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

[Chris Wilson]

Quoting Chris Wilson (2020-08-06 15:35:36)
> 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);

But what about if it returns -1.

Gah.
-Chris