linux-kernel - Re: [PATCH v5 21/23] powerpc: Simplify test in __dma

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Date:	Fri, 5 Feb 2016 10:52:32 +0300
From:	Denis Kirjanov <kda@...ux-powerpc.org>
To:	Christophe Leroy <christophe.leroy@....fr>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Scott Wood <oss@...error.net>, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 21/23] powerpc: Simplify test in __dma_sync()

On 2/4/16, Christophe Leroy <christophe.leroy@....fr> wrote:
>
>
> Le 04/02/2016 12:37, Denis Kirjanov a écrit :
>> On 2/4/16, Christophe Leroy <christophe.leroy@....fr> wrote:
>>> This simplification helps the compiler. We now have only one test
>>> instead of two, so it reduces the number of branches.
>>>
>>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>>> ---
>>> v2: new
>>> v3: no change
>>> v4: no change
>>> v5: no change
>>>
>>>   arch/powerpc/mm/dma-noncoherent.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/arch/powerpc/mm/dma-noncoherent.c
>>> b/arch/powerpc/mm/dma-noncoherent.c
>>> index 169aba4..2dc74e5 100644
>>> --- a/arch/powerpc/mm/dma-noncoherent.c
>>> +++ b/arch/powerpc/mm/dma-noncoherent.c
>>> @@ -327,7 +327,7 @@ void __dma_sync(void *vaddr, size_t size, int
>>> direction)
>>>   		 * invalidate only when cache-line aligned otherwise there is
>>>   		 * the potential for discarding uncommitted data from the cache
>>>   		 */
>>> -		if ((start & (L1_CACHE_BYTES - 1)) || (size & (L1_CACHE_BYTES - 1)))
>>> +		if ((start | end) & (L1_CACHE_BYTES - 1))
>>>   			flush_dcache_range(start, end);
>>>   		else
>>>   			invalidate_dcache_range(start, end);
>> The previous version of address cache-line aligned check reads perfectly
>> fine.
>> What's the benefit of this micro optimization?
> With this optimisation we avoid one unneccessary test and two associated
> jumps. Taking into account that __dma_sync() is one of the top ten CPU
> consummers, I believe it is worth it:
>
> Without the patch:
>
> c000d894:    70 6a 00 0f     andi.   r10,r3,15
> c000d898:    39 29 00 0f     addi    r9,r9,15
> c000d89c:    54 63 00 36     rlwinm  r3,r3,0,0,27
> c000d8a0:    7d 23 48 50     subf    r9,r3,r9
> c000d8a4:    41 82 00 84     beq     c000d928 <__dma_sync+0xb8>
> [...]
> c000d8c0:    7c 00 04 ac     sync
> c000d8c4:    4e 80 00 20     blr
> [...]
> c000d928:    70 8a 00 0f     andi.   r10,r4,15
> c000d92c:    40 a2 ff 7c     bne     c000d8a8 <__dma_sync+0x38>
> c000d930:    55 2a e1 3f     rlwinm. r10,r9,28,4,31
> c000d934:    41 a2 ff 8c     beq     c000d8c0 <__dma_sync+0x50>
>
> With the patch:
>
> c000d894:    7c 89 1b 78     or      r9,r4,r3
> c000d898:    71 2a 00 0f     andi.   r10,r9,15
> c000d89c:    54 63 00 36     rlwinm  r3,r3,0,0,27
> c000d8a0:    38 84 00 0f     addi    r4,r4,15
> c000d8a4:    7c 83 20 50     subf    r4,r3,r4
> c000d8a8:    41 82 00 84     beq     c000d92c <__dma_sync+0xbc>
> [...]
> c000d8c4:    7c 00 04 ac     sync
> c000d8c8:    4e 80 00 20     blr
> [...]
> c000d92c:    54 89 e1 3f     rlwinm. r9,r4,28,4,31
> c000d930:    41 a2 ff 94     beq     c000d8c4 <__dma_sync+0x54>

Yeah, looks better. Did you compile the kernel with default compiler flags?

Thanks!

>
>
> Christophe
>>> --
>>> 2.1.0
>>>
>>> _______________________________________________
>>> Linuxppc-dev mailing list
>>> Linuxppc-dev@...ts.ozlabs.org
>>> https://lists.ozlabs.org/listinfo/linuxppc-dev
>
>