Re: [PATCH] drm: omapdrm: reduce clang stack usage

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

Hi,

On 10/06/2025 12:27, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> The thread sanitizer makes the stack usage explode from extra variable
> spills in dispc_runtime_resume:
> 
> drivers/gpu/drm/omapdrm/dss/dispc.c:4735:27: error: stack frame size (1824) exceeds limit (1280) in 'dispc_runtime_resume' [-Werror,-Wframe-larger-than]
> 
> I could not figure out what exactly is going on here, but I see that
> whenever dispc_restore_context() is not inlined, that function
> and its caller shrink below 900 bytes combined of stack usage.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  drivers/gpu/drm/omapdrm/dss/dispc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/omapdrm/dss/dispc.c b/drivers/gpu/drm/omapdrm/dss/dispc.c
> index 533f70e8a4a6..cf055815077c 100644
> --- a/drivers/gpu/drm/omapdrm/dss/dispc.c
> +++ b/drivers/gpu/drm/omapdrm/dss/dispc.c
> @@ -524,7 +524,7 @@ static void dispc_save_context(struct dispc_device *dispc)
>  	DSSDBG("context saved\n");
>  }
>  
> -static void dispc_restore_context(struct dispc_device *dispc)
> +static noinline_for_stack void dispc_restore_context(struct dispc_device *dispc)
>  {
>  	int i, j;
>  

While I don't think this causes any harm, but... What's going on here?
If I compile with gcc (x86 or arm), I see stack usage in few hundreds of
bytes. If I compile with LLVM=1, the stack usage jumps to over a thousand.

Is clang just broken? I don't see anything special with
dispc_restore_context() or dispc_runtime_resume(), so is this same thing
happening all around the kernel, and we need to sprinkle noinlines
everywhere?

Or do we get some extra debugging feature enabled only on clang with
allmodconfig, and that is eating the stack?

 Tomi