linux-kernel - [PATCH v3 2/6] rslib: Replace hard-coded 1 with alpha

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Date:   Mon, 20 Jun 2022 14:20:11 +0800
From:   Zhang Boyang <zhangboyang.id@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Ferdinand Blomqvist <ferdinand.blomqvist@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Zhang Boyang <zhangboyang.id@...il.com>
Subject: [PATCH v3 2/6] rslib: Replace hard-coded 1 with alpha_to[0]

Currently the rslib allows customizing the finite field by the `gffunc'
parameter of init_rs_non_canonical(). However, there are several places
in rslib use hard-coded 1 instead of alpha_to[0], leading to errors if
gffunc(0) != 1. This patch fixes the problem. One of such `gffunc' might
be gffunc'(x) = swab16(gffunc(swab16(x))), as gffunc'(0) = swab16(1).
This special gffunc'(x) is useful when implementing RS coder for
16 bit foreign-endian symbols.

Signed-off-by: Zhang Boyang <zhangboyang.id@...il.com>
---
 lib/reed_solomon/decode_rs.c    | 4 ++--
 lib/reed_solomon/reed_solomon.c | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/lib/reed_solomon/decode_rs.c b/lib/reed_solomon/decode_rs.c
index 805de84ae83d..6c1d53d1b702 100644
--- a/lib/reed_solomon/decode_rs.c
+++ b/lib/reed_solomon/decode_rs.c
@@ -104,7 +104,7 @@
 
  decode:
 	memset(&lambda[1], 0, nroots * sizeof(lambda[0]));
-	lambda[0] = 1;
+	lambda[0] = alpha_to[0];
 
 	if (no_eras > 0) {
 		/* Init lambda to be the erasure locator polynomial */
@@ -198,7 +198,7 @@
 	memcpy(&reg[1], &lambda[1], nroots * sizeof(reg[0]));
 	count = 0;		/* Number of roots of lambda(x) */
 	for (i = 1, k = iprim - 1; i <= nn; i++, k = rs_modnn(rs, k + iprim)) {
-		q = 1;		/* lambda[0] is always 0 */
+		q = alpha_to[0];	/* lambda[0] is always 0 */
 		for (j = deg_lambda; j > 0; j--) {
 			if (reg[j] != nn) {
 				reg[j] = rs_modnn(rs, reg[j] + j);
diff --git a/lib/reed_solomon/reed_solomon.c b/lib/reed_solomon/reed_solomon.c
index bbc01bad3053..bb4f44c8edba 100644
--- a/lib/reed_solomon/reed_solomon.c
+++ b/lib/reed_solomon/reed_solomon.c
@@ -131,9 +131,9 @@ static struct rs_codec *codec_init(int symsize, int gfpoly, int (*gffunc)(int),
 	rs->iprim = iprim / prim;
 
 	/* Form RS code generator polynomial from its roots */
-	rs->genpoly[0] = 1;
+	rs->genpoly[0] = rs->alpha_to[0];
 	for (i = 0, root = fcr * prim; i < nroots; i++, root += prim) {
-		rs->genpoly[i + 1] = 1;
+		rs->genpoly[i + 1] = rs->alpha_to[0];
 		/* Multiply rs->genpoly[] by  @**(root + x) */
 		for (j = i; j > 0; j--) {
 			if (rs->genpoly[j] != 0) {
-- 
2.30.2