Re: [PATCH 1/5] ARC: uapi: Stop leaking CONFIG_ARC_HAS_SWAPE reference to userspace

[Vineet Gupta <vgupta@kernel.org>]

Hi Arnd,

Long time no see. Happy New Year !

On 12/30/25 13:23, Arnd Bergmann wrote:
> On Tue, Dec 30, 2025, at 08:10, Thomas Weißschuh wrote:
>> UAPI headers are not supposed to leak references to kconfig symbols.
>> These won't be set when building userspace. Hide the kconfig reference
>> behind 'if defined(__KERNEL__)', so it will be stripped by
>> headers_install.sh. The result for userspace will be the same, but the
>> exception in headers_install.sh can also be removed.
>>
>> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> Acked-by: Arnd Bergmann <arnd@arndb.de>
>
> Same as for the nios2 version, I think the nicer solution would
> be to use the asm-generic header for uapi and move this one
> to arch/arc/include/asm/swab.h for kernel-internal use.
>
> In theory, using 'select ARCH_USE_BUILTIN_BSWAP' in Kconfig should
> let the compiler pick the best swap instruction, which would
> be even less code, but I'm not sure if there was a reason that
> nios2 and arc don't already do this.

The builtins didn't exist when the insns were first rolled out in hw.
It seems ARCH_USE_BUILTIN_BSWAP assumes 16,32,64 variants to be available which
surprisingly seems to be true.

foo32:
    j_s.d    [blink]
    swape    r0,r0

foo16:
    exth_s    r0,r0
    asl    r2,r0,8
    lsr_s    r0,r0,8
    or_s    r0,r0,r2
    j_s.d    [blink]
    exth_s    r0,r0

foo64:
    mov_s    r2,r1    ;4
    swape    r1,r0
    j_s.d    [blink]
    swape    r0,r2

And as you said, since all upstream cores support SWAPE, we can really get rid
of all the legacy stuff and just rely on builtins.

Thx,
-Vineet