Every time DM_VERITY_FEC_{MAX,MIN}_RSN are used, they are subtracted
from DM_VERITY_FEC_RSM to get the bounds on the number of roots.
Therefore, replace these with {MIN,MAX}_ROOTS constants which are more
directly useful.  (Note the inversion, where MAX_RSN maps to MIN_ROOTS
and MIN_RSN maps to MAX_ROOTS.)  No functional change.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 drivers/md/dm-verity-fec.c | 6 +++---
 drivers/md/dm-verity-fec.h | 7 +++----
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
index c3f2516f9c2a9..c5f97a49e5b79 100644
--- a/drivers/md/dm-verity-fec.c
+++ b/drivers/md/dm-verity-fec.c
@@ -71,11 +71,11 @@ static int fec_decode_bufs(struct dm_verity *v, struct dm_verity_io *io,
 			   unsigned int block_offset, int neras)
 {
 	int r, corrected = 0, res;
 	struct dm_buffer *buf;
 	unsigned int n, i, j, parity_pos, to_copy;
-	uint16_t par_buf[DM_VERITY_FEC_RSM - DM_VERITY_FEC_MIN_RSN];
+	uint16_t par_buf[DM_VERITY_FEC_MAX_ROOTS];
 	u8 *par, *block;
 	u64 parity_block;
 	struct bio *bio = dm_bio_from_per_bio_data(io, v->ti->per_io_data_size);
 
 	/*
@@ -570,12 +570,12 @@ int verity_fec_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v,
 		}
 		v->fec->start = num_ll;
 
 	} else if (!strcasecmp(arg_name, DM_VERITY_OPT_FEC_ROOTS)) {
 		if (sscanf(arg_value, "%hhu%c", &num_c, &dummy) != 1 || !num_c ||
-		    num_c < (DM_VERITY_FEC_RSM - DM_VERITY_FEC_MAX_RSN) ||
-		    num_c > (DM_VERITY_FEC_RSM - DM_VERITY_FEC_MIN_RSN)) {
+		    num_c < DM_VERITY_FEC_MIN_ROOTS ||
+		    num_c > DM_VERITY_FEC_MAX_ROOTS) {
 			ti->error = "Invalid " DM_VERITY_OPT_FEC_ROOTS;
 			return -EINVAL;
 		}
 		v->fec->roots = num_c;
 
diff --git a/drivers/md/dm-verity-fec.h b/drivers/md/dm-verity-fec.h
index 32ca2bfee1db7..d8d0e81da2701 100644
--- a/drivers/md/dm-verity-fec.h
+++ b/drivers/md/dm-verity-fec.h
@@ -11,12 +11,12 @@
 #include "dm-verity.h"
 #include <linux/rslib.h>
 
 /* Reed-Solomon(M, N) parameters */
 #define DM_VERITY_FEC_RSM		255
-#define DM_VERITY_FEC_MAX_RSN		253
-#define DM_VERITY_FEC_MIN_RSN		231	/* ~10% space overhead */
+#define DM_VERITY_FEC_MIN_ROOTS	2	/* RS(255, 253): ~0.8% space overhead */
+#define DM_VERITY_FEC_MAX_ROOTS	24	/* RS(255, 231): ~10% space overhead */
 
 /* buffers for deinterleaving and decoding */
 #define DM_VERITY_FEC_BUF_RS_BITS	4	/* 1 << RS blocks per buffer */
 
 #define DM_VERITY_OPT_FEC_DEV		"use_fec_from_device"
@@ -45,12 +45,11 @@ struct dm_verity_fec {
 };
 
 /* per-bio data */
 struct dm_verity_fec_io {
 	struct rs_control *rs;	/* Reed-Solomon state */
-	/* erasures for decode_rs8 */
-	int erasures[DM_VERITY_FEC_RSM - DM_VERITY_FEC_MIN_RSN + 1];
+	int erasures[DM_VERITY_FEC_MAX_ROOTS + 1]; /* erasures for decode_rs8 */
 	u8 *output;		/* buffer for corrected output */
 	unsigned int level;		/* recursion level */
 	unsigned int nbufs;		/* number of buffers allocated */
 	/*
 	 * Buffers for deinterleaving RS blocks.  Each buffer has space for
-- 
2.52.0