lib/bitmap: Introduce bitmap_scatter() and bitmap_gather() helpers

These helpers scatters or gathers a bitmap with the help of the mask
position bits parameter.

bitmap_scatter() does the following:
  src:  0000000001011010
                  ||||||
           +------+|||||
           |  +----+||||
           |  |+----+|||
           |  ||   +-+||
           |  ||   |  ||
  mask: ...v..vv...v..vv
        ...0..11...0..10
  dst:  0000001100000010

and bitmap_gather() performs this one:
   mask: ...v..vv...v..vv
   src:  0000001100000010
            ^  ^^   ^   0
            |  ||   |  10
            |  ||   > 010
            |  |+--> 1010
            |  +--> 11010
            +----> 011010
   dst:  0000000000011010

bitmap_gather() can the seen as the reverse bitmap_scatter() operation.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/lkml/20230926052007.3917389-3-andriy.shevchenko@linux.intel.com/Co-developed-by: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Acked-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/bitmap.h

@@ -63,6 +63,8 @@ struct device;
  *  bitmap_shift_left(dst, src, n, nbits)       *dst = *src << n
  *  bitmap_cut(dst, src, first, n, nbits)       Cut n bits from first, copy rest
  *  bitmap_replace(dst, old, new, mask, nbits)  *dst = (*old & ~(*mask)) | (*new & *mask)
+ *  bitmap_scatter(dst, src, mask, nbits)	*dst = map(dense, sparse)(src)
+ *  bitmap_gather(dst, src, mask, nbits)	*dst = map(sparse, dense)(src)
  *  bitmap_remap(dst, src, old, new, nbits)     *dst = map(old, new)(src)
  *  bitmap_bitremap(oldbit, old, new, nbits)    newbit = map(old, new)(oldbit)
  *  bitmap_onto(dst, orig, relmap, nbits)       *dst = orig relative to relmap
@@ -499,6 +501,105 @@ static inline void bitmap_replace(unsigned long *dst,
 		__bitmap_replace(dst, old, new, mask, nbits);
 }
 
+/**
+ * bitmap_scatter - Scatter a bitmap according to the given mask
+ * @dst: scattered bitmap
+ * @src: gathered bitmap
+ * @mask: mask representing bits to assign to in the scattered bitmap
+ * @nbits: number of bits in each of these bitmaps
+ *
+ * Scatters bitmap with sequential bits according to the given @mask.
+ *
+ * Example:
+ * If @src bitmap = 0x005a, with @mask = 0x1313, @dst will be 0x0302.
+ *
+ * Or in binary form
+ * @src			@mask			@dst
+ * 0000000001011010	0001001100010011	0000001100000010
+ *
+ * (Bits 0, 1, 2, 3, 4, 5 are copied to the bits 0, 1, 4, 8, 9, 12)
+ *
+ * A more 'visual' description of the operation:
+ * src:  0000000001011010
+ *                 ||||||
+ *          +------+|||||
+ *          |  +----+||||
+ *          |  |+----+|||
+ *          |  ||   +-+||
+ *          |  ||   |  ||
+ * mask: ...v..vv...v..vv
+ *       ...0..11...0..10
+ * dst:  0000001100000010
+ *
+ * A relationship exists between bitmap_scatter() and bitmap_gather().
+ * bitmap_gather() can be seen as the 'reverse' bitmap_scatter() operation.
+ * See bitmap_scatter() for details related to this relationship.
+ */
+static inline void bitmap_scatter(unsigned long *dst, const unsigned long *src,
+				  const unsigned long *mask, unsigned int nbits)
+{
+	unsigned int n = 0;
+	unsigned int bit;
+
+	bitmap_zero(dst, nbits);
+
+	for_each_set_bit(bit, mask, nbits)
+		__assign_bit(bit, dst, test_bit(n++, src));
+}
+
+/**
+ * bitmap_gather - Gather a bitmap according to given mask
+ * @dst: gathered bitmap
+ * @src: scattered bitmap
+ * @mask: mask representing bits to extract from in the scattered bitmap
+ * @nbits: number of bits in each of these bitmaps
+ *
+ * Gathers bitmap with sparse bits according to the given @mask.
+ *
+ * Example:
+ * If @src bitmap = 0x0302, with @mask = 0x1313, @dst will be 0x001a.
+ *
+ * Or in binary form
+ * @src			@mask			@dst
+ * 0000001100000010	0001001100010011	0000000000011010
+ *
+ * (Bits 0, 1, 4, 8, 9, 12 are copied to the bits 0, 1, 2, 3, 4, 5)
+ *
+ * A more 'visual' description of the operation:
+ * mask: ...v..vv...v..vv
+ * src:  0000001100000010
+ *          ^  ^^   ^   0
+ *          |  ||   |  10
+ *          |  ||   > 010
+ *          |  |+--> 1010
+ *          |  +--> 11010
+ *          +----> 011010
+ * dst:  0000000000011010
+ *
+ * A relationship exists between bitmap_gather() and bitmap_scatter(). See
+ * bitmap_scatter() for the bitmap scatter detailed operations.
+ * Suppose scattered computed using bitmap_scatter(scattered, src, mask, n).
+ * The operation bitmap_gather(result, scattered, mask, n) leads to a result
+ * equal or equivalent to src.
+ *
+ * The result can be 'equivalent' because bitmap_scatter() and bitmap_gather()
+ * are not bijective.
+ * The result and src values are equivalent in that sense that a call to
+ * bitmap_scatter(res, src, mask, n) and a call to
+ * bitmap_scatter(res, result, mask, n) will lead to the same res value.
+ */
+static inline void bitmap_gather(unsigned long *dst, const unsigned long *src,
+				 const unsigned long *mask, unsigned int nbits)
+{
+	unsigned int n = 0;
+	unsigned int bit;
+
+	bitmap_zero(dst, nbits);
+
+	for_each_set_bit(bit, mask, nbits)
+		__assign_bit(n++, dst, test_bit(bit, src));
+}
+
 static inline void bitmap_next_set_region(unsigned long *bitmap,
 					  unsigned int *rs, unsigned int *re,
 					  unsigned int end)

lib/test_bitmap.c

@@ -380,6 +380,47 @@ static void __init test_replace(void)
 	expect_eq_bitmap(bmap, exp3_1_0, nbits);
 }
 
+static const unsigned long sg_mask[] __initconst = {
+	BITMAP_FROM_U64(0x000000000000035aULL),
+};
+
+static const unsigned long sg_src[] __initconst = {
+	BITMAP_FROM_U64(0x0000000000000667ULL),
+};
+
+static const unsigned long sg_gather_exp[] __initconst = {
+	BITMAP_FROM_U64(0x0000000000000029ULL),
+};
+
+static const unsigned long sg_scatter_exp[] __initconst = {
+	BITMAP_FROM_U64(0x000000000000021aULL),
+};
+
+static void __init test_bitmap_sg(void)
+{
+	unsigned int nbits = 64;
+	DECLARE_BITMAP(bmap_gather, 100);
+	DECLARE_BITMAP(bmap_scatter, 100);
+	DECLARE_BITMAP(bmap_tmp, 100);
+	DECLARE_BITMAP(bmap_res, 100);
+
+	/* Simple gather call */
+	bitmap_zero(bmap_gather, 100);
+	bitmap_gather(bmap_gather, sg_src, sg_mask, nbits);
+	expect_eq_bitmap(sg_gather_exp, bmap_gather, nbits);
+
+	/* Simple scatter call */
+	bitmap_zero(bmap_scatter, 100);
+	bitmap_scatter(bmap_scatter, sg_src, sg_mask, nbits);
+	expect_eq_bitmap(sg_scatter_exp, bmap_scatter, nbits);
+
+	/* Scatter/gather relationship */
+	bitmap_zero(bmap_tmp, 100);
+	bitmap_gather(bmap_tmp, bmap_scatter, sg_mask, nbits);
+	bitmap_scatter(bmap_res, bmap_tmp, sg_mask, nbits);
+	expect_eq_bitmap(bmap_scatter, bmap_res, nbits);
+}
+
 #define PARSE_TIME	0x1
 #define NO_LEN		0x2
 
@@ -1252,6 +1293,7 @@ static void __init selftest(void)
 	test_copy();
 	test_bitmap_region();
 	test_replace();
+	test_bitmap_sg();
 	test_bitmap_arr32();
 	test_bitmap_arr64();
 	test_bitmap_parse();