Re: [PATCH] powerpc: define __LITTLE_ENDIAN and __BIG_ENDIAN for math-emu

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Le 20/05/2026 à 20:43, David Laight a écrit :
> On Wed, 20 May 2026 15:14:40 +0200
> "Christophe Leroy (CS GROUP)" <chleroy@kernel.org> wrote:
> 
>> Le 17/05/2026 à 15:54, David Laight a écrit :
>>> On Sun, 17 May 2026 12:14:21 +0800
>>> Mingcong Bai <jeffbai@aosc.io> wrote:
>>>    
>>>> Similar to commit b929926f01f2 ("sh: define __BIG_ENDIAN for math-emu"),
>>>> define __LITTLE_ENDIAN and __BIG_ENDIAN as 0 to mitigate build-time
>>>> warnings:
>>>>
>>>>     ./include/math-emu/double.h:59:21: error: ‘__BIG_ENDIAN’ is not defined, evaluates to ‘0’ [-Werror=undef]
>>>>        59 | #if __BYTE_ORDER == __BIG_ENDIAN
>>>>           |
>>>>
>>>> Cc: stable@vger.kernel.org
>>>> Fixes: 13da9e200fe4 ("Revert "endian: #define __BYTE_ORDER"")
>>>> Reported-by: kernel test robot <lkp@intel.com>
>>>> Closes: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Foe-kbuild-all%2F202507301656.7FEX6J5W-lkp%40intel.com%2F&data=05%7C02%7Cchristophe.leroy2%40cs-soprasteria.com%7C3ed26b8c3d6449fdc29608deb69fac66%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639148994069314641%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=FfFnXxMPXXxoOMM8fYU4df5gMjk3B2dPgQsjwUagaNA%3D&reserved=0
>>>> Signed-off-by: Mingcong Bai <jeffbai@aosc.io>
>>>> ---
>>>>    arch/powerpc/include/asm/sfp-machine.h | 4 +++-
>>>>    1 file changed, 3 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/powerpc/include/asm/sfp-machine.h b/arch/powerpc/include/asm/sfp-machine.h
>>>> index 8b957aabb826d..db8525605c026 100644
>>>> --- a/arch/powerpc/include/asm/sfp-machine.h
>>>> +++ b/arch/powerpc/include/asm/sfp-machine.h
>>>> @@ -319,10 +319,12 @@
>>>>    #define abort()								\
>>>>    	return 0
>>>>    
>>>> -#ifdef __BIG_ENDIAN
>>>> +#ifdef __BIG_ENDIAN__
>>>>    #define __BYTE_ORDER __BIG_ENDIAN
>>>> +#define __LITTLE_ENDIAN 0
>>>>    #else
>>>>    #define __BYTE_ORDER __LITTLE_ENDIAN
>>>> +#define __BIG_ENDIAN 0
>>>>    #endif
>>>
>>> I thought the expected/correct value for __BYTE_ORDER__ was either 1234 or 4321.
>>> (apart from pdp11's 2143).
>>
>> That's the case, in include/linux/kconfig.h we have:
>>
>> #ifdef CONFIG_CPU_BIG_ENDIAN
>> #define __BIG_ENDIAN 4321
>> #else
>> #define __LITTLE_ENDIAN 1234
>> #endif
>>
>> But as far as I understand the problem is that math-emu expects
>> __BIG_ENDIAN to be defined at all time as it has tests like:
>>
>> #if __BYTE_ORDER == __BIG_ENDIAN
> 
> The gcc docs have (https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgcc.gnu.org%2Fonlinedocs%2Fcpp%2FCommon-Predefined-Macros.html&data=05%7C02%7Cchristophe.leroy2%40cs-soprasteria.com%7C3ed26b8c3d6449fdc29608deb69fac66%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639148994069350793%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=D0BlZT73XnqHXHN2ukPFFUQw5lCCwaKfkmp6vMHz0Gk%3D&reserved=0):
>       
> __BYTE_ORDER__
> __ORDER_LITTLE_ENDIAN__
> __ORDER_BIG_ENDIAN__
> __ORDER_PDP_ENDIAN__
> 
>      __BYTE_ORDER__ is defined to one of the values __ORDER_LITTLE_ENDIAN__, __ORDER_BIG_ENDIAN__, or __ORDER_PDP_ENDIAN__ to reflect the layout of multi-byte and multi-word quantities in memory. If __BYTE_ORDER__ is equal to __ORDER_LITTLE_ENDIAN__ or __ORDER_BIG_ENDIAN__, then multi-byte and multi-word quantities are laid out identically: the byte (word) at the lowest address is the least significant or most significant byte (word) of the quantity, respectively. If __BYTE_ORDER__ is equal to __ORDER_PDP_ENDIAN__, then bytes in 16-bit words are laid out in a little-endian fashion, whereas the 16-bit subwords of a 32-bit quantity are laid out in big-endian fashion.
> 
>      You should use these macros for testing like this:
> 
>      /* Test for a little-endian machine */
>      #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> 
> The doc doesn't mention the value, but __ORDER_BIG_ENDIAN__ is 4321 (decimal).
> 
> So the math-emu code is neither following gcc's rules or the kernel ones.
> 
> Your change will break anything that currently does:
> #ifdef __BIG_ENDIAN
> 
> Any change would have to be limited to code that is implementing math-emu.

asm/sfp-machine.h is only included by math-emu it seems, so the change 
should be safe.

Apparently math-emu predates git history, not sure where it comes from.

Christophe