[Intel-gfx] [PATCH igt 2/2] lib: Remove illegal instructions from hang injection

[Chris Wilson]

The idea behind using an illegal instruction was to hang the GPU must
faster than simply using the recursive batch. However, we stopped doing
so on gen8+ as the CS parser was much laxer and allowed the illegal
command through but still interpreted the packet length (jumping over
the recursive batch buffer start that followed). Sandybridge doesn't
just hang the GPU when it encounters an illegal command on the BLT
engine, it hangs the machine. That goes above and beyond testing our
hangcheck + reset, so remove the deadly instructions.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 lib/igt_gt.c | 25 +++++--------------------
 1 file changed, 5 insertions(+), 20 deletions(-)

diff --git a/lib/igt_gt.c b/lib/igt_gt.c
index 6f7daa5e..d5e8b557 100644
--- a/lib/igt_gt.c
+++ b/lib/igt_gt.c
@@ -270,30 +270,15 @@ igt_hang_t igt_hang_ctx(int fd,
 
 	memset(b, 0xc5, sizeof(b));
 
-	/*
-	 * We emit invalid command to provoke a gpu hang.
-	 * If that doesn't work, we do bb start loop.
-	 * Note that the bb start aligment is illegal due this.
-	 * But hey, we are here to hang the gpu so whatever works.
-	 * We skip 0xfffffff on gen9 as it confuses hw in an such a way that
-	 * it will skip over the bb start, causing runaway head and
-	 * thus much slower hang detection.
-	 */
 	len = 2;
-	if (intel_gen(intel_get_drm_devid(fd)) >= 8) {
-		b[0] = MI_NOOP;
+	if (intel_gen(intel_get_drm_devid(fd)) >= 8)
 		len++;
-	} else {
-		b[0] = 0xffffffff;
-	}
-
-	b[1] = MI_BATCH_BUFFER_START | (len - 2);
-	b[1+len] = MI_BATCH_BUFFER_END;
-	b[2+len] = MI_NOOP;
+	b[0] = MI_BATCH_BUFFER_START | (len - 2);
+	b[len] = MI_BATCH_BUFFER_END;
+	b[len+1] = MI_NOOP;
 	gem_write(fd, exec.handle, 0, b, sizeof(b));
 
-	reloc.offset = 8;
-	reloc.delta = 4;
+	reloc.offset = sizeof(uint32_t);
 	reloc.target_handle = exec.handle;
 	reloc.read_domains = I915_GEM_DOMAIN_COMMAND;
 
-- 
2.13.3