[PATCH 21/26] xor: add a better public API

xor_blocks is very annoying to use, because it is limited to 4 + 1
sources / destinations, has an odd argument order and is completely
undocumented.

Lift the code that loops around it from btrfs and async_tx/async_xor into
common code under the name xor_gen and properly document it.

Signed-off-by: Christoph Hellwig <[email protected]>
---
 include/linux/raid/xor.h |  2 ++
 lib/raid/xor/xor-core.c  | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/include/linux/raid/xor.h b/include/linux/raid/xor.h
index 02bda8d99534..6d9a39fd85dd 100644
--- a/include/linux/raid/xor.h
+++ b/include/linux/raid/xor.h
@@ -7,4 +7,6 @@
 extern void xor_blocks(unsigned int count, unsigned int bytes,
 	void *dest, void **srcs);
 
+void xor_gen(void *dest, void **srcs, unsigned int src_cnt, unsigned int bytes);
+
 #endif /* _XOR_H */
diff --git a/lib/raid/xor/xor-core.c b/lib/raid/xor/xor-core.c
index 47e09ae954b2..fdd0cf5584e7 100644
--- a/lib/raid/xor/xor-core.c
+++ b/lib/raid/xor/xor-core.c
@@ -46,6 +46,38 @@ xor_blocks(unsigned int src_count, unsigned int bytes, void *dest, void **srcs)
 }
 EXPORT_SYMBOL(xor_blocks);
 
+/**
+ * xor_gen - generate RAID-style XOR information
+ * @dest:	destination vector
+ * @srcs:	source vectors
+ * @src_cnt:	number of source vectors
+ * @bytes:	length in bytes of each vector
+ *
+ * Performs bit-wise XOR operation into @dest for each of the @src_cnt vectors
+ * in @srcs for a length of @bytes bytes.  @src_count must be non-zero, and the
+ * memory pointed to by @dest and each member of @srcs must be at least 64-byte
+ * aligned.  @bytes must be non-zero and a multiple of 512.
+ *
+ * Note: for typical RAID uses, @dest either needs to be zeroed, or filled with
+ * the first disk, which then needs to be removed from @srcs.
+ */
+void xor_gen(void *dest, void **srcs, unsigned int src_cnt, unsigned int bytes)
+{
+	unsigned int src_off = 0;
+
+	WARN_ON_ONCE(bytes & 511);
+
+	while (src_cnt > 0) {
+		unsigned int this_cnt = min(src_cnt, MAX_XOR_BLOCKS);
+
+		xor_blocks(this_cnt, bytes, dest, srcs + src_off);
+
+		src_cnt -= this_cnt;
+		src_off += this_cnt;
+	}
+}
+EXPORT_SYMBOL(xor_gen);
+
 /* Set of all registered templates.  */
 static struct xor_block_template *__initdata template_list;
 static bool __initdata xor_forced = false;
-- 
2.47.3