[RFC PATCH] asm-generic: Unify uapi bitsperlong.h

[Tiezhu Yang]

Hi all,

On 05/09/2023 05:37 PM, Arnd Bergmann wrote:
> On Tue, May 9, 2023, at 09:05, Tiezhu Yang wrote:
>> Now we specify the minimal version of GCC as 5.1 and Clang/LLVM as 11.0.0
>> in Documentation/process/changes.rst, __CHAR_BIT__ and __SIZEOF_LONG__ are
>> usable, just define __BITS_PER_LONG as (__CHAR_BIT__ * __SIZEOF_LONG__) in
>> asm-generic uapi bitsperlong.h, simpler, works everywhere.
>>
>> Remove all the arch specific uapi bitsperlong.h which will be generated as
>> arch/*/include/generated/uapi/asm/bitsperlong.h.
>>
>> Suggested-by: Xi Ruoyao <xry111 at xry111.site>
>> Link:
>> https://lore.kernel.org/all/d3e255e4746de44c9903c4433616d44ffcf18d1b.camel@xry111.site/
>> Signed-off-by: Tiezhu Yang <yangtiezhu at loongson.cn>
>
> I originally introduced the bitsperlong.h header, and I'd love to
> see it removed if it's no longer needed. Your patch certainly
> seems like it does this well.
>
> There is one minor obstacle to this, which is that the compiler
> requirements for uapi headers are not the same as for kernel
> internal code. In particular, the uapi headers may be included
> by user space code that is built with an older compiler version,
> or with a compiler that is not gcc or clang.
>
> I think we are completely safe on the architectures that were
> added since the linux-3.x days (arm64, riscv, csky, openrisc,
> loongarch, nios2, and hexagon), but for the older ones there
> is a regression risk. Especially on targets that are not that
> actively maintained (sparc, alpha, ia64, sh, ...) there is
> a good chance that users are stuck on ancient toolchains.
>
> It's probably also a safe assumption that anyone with an older
> libc version won't be using the latest kernel headers, so
> I think we can still do this across architectures if both
> glibc and musl already require a compiler that is new enough,
> or alternatively if we know that the kernel headers require
> a new compiler for other reasons and nobody has complained.
>
> For glibc, it looks the minimum compiler version was raised
> from gcc-5 to gcc-8 four years ago, so we should be fine.
>
> In musl, the documentation states that at least gcc-3.4 or
> clang-3.2 are required, which probably predate the
> __SIZEOF_LONG__ macro. On the other hand, musl was only
> released in 2011, and building musl itself explicitly
> does not require kernel uapi headers, so this may not
> be too critical.
>
> There is also uClibc, but I could not find any minimum
> supported compiler version for that. Most commonly, this
> one is used for cross-build environments, so it's also
> less likely to have libc/gcc/headers being wildly out of
> sync. Not sure.
>
>       Arnd
>
> [1] https://sourceware.org/pipermail/libc-alpha/2019-January/101010.html
>

Thanks Arnd for the detailed reply.
Any more comments? What should I do in the next step?

Thanks,
Tiezhu