dm stripe: optimize sector division

Optimize sector division: If the number of stripes is a power of two,
we can do shift and mask instead of division.

Signed-off-by: Mikulas Patocka <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>
Signed-off-by: Alasdair G Kergon <[e-mail]>

drivers/md/dm-stripe.c

···
 
 struct stripe_c {
 	uint32_t stripes;
+	int stripes_shift;
+	sector_t stripes_mask;
 
 	/* The size of this target / num. stripes */
 	sector_t stripe_width;
···
 
 	/* Set pointer to dm target; used in trigger_event */
 	sc->ti = ti;
-
 	sc->stripes = stripes;
 	sc->stripe_width = width;
+
+	if (stripes & (stripes - 1))
+		sc->stripes_shift = -1;
+	else {
+		sc->stripes_shift = ffs(stripes) - 1;
+		sc->stripes_mask = ((sector_t) stripes) - 1;
+	}
+
 	ti->split_io = chunk_size;
 	ti->num_flush_requests = stripes;
 
+	sc->chunk_shift = ffs(chunk_size) - 1;
 	sc->chunk_mask = ((sector_t) chunk_size) - 1;
-	for (sc->chunk_shift = 0; chunk_size; sc->chunk_shift++)
-		chunk_size >>= 1;
-	sc->chunk_shift--;
 
 	/*
 	 * Get the stripe destinations.
···
 	sector_t offset = dm_target_offset(sc->ti, sector);
 	sector_t chunk = offset >> sc->chunk_shift;
 
-	*stripe = sector_div(chunk, sc->stripes);
+	if (sc->stripes_shift < 0)
+		*stripe = sector_div(chunk, sc->stripes);
+	else {
+		*stripe = chunk & sc->stripes_mask;
+		chunk >>= sc->stripes_shift;
+	}
+
 	*result = (chunk << sc->chunk_shift) | (offset & sc->chunk_mask);
 }