[PATCH v2 4/6] bitmap: Introduce bitmap_off()

[Herve Codina <herve.codina@bootlin.com>]

The bitmap_onto() function translates one bitmap relative to another but
no function are present to perform the reverse translation.

Introduce bitmap_off() to fill this hole.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
---
 include/linux/bitmap.h |  3 +++
 lib/bitmap.c           | 42 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 99451431e4d6..5ecfcbbc91f4 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -65,6 +65,7 @@ struct device;
  *  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
+ *  bitmap_off(dst, orig, relmap, nbits)        *dst = bitmap_onto() reverse operation
  *  bitmap_fold(dst, orig, sz, nbits)           dst bits = orig bits mod sz
  *  bitmap_parse(buf, buflen, dst, nbits)       Parse bitmap dst from kernel buf
  *  bitmap_parse_user(ubuf, ulen, dst, nbits)   Parse bitmap dst from user buf
@@ -208,6 +209,8 @@ int bitmap_bitremap(int oldbit,
 		const unsigned long *old, const unsigned long *new, int bits);
 void bitmap_onto(unsigned long *dst, const unsigned long *orig,
 		const unsigned long *relmap, unsigned int bits);
+void bitmap_off(unsigned long *dst, const unsigned long *orig,
+		const unsigned long *relmap, unsigned int bits);
 void bitmap_fold(unsigned long *dst, const unsigned long *orig,
 		unsigned int sz, unsigned int nbits);
 
diff --git a/lib/bitmap.c b/lib/bitmap.c
index 2feccb5047dc..71343967335e 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -682,6 +682,48 @@ void bitmap_onto(unsigned long *dst, const unsigned long *orig,
 }
 EXPORT_SYMBOL(bitmap_onto);
 
+/**
+ * bitmap_off - revert operation done by bitmap_onto()
+ *     @dst: resulting translated bitmap
+ *     @orig: original untranslated bitmap
+ *     @relmap: bitmap relative to which translated
+ *     @bits: number of bits in each of these bitmaps
+ *
+ * Suppose onto computed using bitmap_onto(onto, src, relmap, n)
+ * The operation bitmap_off(result, onto, relmap, n) leads to a
+ * result equal or equivalent to src.
+ *
+ * The result can be 'equivalent' because bitmap_onto() and
+ * bitmap_off() are not bijective.
+ * The result and src values are equivalent in that sense that a
+ * call to bitmap_onto(onto, src, relmap, n) and a call to
+ * bitmap_onto(onto, result, relmap, n) will lead to the same onto
+ * value.
+ *
+ * If either of @orig or @relmap is empty (no set bits), then @dst
+ * will be returned empty.
+ *
+ * All bits in @dst not set by the above rule are cleared.
+ */
+void bitmap_off(unsigned long *dst, const unsigned long *orig,
+		const unsigned long *relmap, unsigned int bits)
+{
+	unsigned int n, m;      /* same meaning as in above comment */
+
+	if (dst == orig)        /* following doesn't handle inplace mappings */
+		return;
+	bitmap_zero(dst, bits);
+
+	m = 0;
+	for_each_set_bit(n, relmap, bits) {
+		/* m == bitmap_pos_to_ord(relmap, n, bits) */
+		if (test_bit(n, orig))
+			set_bit(m, dst);
+		m++;
+	}
+}
+EXPORT_SYMBOL(bitmap_off);
+
 #ifdef CONFIG_NUMA
 /**
  * bitmap_fold - fold larger bitmap into smaller, modulo specified size
-- 
2.43.0