[Piglit] problem with msaa dual source blending tests

[Vadim Girlin]

On Wed, 2012-08-22 at 11:16 -0700, Anuj Phogat wrote:
> On Wed, Aug 22, 2012 at 12:48 AM, Vadim Girlin <vadimgirlin at gmail.com> wrote:
> > Hi,
> >
> > It seems there is a problem with alpha-to-coverage-dual-src-blend test.
> >
> > AFAICS the test uses DrawPixels with dual source blending and
> > fixed-function fragment pipeline. According to GL and
> > ARB_blend_func_extended specs:
> >
> > "Rendering using any of the blend functions that consume the second
> > input color (SRC1_COLOR, ONE_MINUS_SRC1_COLOR, SRC1_ALPHA or
> > ONE_MINUS_SRC1_ALPHA) using fixed function will produce undefined
> > results."
> >
> > So it looks like the test relies on the undefined behaviour. Here is the
> > part of the stack for that DrawPixels call:
> >
> >         12 visualize_image() common.cpp:1775
> >         11 draw_image_to_window_system_fb() draw-buffers-common.cpp:704
> >         10 draw_reference_image() draw-buffers-common.cpp:823
> >         9  piglit_display() alpha-to-coverage-dual-src-blend.cpp:104
> >
> > Probably blending should be temporarily disabled in that case.
> Yes. Blending can be temporarily disabled in visualize_image function.
> I'll send out a small patch to fix this. Patch won't make any difference
> to the test output. Test is already passing on AMD, Intel and NVIDIA drivers.
> Thanks for noticing this.

In fact there was some difference on AMD hardware with r600g, that's how
I noticed it :) . It required special handling to convert one kind of
"undefined results" (GPU lockup leading to system reboot) to another by
reproducing the behaviour of other drivers for compatibility:

http://cgit.freedesktop.org/mesa/mesa/commit/?id=8d1a9a984f33d8e45f932a9f47cdd57da617a919

But some driver could simply ignore invalid draw call, and I think the
test without a fix would then result in false "FAIL".

Thanks for fixing this.

Vadim