Some phy modes such as QSGMII multiplex several MAC<->PHY links on one
single physical interface. QSGMII used to be the only one supported, but
other modes such as QUSGMII also carry multiple links.

This helper allows getting the number of links that are multiplexed
on a given interface.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
---
V1->V2 : New patch
V2->V3 : Made PHY_INTERFACE_MODE_INTERNAL 1 port, and added the MAX
case.

 drivers/net/phy/phy-core.c | 52 ++++++++++++++++++++++++++++++++++++++
 include/linux/phy.h        |  2 ++
 2 files changed, 54 insertions(+)

diff --git a/drivers/net/phy/phy-core.c b/drivers/net/phy/phy-core.c
index 1f2531a1a876..f8ec12d3d6ae 100644
--- a/drivers/net/phy/phy-core.c
+++ b/drivers/net/phy/phy-core.c

@@ -74,6 +74,58 @@ const char *phy_duplex_to_str(unsigned int duplex)
 }
 EXPORT_SYMBOL_GPL(phy_duplex_to_str);
 
+/**
+ * phy_interface_num_ports - Return the number of links that can be carried by
+ *			     a given MAC-PHY physical link. Returns 0 if this is
+ *			     unknown, the number of links else.
+ *
+ * @interface: The interface mode we want to get the number of ports
+ */
+int phy_interface_num_ports(phy_interface_t interface)
+{
+	switch (interface) {
+	case PHY_INTERFACE_MODE_NA:
+		return 0;
+	case PHY_INTERFACE_MODE_INTERNAL: