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.

[hch@lst.de: make xor_blocks less annoying to use]
 Link: https://lkml.kernel.org/r/20260327061704.3707577-24-hch@lst.de
Link: https://lkml.kernel.org/r/20260327061704.3707577-23-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Biggers <ebiggers@kernel.org>
Tested-by: Eric Biggers <ebiggers@kernel.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "Borislav Petkov (AMD)" <bp@alien8.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Chris Mason <clm@fb.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: David Sterba <dsterba@suse.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jason A. Donenfeld <jason@zx2c4.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: Li Nan <linan122@huawei.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Magnus Lindholm <linmag7@gmail.com>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Richard Weinberger <richard@nod.at>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Song Liu <song@kernel.org>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Ted Ts'o <tytso@mit.edu>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: WANG Xuerui <kernel@xen0n.name>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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 */

lib/raid/xor/xor-core.c

@@ -46,6 +46,40 @@ 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_cnt 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(in_interrupt());
+	WARN_ON_ONCE(bytes == 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;