[PATCH] drm/selftests/mm: reduce per-function stack usage

[Chris Wilson]

Quoting Arnd Bergmann (2020-05-29 21:15:26)
> The check_reserve_boundaries() function has a large array on the stack,
> over 500 bytes. It gets inlined into __igt_reserve, which has multiple
> other large structures as well but stayed just under the stack size
> warning limit of 1024 bytes until one more member got added to struct
> drm_mm_node, causing a warning:
> 
> drivers/gpu/drm/selftests/test-drm_mm.c:371:12: error:
> stack frame size of 1032 bytes in function '__igt_reserve' [-Werror,-Wframe-larger-than=]
> 
> As far as I can tell, this is not nice but will not be called from
> a context that is already low for the kernel stack, so just annotate
> the inner function as noinline_for_stack to ensure that each function
> by itself stays under the warning limit.
> 
> Fixes: 0cdea4455acd ("drm/mm: optimize rb_hole_addr rbtree search")
> Signed-off-by: Arnd Bergmann <arnd at arndb.de>
> ---
>  drivers/gpu/drm/selftests/test-drm_mm.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/selftests/test-drm_mm.c b/drivers/gpu/drm/selftests/test-drm_mm.c
> index 9aabe82dcd3a..30108c330db8 100644
> --- a/drivers/gpu/drm/selftests/test-drm_mm.c
> +++ b/drivers/gpu/drm/selftests/test-drm_mm.c
> @@ -323,9 +323,8 @@ static bool expect_reserve_fail(struct drm_mm *mm, struct drm_mm_node *node)
>         return false;
>  }
>  
> -static bool check_reserve_boundaries(struct drm_mm *mm,
> -                                    unsigned int count,
> -                                    u64 size)
> +static noinline_for_stack bool
> +check_reserve_boundaries(struct drm_mm *mm, unsigned int count, u64 size)
>  {
>         const struct boundary {

It's this const [] right? Hmm, if we felt adventurous that could be a
small set of multiplication factors (0, -1, 1, count, count+1, ...) and
made static.
-Chris