[PATCH] drm/amd/display: Increase frame warning limit for clang in dml2

[Hamza Mahfooz]

On 11/2/23 12:24, Nathan Chancellor wrote:
> When building ARCH=x86_64 allmodconfig with clang, which have sanitizers
> enabled, there is a warning about a large stack frame.
> 
>    drivers/gpu/drm/amd/amdgpu/../display/dc/dml2/display_mode_core.c:6265:13: error: stack frame size (2520) exceeds limit (2048) in 'dml_prefetch_check' [-Werror,-Wframe-larger-than]
>     6265 | static void dml_prefetch_check(struct display_mode_lib_st *mode_lib)
>          |             ^
>    1 error generated.
> 
> Notably, GCC 13.2.0 does not do too much of a better job, as it is right
> at the current limit of 2048:
> 
>    drivers/gpu/drm/amd/amdgpu/../display/dc/dml2/display_mode_core.c: In function 'dml_prefetch_check':
>    drivers/gpu/drm/amd/amdgpu/../display/dc/dml2/display_mode_core.c:6705:1: error: the frame size of 2048 bytes is larger than 1800 bytes [-Werror=frame-larger-than=]
>     6705 | }
>          | ^
> 
> In the past, these warnings have been avoided by reducing the number of
> parameters to various functions so that not as many arguments need to be
> passed on the stack. However, these patches take a good amount of effort
> to write despite being mechanical due to code structure and complexity
> and they are never carried forward to new generations of the code so
> that effort has to be expended every new hardware generation, which
> becomes harder to justify as time goes on.
> 
> There is some effort to improve clang's code generation but that may
> take some time between code review, shifting priorities, and release
> cycles. To avoid having a noticeable or lengthy breakage in
> all{mod,yes}config, which are easy testing targets that have -Werror
> enabled, increase the limit for clang by 50% so that cases of extremely
> poor code generation can still be caught while not breaking the majority
> of builds. When clang's code generation improves, the limit increase can
> be restricted to older clang versions.
> 
> Signed-off-by: Nathan Chancellor <nathan at kernel.org>
> ---
> If there is another DRM pull before 6.7-rc1, it would be much
> appreciated if this could make that so that other trees are not
> potentially broken by this. If not, no worries, as it was my fault for
> not sending this sooner.
> ---
>   drivers/gpu/drm/amd/display/dc/dml2/Makefile | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/dc/dml2/Makefile b/drivers/gpu/drm/amd/display/dc/dml2/Makefile
> index 70ae5eba624e..dff8237c0999 100644
> --- a/drivers/gpu/drm/amd/display/dc/dml2/Makefile
> +++ b/drivers/gpu/drm/amd/display/dc/dml2/Makefile
> @@ -60,7 +60,7 @@ endif
>   endif
>   
>   ifneq ($(CONFIG_FRAME_WARN),0)
> -frame_warn_flag := -Wframe-larger-than=2048
> +frame_warn_flag := -Wframe-larger-than=$(if $(CONFIG_CC_IS_CLANG),3072,2048)

I would prefer checking for `CONFIG_KASAN || CONFIG_KCSAN` instead
since the stack usage shouldn't change much if both of those are disabled.

>   endif
>   
>   CFLAGS_$(AMDDALPATH)/dc/dml2/display_mode_core.o := $(dml2_ccflags) $(frame_warn_flag)
> 
> ---
> base-commit: 21e80f3841c01aeaf32d7aee7bbc87b3db1aa0c6
> change-id: 20231102-amdgpu-dml2-increase-frame-size-warning-for-clang-c93bd2d6a871
> 
> Best regards,
-- 
Hamza