[Mesa-dev] [PATCH] mesa: Correctly use glDrawBuffers for multiple buffers and glDrawBuffer for one buffer.

[Pavel Popov]

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 situation realization
for glDrawBuffers is used for any number of buffers. Even for one. But glDrawBuffer have to be
used for one buffer instead of glDrawBuffers.

Piglit test 'gl30basic' fails with assert with debug Mesa and pass with release
    'main/buffers.c:520: _mesa_drawbuffers: Assertion `__builtin_popcount(destMask[buf]) == 1' failed.'
Probably some other tests also can be affected.

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..a640360 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) {
       GLuint count = 0, destMask0 = destMask[0];
       while (destMask0) {
          GLint bufIndex = ffs(destMask0) - 1;
-- 
1.9.1


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