We have had a customer request to be able to run individual self-tests in
a future version of our driver.  Now we could do this any way we want in
an out-of-tree driver, but I'd rather find some way of doing this that
will also be acceptable in-tree.  I think that would mean extending the
ethtool self-test API in some way.

One possibility might be to add a new flag and to assign the reserved field
as a test index:

--- a/include/linux/ethtool.h
+++ b/include/linux/ethtool.h

@@ -235,13 +235,14 @@ struct ethtool_gstrings {
 enum ethtool_test_flags {
 	ETH_TEST_FL_OFFLINE	= (1 << 0),	/* online / offline */
 	ETH_TEST_FL_FAILED	= (1 << 1),	/* test passed / failed */
+	ETH_TEST_FL_SINGLE	= (1 << 2),	/* run single test */
 };
 
 /* for requesting NIC test and getting results*/
 struct ethtool_test {
 	__u32	cmd;		/* ETHTOOL_TEST */
 	__u32	flags;		/* ETH_TEST_FL_xxx */
-	__u32	reserved;
+	__u32	index;		/* test index if ETH_TEST_FL_SINGLE */
 	__u32	len;		/* result length, in number of u64 elements */
 	__u64	data[0];
 };

Clients would need to check len on return since the ethtool core and driver
ethtool functions ignore unknown flags.  I am assuming that running all
online self-tests instead of a single self-test will be harmless.

Ideally the ethtool core would allocate only the necessary memory for the
results of the selected test set or single test, but it would need to know
whether the driver also supports this extension.