mptcp: clarify when options can be used

RFC8684 doesn't seem to clearly specify which MPTCP options can be used
together.

Some options are mutually exclusive -- e.g. MP_CAPABLE and MP_JOIN --,
some can be used together -- e.g. DSS + MP_PRIO --, some can but we
prefer not to -- e.g. DSS + ADD_ADDR -- and some have to be used
together at some points -- e.g. MP_FAIL and DSS.

We need to clarify this as a base before allowing other modifications.

For example, does it make sense to send a RM_ADDR with an MPC or MPJ?
This remains open for possible future discussions.

Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/options.c

@@ -1267,8 +1267,27 @@ void mptcp_write_options(__be32 *ptr, const struct tcp_sock *tp,
 	const struct sock *ssk = (const struct sock *)tp;
 	struct mptcp_subflow_context *subflow;
 
-	/* DSS, MPC, MPJ, ADD_ADDR, FASTCLOSE and RST are mutually exclusive,
-	 * see mptcp_established_options*()
+	/* Which options can be used together?
+	 *
+	 * X: mutually exclusive
+	 * O: often used together
+	 * C: can be used together in some cases
+	 * P: could be used together but we prefer not to (optimisations)
+	 *
+	 *  Opt: | MPC  | MPJ  | DSS  | ADD  |  RM  | PRIO | FAIL |  FC  |
+	 * ------|------|------|------|------|------|------|------|------|
+	 *  MPC  |------|------|------|------|------|------|------|------|
+	 *  MPJ  |  X   |------|------|------|------|------|------|------|
+	 *  DSS  |  X   |  X   |------|------|------|------|------|------|
+	 *  ADD  |  X   |  X   |  P   |------|------|------|------|------|
+	 *  RM   |  C   |  C   |  C   |  P   |------|------|------|------|
+	 *  PRIO |  X   |  C   |  C   |  C   |  C   |------|------|------|
+	 *  FAIL |  X   |  X   |  C   |  X   |  X   |  X   |------|------|
+	 *  FC   |  X   |  X   |  X   |  X   |  X   |  X   |  X   |------|
+	 *  RST  |  X   |  X   |  X   |  X   |  X   |  X   |  O   |  O   |
+	 * ------|------|------|------|------|------|------|------|------|
+	 *
+	 * The same applies in mptcp_established_options() function.
 	 */
 	if (likely(OPTION_MPTCP_DSS & opts->suboptions)) {
 		struct mptcp_ext *mpext = &opts->ext_copy;