com>
---
 src/mesa/main/buffers.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/mesa/main/buffers.c b/src/mesa/main/buffers.c
index b13a7af..95a8722 100644
--- a/src/mesa/main/buffers.c
+++ b/src/mesa/main/buffers.c
@@ -480,6 +480,7 @@ _mesa_drawbuffers(struct gl_context *ctx, GLuint n, const GLenum *buffers,
    struct gl_framebuffer *fb = ctx->DrawBuffer;
    GLbitfield mask[MAX_DRAW_BUFFERS];
    GLuint buf;
+   GLuint m = n;
 
    if (!destMask) {
       /* compute destMask values now */
@@ -489,15 +490,17 @@ _mesa_drawbuffers(struct gl_context *ctx, GLuint n, const GLenum *buffers,
          mask[output] = draw_buffer_enum_to_bitmask(ctx, buffers[output]);
          ASSERT(mask[output] != BAD_MASK);
          mask[output] &= supportedMask;
+         if (mask[output] == 0)
+            m--;
       }
       destMask = mask;
    }
 
    /*
-    * If n==1, destMask[0] may have up to four bits set.
+    * If m==1, destMask[0] may have up to four bits set.
     * Otherwise, destMask[x] can only have one bit set.
     */
-   if (n == 1) {
+   if (m == 1 && destMask[0]) {
       GLuint count = 0, destMask0 = destMask[0];
       while (destMask0) {
          GLint bufIndex = ffs(destMask0) - 1;
-- 
1.9.1


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