Re: [PATCH v5 21/23] powerpc: Simplify test in __dma_sync()

[Christophe Leroy]

Le 05/02/2016 08:52, Denis Kirjanov a Ãcrit :
> On 2/4/16, Christophe Leroy <christophe.leroy@xxxxxx> wrote:
> > Le 04/02/2016 12:37, Denis Kirjanov a Ãcrit :
> > > On 2/4/16, Christophe Leroy <christophe.leroy@xxxxxx> 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@xxxxxx>
> > > > ---
> > > > 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:
> Yeah, looks better. Did you compile the kernel with default compiler flags?
>
> Thanks!
Yes I did

Christophe