[Intel-gfx] [PATCH] drm/i915/selftests: Prevent selecting 0 for our random width/align

[Chris Wilson]

When igt_random_offset() is a given a range of [0, PAGE_SIZE], it is
allowed to return 0. However, attempting to use a size of 0 for the
igt_lmem_write_cpu() byte poking, leads to call igt_random_offset() with
a range of [offset, offset + 0] and ask it to find a length of 4 within
it. This triggers the bug on that the requested length should fit within
the range!

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld at intel.com>
---
I feel into the same trap of not fixing up the random return of 0
---
 drivers/gpu/drm/i915/selftests/intel_memory_region.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/selftests/intel_memory_region.c b/drivers/gpu/drm/i915/selftests/intel_memory_region.c
index 6e80d99048e4..54e683bb220b 100644
--- a/drivers/gpu/drm/i915/selftests/intel_memory_region.c
+++ b/drivers/gpu/drm/i915/selftests/intel_memory_region.c
@@ -522,9 +522,10 @@ static int igt_lmem_write_cpu(void *arg)
 		goto out_unpin;
 	}
 
-	/* We want to throw in a random width/align */
-	bytes[0] = igt_random_offset(&prng, 0, PAGE_SIZE, sizeof(u32),
-				     sizeof(u32));
+	/* A random multiple of u32, picked between [64, PAGE_SIZE - 64] */
+	bytes[0] = 1 + prandom_u32_state(&prng);
+	bytes[0] = round_up(bytes[0], 64) & (PAGE_SIZE - 1);
+	GEM_BUG_ON(!IS_ALIGNED(bytes[0], sizeof(u32)));
 
 	i = 0;
 	do {
-- 
2.20.1