[Intel-gfx] [PATCH i-g-t] lib: don't hang on blt on snb

[Chris Wilson]

Quoting Daniel Vetter (2017-08-04 17:07:22)
> We now have full (or a lot at least) igt running in beta CI, and snb
> blt hangs are really unhappy:
> 
> - drv_hangman at error-state-capture-blt and gem_exec_capture at capture-blt
>   reliably result in insta-machine death when we try to reset the gpu,
>   both on the CI snb and the one I have here.
> 
> - Other testcases also randomly (and sometimes rather rarely) die on
>   snb.
> 
> We can't use the endless batch because that results in a reset failure
> and wedged gpu, so also not really better.

It shouldn't be the recursion, but the invalid instruction we use to try
and trigger the hang quicker (otherwise hangcheck may see the advancing
ACTHD and give us longer to escape the loop).

In gem_exec_capture we shouldn't even need that invalid instruction, we
just need the busy batch as we pull the trigger ourselves, and if that
fails to reset a simple recursive batch we have some issues to resolve.
-Chris