Re: [PATCH v4 1/2] include/uapi/linux/swab.h: move default implementation for swab macros into asm-generic

[David Laight <david.laight.linux@gmail.com>]

On Sat, 26 Apr 2025 16:56:18 +0200
Ignacio Encinas <ignacio@iencinas.com> wrote:

> Move the default byteswap implementation into asm-generic so that it can
> be included from arch code.
> 
> This is required by RISC-V in order to have a fallback implementation
> without duplicating it.
> 
> Signed-off-by: Ignacio Encinas <ignacio@iencinas.com>
> ---
>  include/uapi/asm-generic/swab.h | 33 +++++++++++++++++++++++++++++++++
>  include/uapi/linux/swab.h       | 33 +--------------------------------
>  2 files changed, 34 insertions(+), 32 deletions(-)
> 
> diff --git a/include/uapi/asm-generic/swab.h b/include/uapi/asm-generic/swab.h
> index f2da4e4fd4d1..232e81661dc5 100644
> --- a/include/uapi/asm-generic/swab.h
> +++ b/include/uapi/asm-generic/swab.h
> @@ -3,6 +3,7 @@
>  #define _ASM_GENERIC_SWAB_H
>  
>  #include <asm/bitsperlong.h>
> +#include <linux/types.h>
>  
>  /*
>   * 32 bit architectures typically (but not always) want to
> @@ -16,4 +17,36 @@
>  #endif
>  #endif
>  
> +/*
> + * casts are necessary for constants, because we never know how for sure
> + * how U/UL/ULL map to __u16, __u32, __u64. At least not in a portable way.

I know you are just moving the code, but that comment just isn't right.
Linux pretty much assumes that ULL is 64bit and U 32bit (UL varies).

So the UL constants should just be U ones (int isn't going to be 16 bits).

Not only that, but the code requires that the (__unn) casts don't
truncate the values. Performing the maths on a larger type isn't
going to change the value of the result.

Then we get to the integer promotion that does an implicit conversion
of the return of all the (__u16) casts back to signed integer.
So it may be better to leave/make the result of swap16() unsigned int
rather than casting it to __u16 and getting it promoted to int.

The only plausibly necessary cast is a (__u32) one in the result
of (except swap64()) to stop the compiler doing 64bit maths with the
result when the constant has a 64bit type (and all the other casts are
removed).

	David

> + */
> +#define ___constant_swab16(x) ((__u16)(				\
> +	(((__u16)(x) & (__u16)0x00ffU) << 8) |			\
> +	(((__u16)(x) & (__u16)0xff00U) >> 8)))
> +
> +#define ___constant_swab32(x) ((__u32)(				\
> +	(((__u32)(x) & (__u32)0x000000ffUL) << 24) |		\
> +	(((__u32)(x) & (__u32)0x0000ff00UL) <<  8) |		\
> +	(((__u32)(x) & (__u32)0x00ff0000UL) >>  8) |		\
> +	(((__u32)(x) & (__u32)0xff000000UL) >> 24)))
> +
> +#define ___constant_swab64(x) ((__u64)(				\
> +	(((__u64)(x) & (__u64)0x00000000000000ffULL) << 56) |	\
> +	(((__u64)(x) & (__u64)0x000000000000ff00ULL) << 40) |	\
> +	(((__u64)(x) & (__u64)0x0000000000ff0000ULL) << 24) |	\
> +	(((__u64)(x) & (__u64)0x00000000ff000000ULL) <<  8) |	\
> +	(((__u64)(x) & (__u64)0x000000ff00000000ULL) >>  8) |	\
> +	(((__u64)(x) & (__u64)0x0000ff0000000000ULL) >> 24) |	\
> +	(((__u64)(x) & (__u64)0x00ff000000000000ULL) >> 40) |	\
> +	(((__u64)(x) & (__u64)0xff00000000000000ULL) >> 56)))
> +
> +#define ___constant_swahw32(x) ((__u32)(			\
> +	(((__u32)(x) & (__u32)0x0000ffffUL) << 16) |		\
> +	(((__u32)(x) & (__u32)0xffff0000UL) >> 16)))
> +
> +#define ___constant_swahb32(x) ((__u32)(			\
> +	(((__u32)(x) & (__u32)0x00ff00ffUL) << 8) |		\
> +	(((__u32)(x) & (__u32)0xff00ff00UL) >> 8)))
> +
>  #endif /* _ASM_GENERIC_SWAB_H */
> diff --git a/include/uapi/linux/swab.h b/include/uapi/linux/swab.h
> index 01717181339e..ca808c492996 100644
> --- a/include/uapi/linux/swab.h
> +++ b/include/uapi/linux/swab.h
> @@ -6,38 +6,7 @@
>  #include <linux/stddef.h>
>  #include <asm/bitsperlong.h>
>  #include <asm/swab.h>
> -
> -/*
> - * casts are necessary for constants, because we never know how for sure
> - * how U/UL/ULL map to __u16, __u32, __u64. At least not in a portable way.
> - */
> -#define ___constant_swab16(x) ((__u16)(				\
> -	(((__u16)(x) & (__u16)0x00ffU) << 8) |			\
> -	(((__u16)(x) & (__u16)0xff00U) >> 8)))
> -
> -#define ___constant_swab32(x) ((__u32)(				\
> -	(((__u32)(x) & (__u32)0x000000ffUL) << 24) |		\
> -	(((__u32)(x) & (__u32)0x0000ff00UL) <<  8) |		\
> -	(((__u32)(x) & (__u32)0x00ff0000UL) >>  8) |		\
> -	(((__u32)(x) & (__u32)0xff000000UL) >> 24)))
> -
> -#define ___constant_swab64(x) ((__u64)(				\
> -	(((__u64)(x) & (__u64)0x00000000000000ffULL) << 56) |	\
> -	(((__u64)(x) & (__u64)0x000000000000ff00ULL) << 40) |	\
> -	(((__u64)(x) & (__u64)0x0000000000ff0000ULL) << 24) |	\
> -	(((__u64)(x) & (__u64)0x00000000ff000000ULL) <<  8) |	\
> -	(((__u64)(x) & (__u64)0x000000ff00000000ULL) >>  8) |	\
> -	(((__u64)(x) & (__u64)0x0000ff0000000000ULL) >> 24) |	\
> -	(((__u64)(x) & (__u64)0x00ff000000000000ULL) >> 40) |	\
> -	(((__u64)(x) & (__u64)0xff00000000000000ULL) >> 56)))
> -
> -#define ___constant_swahw32(x) ((__u32)(			\
> -	(((__u32)(x) & (__u32)0x0000ffffUL) << 16) |		\
> -	(((__u32)(x) & (__u32)0xffff0000UL) >> 16)))
> -
> -#define ___constant_swahb32(x) ((__u32)(			\
> -	(((__u32)(x) & (__u32)0x00ff00ffUL) << 8) |		\
> -	(((__u32)(x) & (__u32)0xff00ff00UL) >> 8)))
> +#include <asm-generic/swab.h>
>  
>  /*
>   * Implement the following as inlines, but define the interface using
> 


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