diff --git a/include/mbedtls/cipher.h b/include/mbedtls/cipher.h
index 1cafa6e..ee9c8e6 100644
--- a/include/mbedtls/cipher.h
+++ b/include/mbedtls/cipher.h
@@ -62,6 +62,8 @@
 #define MBEDTLS_ERR_CIPHER_AUTH_FAILED          -0x6300  /**< Authentication failed (for AEAD modes). */
 #define MBEDTLS_ERR_CIPHER_INVALID_CONTEXT      -0x6380  /**< The context is invalid. For example, because it was freed. */
 
+// TODO [TR] for #4029: can't remove it because it's still used in the code.
+// see other TODOS
 /* MBEDTLS_ERR_CIPHER_HW_ACCEL_FAILED is deprecated and should not be used. */
 #define MBEDTLS_ERR_CIPHER_HW_ACCEL_FAILED      -0x6400  /**< Cipher hardware accelerator failed. */