[U-Boot] [PATCH] MIPS: fix mips_cache fallback without __builtin_mips_cache

[Daniel Schwierzeck]

Am 05.03.2016 um 04:15 schrieb Matthias Schiffer:
> The "R" constraint supplies the address of an variable in a register. Use
> "r" instead and adjust asm to supply the content of addr in a register
> instead.
> 
> Fixes: 2b8bcc5a ("MIPS: avoid .set ISA for cache operations")
> Signed-off-by: Matthias Schiffer <mschiffer at universe-factory.net>
> Cc: Paul Burton <paul.burton at imgtec.com>
> Cc: Daniel Schwierzeck <daniel.schwierzeck at gmail.com>
> ---
> 
> Hi,
> I've noticed this when reading the code to understand how the cache
> instruction is used. I'm not sure if this bug had any practical
> consequences, or if nowadays all relevant compilers have
> __builtin_mips_cache anyways.
> 
> Please keep me in Cc in follow-up mails, I'm not subscribed to the u-boot
> ML.
> 
> Matthias

I've disabled the builtin code and compared dissaemblies with and without your patch. Without your patch, gcc adds an additional store instruction before each cache instruction. 

E.g. for flush_dcache_range():

  18:	afa20008 	sw	v0,8(sp)
  1c:	bfb50008 	cache	0x15,8(sp)

vs.

  14:	bc550000 	cache	0x15,0(v0)

The cache operation works anyway, but with your patch better code is generated.

> 
> 
>  arch/mips/include/asm/cacheops.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/mips/include/asm/cacheops.h b/arch/mips/include/asm/cacheops.h
> index a3b07c6..002b839 100644
> --- a/arch/mips/include/asm/cacheops.h
> +++ b/arch/mips/include/asm/cacheops.h
> @@ -16,7 +16,7 @@ static inline void mips_cache(int op, const volatile void *addr)
>  #ifdef __GCC_HAVE_BUILTIN_MIPS_CACHE
>  	__builtin_mips_cache(op, addr);
>  #else
> -	__asm__ __volatile__("cache %0, %1" : : "i"(op), "R"(addr));
> +	__asm__ __volatile__("cache %0, 0(%1)" : : "i"(op), "r"(addr));
>  #endif
>  }
>  
> 

applied to u-boot-mips/next, thanks!

-- 
- Daniel