net: phylink: add rxc_always_on flag to phylink_pcs

Some MAC drivers (e.g. stmmac) require a continuous receive clock signal to
be generated by a PCS that is handled by a standalone PCS driver.

Such a PCS driver does not have access to a PHY device, thus cannot check
the PHY_F_RXC_ALWAYS_ON flag. They cannot check max_requires_rxc in the
phylink config either, since it is a private member. Therefore, a new flag
is needed to signal to the PCS that it should keep the RX clock signal up
at all times.
Co-developed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Romain Gantois <romain.gantois@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20240326-rxc_bugfix-v6-2-24a74e5c761f@bootlin.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/phylink.c

@@ -1042,6 +1042,21 @@ static void phylink_pcs_poll_start(struct phylink *pl)
 		mod_timer(&pl->link_poll, jiffies + HZ);
 }
 
+int phylink_pcs_pre_init(struct phylink *pl, struct phylink_pcs *pcs)
+{
+	int ret = 0;
+
+	/* Signal to PCS driver that MAC requires RX clock for init */
+	if (pl->config->mac_requires_rxc)
+		pcs->rxc_always_on = true;
+
+	if (pcs->ops->pcs_pre_init)
+		ret = pcs->ops->pcs_pre_init(pcs);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(phylink_pcs_pre_init);
+
 static void phylink_mac_config(struct phylink *pl,
 			       const struct phylink_link_state *state)
 {

include/linux/phylink.h

@@ -396,6 +396,10 @@ struct phylink_pcs_ops;
  * @phylink: pointer to &struct phylink_config
  * @neg_mode: provide PCS neg mode via "mode" argument
  * @poll: poll the PCS for link changes
+ * @rxc_always_on: The MAC driver requires the reference clock
+ *                 to always be on. Standalone PCS drivers which
+ *                 do not have access to a PHY device can check
+ *                 this instead of PHY_F_RXC_ALWAYS_ON.
  *
  * This structure is designed to be embedded within the PCS private data,
  * and will be passed between phylink and the PCS.
@@ -408,6 +412,7 @@ struct phylink_pcs {
 	struct phylink *phylink;
 	bool neg_mode;
 	bool poll;
+	bool rxc_always_on;
 };
 
 /**
@@ -422,6 +427,8 @@ struct phylink_pcs {
  * @pcs_an_restart: restart 802.3z BaseX autonegotiation.
  * @pcs_link_up: program the PCS for the resolved link configuration
  *               (where necessary).
+ * @pcs_pre_init: configure PCS components necessary for MAC hardware
+ *                initialization e.g. RX clock for stmmac.
  */
 struct phylink_pcs_ops {
 	int (*pcs_validate)(struct phylink_pcs *pcs, unsigned long *supported,
@@ -441,6 +448,7 @@ struct phylink_pcs_ops {
 	void (*pcs_an_restart)(struct phylink_pcs *pcs);
 	void (*pcs_link_up)(struct phylink_pcs *pcs, unsigned int neg_mode,
 			    phy_interface_t interface, int speed, int duplex);
+	int (*pcs_pre_init)(struct phylink_pcs *pcs);
 };
 
 #if 0 /* For kernel-doc purposes only. */
@@ -546,6 +554,34 @@ void pcs_an_restart(struct phylink_pcs *pcs);
  */
 void pcs_link_up(struct phylink_pcs *pcs, unsigned int neg_mode,
 		 phy_interface_t interface, int speed, int duplex);
+
+/**
+ * pcs_pre_init() - Configure PCS components necessary for MAC initialization
+ * @pcs: a pointer to a &struct phylink_pcs.
+ *
+ * This function can be called by MAC drivers through the
+ * phylink_pcs_pre_init() wrapper, before their hardware is initialized. It
+ * should not be called after the link is brought up, as reconfiguring the PCS
+ * at this point could break the link.
+ *
+ * Some MAC devices require specific hardware initialization to be performed by
+ * their associated PCS device before they can properly initialize their own
+ * hardware. An example of this is the initialization of stmmac controllers,
+ * which requires an active REF_CLK signal to be provided by the PHY/PCS.
+ *
+ * By calling phylink_pcs_pre_init(), MAC drivers can ensure that the PCS is
+ * setup in a way that allows for successful hardware initialization.
+ *
+ * The specific configuration performed by pcs_pre_init() is dependent on the
+ * model of PCS and the requirements of the MAC device attached to it. PCS
+ * driver authors should consider whether their target device is to be used in
+ * conjunction with a MAC device whose driver calls phylink_pcs_pre_init(). MAC
+ * driver authors should document their requirements for the PCS
+ * pre-initialization.
+ *
+ */
+int pcs_pre_init(struct phylink_pcs *pcs);
+
 #endif
 
 struct phylink *phylink_create(struct phylink_config *,
@@ -565,6 +601,8 @@ void phylink_disconnect_phy(struct phylink *);
 void phylink_mac_change(struct phylink *, bool up);
 void phylink_pcs_change(struct phylink_pcs *, bool up);
 
+int phylink_pcs_pre_init(struct phylink *pl, struct phylink_pcs *pcs);
+
 void phylink_start(struct phylink *);
 void phylink_stop(struct phylink *);