Search notes:

include/uapi/asm-generic/int-ll64.h

Integer declarations for architectures which use long long for 64-bit types:

__s8, __u8
__s16, __u16
__s32, __u32
__s64, __u64

See also

include/asm-generic/int-ll64.h

include/uapi/asm-generic/int-l64.h