nfs4_proc_exchange_id() can return -EINVAL if the server
reported NFS4INVAL (which I have seen in a packet trace),
or nfs4_check_cl_exchange_flags() exchange flags detects
a problem.
Each of these mean that NFSv4.1 later cannot be use, but
they should not prevent fallback to NFSv4.0.

Currently this EINVAL error is returned by nfs4_proc_exchange_id() to
nfs41_discover_server_trunking(), and thence to
nfs4_discover_server_trunking().
nfs4_discover_server_trunking() doesn't understand EINVAL, so converts
it to EIO which causes mount.nfs to think something is horribly wrong
and to give up.

It would be a more graceful failure if nfs4_discover_server_trunking()
mapped -EINVAL to -EPROTONOSUPPORT - a failure to negotiate a client
ID clearly shows that NFSv4.1 cannot be supported, but isn't as
general a failure as EIO.

Signed-off-by: NeilBrown <neilb@suse.com>
---
 fs/nfs/nfs4state.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/nfs/nfs4state.c b/fs/nfs/nfs4state.c
index 91a4d4eeb235..7e237bb9c699 100644
--- a/fs/nfs/nfs4state.c
+++ b/fs/nfs/nfs4state.c
@@ -2219,6 +2219,8 @@ int nfs4_discover_server_trunking(struct nfs_client *clp,
 		clnt = clp->cl_rpcclient;
 		goto again;
 
+	case -NFS4INVAL:
+		/* Server confused - assume this minor isn't supported */
 	case -NFS4ERR_MINOR_VERS_MISMATCH:
 		status = -EPROTONOSUPPORT;
 		break;
-- 
2.14.0.rc0.dirty