Subject: Re: [PATCH 21/25] xor: add a better public API

From: Eric Biggers <[email-masked]>
Date: Fri, 27 Feb 2026 22:50:38 -0800

On Thu, Feb 26, 2026 at 07:10:33AM -0800, Christoph Hellwig wrote:
> 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-masked]>
> ---
>  include/linux/raid/xor.h |  3 +++
>  lib/raid/xor/xor-core.c  | 28 ++++++++++++++++++++++++++++
>  2 files changed, 31 insertions(+)
> 
> diff --git a/include/linux/raid/xor.h b/include/linux/raid/xor.h
> index 02bda8d99534..4735a4e960f9 100644
> --- a/include/linux/raid/xor.h
> +++ b/include/linux/raid/xor.h
> @@ -7,4 +7,7 @@
>  extern void xor_blocks(unsigned int count, unsigned int bytes,
>  	void *dest, void **srcs);
>  
> +void xor_gen(void *dest, void **srcss, unsigned int src_cnt,
> +		unsigned int bytes);

srcss => srcs

Ideally the source vectors would be 'const' as well.

> +/**
> + * 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.
> + *
> + * 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;
> +
> +	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);

The alignment requirements on the vectors should be documented, as
should which values of bytes are accepted.  It looks like, at the very
least, the vectors have to be 32-byte aligned and the length has to be a
nonzero multiple of 512 bytes.  But I didn't check every implementation.

Also, the requirement on the calling context (e.g. !is_interrupt())
should be documented as well.

- Eric