Re: [PATCH v2 09/17] m68k: Implement xor_unlock_is_negative_byte

[Greg Ungerer <gerg@linux-m68k.org>]

On 5/10/23 02:53, Matthew Wilcox (Oracle) wrote:
> Using EOR to clear the guaranteed-to-be-set lock bit will test the
> negative flag just like the x86 implementation.  This should be
> more efficient than the generic implementation in filemap.c.  It
> would be better if m68k had __GCC_ASM_FLAG_OUTPUTS__.
> 
> Coldfire doesn't have a byte-sized EOR, so we test bit 7 after the
> EOR, which is a second memory access, but it's slightly better than
> the current C code.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Acked-by: Greg Ungerer <gerg@linux-m68k.org>


> ---
>   arch/m68k/include/asm/bitops.h | 22 ++++++++++++++++++++++
>   1 file changed, 22 insertions(+)
> 
> diff --git a/arch/m68k/include/asm/bitops.h b/arch/m68k/include/asm/bitops.h
> index e984af71df6b..80ee36095905 100644
> --- a/arch/m68k/include/asm/bitops.h
> +++ b/arch/m68k/include/asm/bitops.h
> @@ -319,6 +319,28 @@ arch___test_and_change_bit(unsigned long nr, volatile unsigned long *addr)
>   	return test_and_change_bit(nr, addr);
>   }
>   
> +static inline bool xor_unlock_is_negative_byte(unsigned long mask,
> +		volatile unsigned long *p)
> +{
> +#ifdef CONFIG_COLDFIRE
> +	__asm__ __volatile__ ("eorl %1, %0"
> +		: "+m" (*p)
> +		: "d" (mask)
> +		: "memory");
> +	return *p & (1 << 7);
> +#else
> +	char result;
> +	char *cp = (char *)p + 3;	/* m68k is big-endian */
> +
> +	__asm__ __volatile__ ("eor.b %1, %2; smi %0"
> +		: "=d" (result)
> +		: "di" (mask), "o" (*cp)
> +		: "memory");
> +	return result;
> +#endif
> +}
> +#define xor_unlock_is_negative_byte xor_unlock_is_negative_byte
> +
>   /*
>    *	The true 68020 and more advanced processors support the "bfffo"
>    *	instruction for finding bits. ColdFire and simple 68000 parts