scsi: sbitmap: Add sbitmap_calculate_shift() helper

Move code for calculating default shift into a public helper which can be
used by SCSI.

Link: https://lore.kernel.org/r/20210122023317.687987-7-ming.lei@redhat.com
Cc: Omar Sandoval <osandov@fb.com>
Cc: Kashyap Desai <kashyap.desai@broadcom.com>
Cc: Sumanesh Samanta <sumanesh.samanta@broadcom.com>
Cc: Ewan D. Milne <emilne@redhat.com>
Tested-by: Sumanesh Samanta <sumanesh.samanta@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

include/linux/sbitmap.h

@@ -332,6 +332,24 @@ static inline int sbitmap_test_bit(struct sbitmap *sb, unsigned int bitnr)
 	return test_bit(SB_NR_TO_BIT(sb, bitnr), __sbitmap_word(sb, bitnr));
 }
 
+static inline int sbitmap_calculate_shift(unsigned int depth)
+{
+	int	shift = ilog2(BITS_PER_LONG);
+
+	/*
+	 * If the bitmap is small, shrink the number of bits per word so
+	 * we spread over a few cachelines, at least. If less than 4
+	 * bits, just forget about it, it's not going to work optimally
+	 * anyway.
+	 */
+	if (depth >= 4) {
+		while ((4U << shift) > depth)
+			shift--;
+	}
+
+	return shift;
+}
+
 /**
  * sbitmap_show() - Dump &struct sbitmap information to a &struct seq_file.
  * @sb: Bitmap to show.

lib/sbitmap.c

@@ -87,19 +87,9 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
 	unsigned int bits_per_word;
 	unsigned int i;
 
-	if (shift < 0) {
-		shift = ilog2(BITS_PER_LONG);
-		/*
-		 * If the bitmap is small, shrink the number of bits per word so
-		 * we spread over a few cachelines, at least. If less than 4
-		 * bits, just forget about it, it's not going to work optimally
-		 * anyway.
-		 */
-		if (depth >= 4) {
-			while ((4U << shift) > depth)
-				shift--;
-		}
-	}
+	if (shift < 0)
+		shift = sbitmap_calculate_shift(depth);
+
 	bits_per_word = 1U << shift;
 	if (bits_per_word > BITS_PER_LONG)
 		return -EINVAL;