diff --git a/include/mbedtls/md.h b/include/mbedtls/md.h
index e4354ba..882ff74 100644
--- a/include/mbedtls/md.h
+++ b/include/mbedtls/md.h
@@ -38,6 +38,8 @@
 #define MBEDTLS_ERR_MD_ALLOC_FAILED                       -0x5180  /**< Failed to allocate memory. */
 #define MBEDTLS_ERR_MD_FILE_IO_ERROR                      -0x5200  /**< Opening or reading of file failed. */
 
+// TODO [TR] for #4029: can't remove it because it's still used in the code.
+// see other TODOs
 /* MBEDTLS_ERR_MD_HW_ACCEL_FAILED is deprecated and should not be used. */
 #define MBEDTLS_ERR_MD_HW_ACCEL_FAILED                    -0x5280  /**< MD hardware accelerator failed. */