Re: [PATCH] arm: clang: use -mthumb when -mthumb-interwork is not supported

[Tom Rini]

On Fri, Oct 30, 2026 at 10:37:38AM +0300, Yalçın Can wrote:

> With SYS_THUMB_BUILD, config.mk asks for "-mthumb -mthumb-interwork" and
> falls back to ARM code (-marm) if the compiler rejects the pair. Clang has
> no -mthumb-interwork (interworking is always on), so clang builds come out
> as ARM code, about 30 % larger than gcc's Thumb-2 build.
> 
> Try plain -mthumb before falling back to -marm.
> 
> Tested with clang 19 on mk808_defconfig: 467 KB instead of ~600 KB, boots
> on an MK808B (RK3066); the interworking veneers around save_boot_params
> were checked in the disassembly.
> 
> Co-Authored-By: Claude Opus 5.5 <nore...@anthropic.com>
> Signed-off-by: Yalçın Can <ylc...@gmail.com>
> ---
>  arch/arm/config.mk | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/config.mk b/arch/arm/config.mk
> index bce9a31e..b14c5590 100644
> --- a/arch/arm/config.mk
> +++ b/arch/arm/config.mk
> @@ -48,9 +48,10 @@ ifeq ($(CONFIG_$(PHASE_)SYS_THUMB_BUILD),y)
>  AFLAGS_IMPLICIT_IT   := $(call as-option,-Wa$(comma)-mimplicit-it=always)
>  PF_CPPFLAGS_ARM              := $(AFLAGS_IMPLICIT_IT) \
>                       $(call cc-option, -mthumb -mthumb-interwork,\
> +                     $(call cc-option, -mthumb,\
>                       $(call cc-option,-marm,)\
>                       $(call cc-option,-mno-thumb-interwork,)\
> -             )
> +             ))
>  else ifneq ($(CONFIG_ARM64),y)
>  PF_CPPFLAGS_ARM := $(call cc-option,-marm,) \
>               $(call cc-option,-mno-thumb-interwork,)

This is a case where I'm glad someone looked at the overall failure.
It's maybe why using clang overflows on a bunch of other targets, too.
The fix however, is wrong. Looking at where everything came from, we
need to look at the linux kernel, and then can see:

commit 22905a24306c8c312c2d66da9f90d09af0414f81
Author: Stefan Agner <ste...@agner.ch>
Date:   Tue May 8 22:49:49 2018 +0100

    ARM: 8766/1: drop no-thumb-interwork in EABI mode

    According to GCC documentation -m(no-)thumb-interwork is
    meaningless in AAPCS configurations. Also clang does not
    support the flag:
      clang-5.0: error: unknown argument: '-mno-thumb-interwork'

    Just drop -mno-thumb-interwork in AEABI configuration.

    Signed-off-by: Stefan Agner <ste...@agner.ch>
    Signed-off-by: Russell King <rmk+ker...@armlinux.org.uk>

And we should just do the same in U-Boot and reference the kernel
having made this correction in the above commit.

-- 
Tom

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