Re: [PATCH] media: mediatek: vcodec: mark vdec_vp9_slice_map_counts_eob_coef noinline

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Oct 18, 2024 at 03:14:42PM +0000, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> With KASAN enabled, clang fails to optimize the inline version of
> vdec_vp9_slice_map_counts_eob_coef() properly, leading to kilobytes
> of temporary values spilled to the stack:
> 
> drivers/media/platform/mediatek/vcodec/decoder/vdec/vdec_vp9_req_lat_if.c:1526:12: error: stack frame size (2160) exceeds limit (2048) in 'vdec_vp9_slice_update_prob' [-Werror,-Wframe-larger-than]
> 
> This seems to affect all versions of clang including the latest (clang-20),
> but the degree of stack overhead is different per release.
> 
> Marking the function as noinline_for_stack is harmless here and avoids
> the problem completely.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> I have not come to a conclusion on how exactly clang fails to do this
> right, but can provide the .config and/or preprocessed source files
> and command line if we think this should be fixed in clang.

I think this might be related to the issue I reported to upstream LLVM,
as a regression within the past couple of weeks:

https://github.com/llvm/llvm-project/issues/111903

If this is a reasonable workaround, it might be worth doing but I will
probably wait until after the LLVM Developers Meeting next week to ping
the thread to have a better chance of visibility. If we want to work
around this in the kernel, we should Cc stable, as this warning is
present there too.

> ---
>  .../mediatek/vcodec/decoder/vdec/vdec_vp9_req_lat_if.c         | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/platform/mediatek/vcodec/decoder/vdec/vdec_vp9_req_lat_if.c b/drivers/media/platform/mediatek/vcodec/decoder/vdec/vdec_vp9_req_lat_if.c
> index eea709d93820..47c302745c1d 100644
> --- a/drivers/media/platform/mediatek/vcodec/decoder/vdec/vdec_vp9_req_lat_if.c
> +++ b/drivers/media/platform/mediatek/vcodec/decoder/vdec/vdec_vp9_req_lat_if.c
> @@ -1188,7 +1188,8 @@ static int vdec_vp9_slice_setup_lat(struct vdec_vp9_slice_instance *instance,
>  	return ret;
>  }
>  
> -static
> +/* clang stack usage explodes if this is inlined */
> +static noinline_for_stack
>  void vdec_vp9_slice_map_counts_eob_coef(unsigned int i, unsigned int j, unsigned int k,
>  					struct vdec_vp9_slice_frame_counts *counts,
>  					struct v4l2_vp9_frame_symbol_counts *counts_helper)
> -- 
> 2.39.5
>