net: dsa: tag_sja1105: be dsa_loop-safe

Add support for tag_sja1105 running on non-sja1105 DSA ports, by making
sure that every time we dereference dp->priv, we check the switch's
dsa_switch_ops (otherwise we access a struct sja1105_port structure that
is in fact something else).

This adds an unconditional build-time dependency between sja1105 being
built as module => tag_sja1105 must also be built as module. This was
there only for PTP before.

Some sane defaults must also take place when not running on sja1105
hardware. These are:

- sja1105_xmit_tpid: the sja1105 driver uses different VLAN protocols
  depending on VLAN awareness and switch revision (when an encapsulated
  VLAN must be sent). Default to 0x8100.

- sja1105_rcv_meta_state_machine: this aggregates PTP frames with their
  metadata timestamp frames. When running on non-sja1105 hardware, don't
  do that and accept all frames unmodified.

- sja1105_defer_xmit: calls sja1105_port_deferred_xmit in sja1105_main.c
  which writes a management route over SPI. When not running on sja1105
  hardware, bypass the SPI write and send the frame as-is.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/dsa/sja1105.h

@@ -88,4 +88,22 @@ static inline void sja1110_process_meta_tstamp(struct dsa_switch *ds, int port,
 
 #endif /* IS_ENABLED(CONFIG_NET_DSA_SJA1105_PTP) */
 
+#if IS_ENABLED(CONFIG_NET_DSA_SJA1105)
+
+extern const struct dsa_switch_ops sja1105_switch_ops;
+
+static inline bool dsa_port_is_sja1105(struct dsa_port *dp)
+{
+	return dp->ds->ops == &sja1105_switch_ops;
+}
+
+#else
+
+static inline bool dsa_port_is_sja1105(struct dsa_port *dp)
+{
+	return false;
+}
+
+#endif
+
 #endif /* _NET_DSA_SJA1105_H */