Re: Bug#993550: gtk4: gsk repeat, repeat-negative-coords tests fail with ngl renderer on mips*el

Control: reassign -1 src:mesa
Control: affects -1 + src:gtk4

On Fri, 03 Sep 2021 at 00:19:29 +0100, Simon McVittie wrote:
> GTK 4 has a new scene-graph-based rendering model, "GSK", with an OpenGL
> preferred implementation and a Cairo fallback. Its regression tests draw
> various combinations of "render nodes" and check the results against
> reference PNG images.
> 
> When using the "new" OpenGL renderer, "ngl", there's a weird rendering
> glitch on mips*el on two tests involving repeating a pattern: the top
> left pixel in each 2x2 block is darker than the other three.
> For more details and comparison images:
> https://gitlab.gnome.org/GNOME/gtk/-/issues/4228
> 
> Is there anything unusual about the OpenGL implementation on mips*el
> that would cause this sort of thing? It seems to be using Mesa swrast_dri.so
> (which I think is llvmpipe?), the same as any other machine without a GPU.

This seems likely to be a mips*-specific bug in Mesa's llvmpipe or some
dependency of llvmpipe (maybe LLVM?), rather than directly a GTK bug: I
can avoid the test failure by running tests with GALLIUM_DRIVER=softpipe
and LIBGL_ALWAYS_SOFTWARE=true in the environment.

I've set up the GTK build to use GALLIUM_DRIVER=softpipe and
LIBGL_ALWAYS_SOFTWARE=true for build-time tests, and I don't intend to
investigate this further from GTK's point of view. Investigation by the
mips porting team would be appreciated.

    smcv