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

Le 05/02/2016 08:52, Denis Kirjanov a écrit :

On 2/4/16, Christophe Leroy [off-list ref] wrote:

quoted

Le 04/02/2016 12:37, Denis Kirjanov a écrit :

quoted

On 2/4/16, Christophe Leroy [off-list ref] wrote:

quoted

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 <redacted>
---
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