[PATCH 14/22] dm-verity-fec: simplify computation of ileaved

[Eric Biggers <ebiggers@kernel.org>]

fec_read_bufs() just iterates over a sequence of message blocks with
step size region_blocks.  At each step, 'ileaved' is just the offset (in
bytes) to one of these blocks.  Compute it in the straightforward way,
eliminating fec_interleave().

In more detail, previously the code computed
'ileaved = (n / k) + (n % k) * (region_blocks * block_size)'
where n = rsb * k + i and 0 <= i < k.  Substituting 'n' gives:

    ileaved = ((rsb * k + i) / k) + ((rsb * k + i) % k) * region_blocks * block_size
            = rsb + (i * region_blocks * block_size)

The result is more efficient and easier to understand.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 drivers/md/dm-verity-fec.c | 13 +------------
 1 file changed, 1 insertion(+), 12 deletions(-)

diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
index 63eeef26a3999..3122017569718 100644
--- a/drivers/md/dm-verity-fec.c
+++ b/drivers/md/dm-verity-fec.c
@@ -21,21 +21,10 @@
 static inline unsigned int fec_max_nbufs(struct dm_verity *v)
 {
 	return 1 << (v->data_dev_block_bits - DM_VERITY_FEC_BUF_RS_BITS);
 }
 
-/*
- * Return an interleaved offset for a byte in RS block.
- */
-static inline u64 fec_interleave(struct dm_verity *v, u64 offset)
-{
-	u32 mod;
-
-	mod = do_div(offset, v->fec->rs_k);
-	return offset + mod * (v->fec->region_blocks << v->data_dev_block_bits);
-}
-
 /* Loop over each allocated buffer. */
 #define fec_for_each_buffer(io, __i) \
 	for (__i = 0; __i < (io)->nbufs; __i++)
 
 /* Loop over each RS message in each allocated buffer. */
@@ -202,11 +191,11 @@ static int fec_read_bufs(struct dm_verity *v, struct dm_verity_io *io,
 	/*
 	 * read each of the rs_k data blocks that are part of the RS block, and
 	 * interleave contents to available bufs
 	 */
 	for (i = 0; i < v->fec->rs_k; i++) {
-		ileaved = fec_interleave(v, rsb * v->fec->rs_k + i);
+		ileaved = rsb + i * (v->fec->region_blocks << v->data_dev_block_bits);
 
 		/*
 		 * target is the data block we want to correct, target_index is
 		 * the index of this block within the rs_k RS blocks
 		 */
-- 
2.52.0