diff --git a/tests/suites/test_suite_constant_time.function b/tests/suites/test_suite_constant_time.function
index 9df9432eb5..262f841bca 100644
--- a/tests/suites/test_suite_constant_time.function
+++ b/tests/suites/test_suite_constant_time.function
@@ -260,6 +260,17 @@ exit:
 /* END_CASE */
 
 /* BEGIN_CASE depends_on:MBEDTLS_NIST_KW_C */
+
+/**
+ * Generate two arrays of the given size, and test mbedtls_ct_memcmp_partial
+ * over them. The arrays will be identical, except that one byte may be specified
+ * to be different.
+ *
+ * \p diff      Index of byte that differs (if out of range, the arrays will match).
+ * \p size      Size of arrays to compare
+ * \p skip_head Leading bytes to skip, as per mbedtls_ct_memcmp_partial
+ * \p skip_tail Trailing bytes to skip, as per mbedtls_ct_memcmp_partial
+ */
 void mbedtls_ct_memcmp_partial(int diff, int size, int skip_head, int skip_tail)
 {
     uint8_t *a = NULL, *b = NULL;