diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index 7d0f977..f6ca04c 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -67,6 +67,8 @@
 #define MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE -0x3980  /**< Unavailable feature, e.g. RSA disabled for RSA key. */
 #define MBEDTLS_ERR_PK_SIG_LEN_MISMATCH    -0x3900  /**< The buffer contains a valid signature followed by more data. */
 
+// TODO [TR] for #4029 can't remove it because it's still used in the code.
+// see the other TODOs
 /* MBEDTLS_ERR_PK_HW_ACCEL_FAILED is deprecated and should not be used. */
 #define MBEDTLS_ERR_PK_HW_ACCEL_FAILED     -0x3880  /**< PK hardware accelerator failed. */