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

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Message-ID: <CAOJe8K2qmRARai6okSXtvpkt2JOfJCrqwUOinDAyo2Qoypd7uw@mail.gmail.com>
Date:	Thu, 4 Feb 2016 14:37:32 +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:
> 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?
> --
> 2.1.0
>
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