fix comment, ChangeLog & migration-guide for MBEDTLS_ECP_FIXED_POINT_OPTIM
Signed-off-by: kXuan <kxuanobj@gmail.com>
diff --git a/ChangeLog.d/issue4128.txt b/ChangeLog.d/issue4128.txt
index 4217403..bc41874 100644
--- a/ChangeLog.d/issue4128.txt
+++ b/ChangeLog.d/issue4128.txt
@@ -1,4 +1,4 @@
API changes
- * The option `MBEDTLS_ECP_FIXED_POINT_OPTIM` use pre-computed comb tables
- instead of computing tables in runtime. Thus, this option now use more
- ROM, and it does not increase RAM usage in runtime anymore.
+ * The option MBEDTLS_ECP_FIXED_POINT_OPTIM use pre-computed comb tables
+ instead of computing tables in runtime. Thus, this option now increase
+ code size, and it does not increase RAM usage in runtime anymore.
diff --git a/docs/3.0-migration-guide.d/modify_MBEDTLS_ECP_FIXED_POINT_OPTIM_behaviour.md b/docs/3.0-migration-guide.d/modify_MBEDTLS_ECP_FIXED_POINT_OPTIM_behaviour.md
index 02a083c..91bec72 100644
--- a/docs/3.0-migration-guide.d/modify_MBEDTLS_ECP_FIXED_POINT_OPTIM_behaviour.md
+++ b/docs/3.0-migration-guide.d/modify_MBEDTLS_ECP_FIXED_POINT_OPTIM_behaviour.md
@@ -1,8 +1,11 @@
Change MBEDTLS_ECP_FIXED_POINT_OPTIM behaviour
------------------------------------------------------
-The option MBEDTLS_ECP_FIXED_POINT_OPTIM now use more ROM and does not increase
-peak RAM usage anymore.
+The option `MBEDTLS_ECP_FIXED_POINT_OPTIM` now increase code size and it does
+not increase peak RAM usage anymore.
-If you are limited by ROM space, you can define MBEDTLS_ECP_FIXED_POINT_OPTIM
-to `0` in your config file. This will save about 50 KiB ROM space.
+If you are limited by code size, you can define `MBEDTLS_ECP_FIXED_POINT_OPTIM`
+to `0` in your config file. The impact depends on the number and size of
+enabled curves. For example, for P-256 the difference is 1KB; see the documentation
+of this option for details.
+
diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index 0c351fa..f53a828 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -276,15 +276,16 @@
#if !defined(MBEDTLS_ECP_FIXED_POINT_OPTIM)
/*
- * Trade ROM usage for speed on fixed-point multiplication.
+ * Trade code size for speed on fixed-point multiplication.
*
* This speeds up repeated multiplication of the generator (that is, the
* multiplication in ECDSA signatures, and half of the multiplications in
* ECDSA verification and ECDHE) by a factor roughly 3 to 4.
*
- * The cost is increasing ROM usage by a factor roughly 2.
+ * For each n-bit Short Weierstrass curve that is enabled, this adds 4n bytes
+ * of code size if n < 384 and 8n otherwise.
*
- * Change this value to 0 to reduce ROM usage.
+ * Change this value to 0 to reduce code size.
*/
#define MBEDTLS_ECP_FIXED_POINT_OPTIM 1 /**< Enable fixed-point speed-up. */
#endif /* MBEDTLS_ECP_FIXED_POINT_OPTIM */