[PATCH net-next v2] net: phylink: add phylink_pcs_loopback() method for PCS loopback support

[Zxyan Zhu <zxyan0222@gmail.com>]

Some Ethernet MAC drivers (such as stmmac) need to perform PCS-level
loopback for selftest purposes when no external PHY is present. Add a
new phylink_pcs_loopback() method that allows MAC drivers to request
PCS loopback mode through the phylink framework.

Implement the pcs_loopback callback in the stmmac integrated PCS
using the GMAC_AN_CTRL_ELE register bit. Add a helper function
stmmac_selftest_get_pcs() in stmmac selftests to unify PCS lookup
across glue driver PCS and integrated PCS. Update the selftest
enable/disable paths to use the new helper.

Signed-off-by: Zxyan Zhu <zxyan0222@gmail.com>
---
 .../net/ethernet/stmicro/stmmac/stmmac_pcs.c  | 17 +++++++
 .../stmicro/stmmac/stmmac_selftests.c         | 51 ++++++++++++++++---
 drivers/net/phy/phylink.c                     | 19 +++++++
 include/linux/phylink.h                       | 16 ++++++
 4 files changed, 95 insertions(+), 8 deletions(-)

v2:
  - Implement pcs_loopback callback in stmmac integrated PCS using
    GMAC_AN_CTRL_ELE register bit.
  - Add stmmac_selftest_get_pcs() helper to unify PCS lookup across glue
    driver PCS and integrated PCS.
  - Add NULL check in phylink_pcs_loopback() wrapper.
v1: https://lore.kernel.org/netdev/20260724085224.3321663-1-zxyan0222@gmail.com/

Note on loopback direction:
The pcs_loopback callback uses a simple enable/disable boolean, matching
the existing phy_loopback() API. Direction is not needed at this level:
selftest loopback is always host-side (TX looped back to RX). The
future ethtool user-space loopback API can handle direction selection
at a higher layer.

Note on phylink_set_loopback() vs phylink_pcs_loopback():
This patch follows the same pattern as phy_loopback() - a direct
callback on the PCS object. Keeping the API minimal avoids introducing
a new phylink-level loopback abstraction with location enumeration
before Björn's ethtool loopback series lands upstream. Once that
series is merged, phylink_pcs_loopback() can serve as a building block
for a higher-level phylink_set_loopback() if needed.

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_pcs.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_pcs.c
index df37af5ab837..90a21a2f7e87 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_pcs.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_pcs.c
@@ -168,6 +168,22 @@ static void dwmac_integrated_pcs_an_restart(struct phylink_pcs *pcs)
 	}
 }
 
+static int dwmac_integrated_pcs_loopback(struct phylink_pcs *pcs, bool enable)
+{
+	struct stmmac_pcs *spcs = phylink_pcs_to_stmmac_pcs(pcs);
+	void __iomem *an_control = spcs->base + GMAC_AN_CTRL(0);
+	u32 ctrl;
+
+	ctrl = readl(an_control);
+	if (enable)
+		ctrl |= GMAC_AN_CTRL_ELE;
+	else
+		ctrl &= ~GMAC_AN_CTRL_ELE;
+	writel(ctrl, an_control);
+
+	return 0;
+}
+
 static const struct phylink_pcs_ops dwmac_integrated_pcs_ops = {
 	.pcs_inband_caps = dwmac_integrated_pcs_inband_caps,
 	.pcs_enable = dwmac_integrated_pcs_enable,
@@ -175,6 +191,7 @@ static const struct phylink_pcs_ops dwmac_integrated_pcs_ops = {
 	.pcs_get_state = dwmac_integrated_pcs_get_state,
 	.pcs_config = dwmac_integrated_pcs_config,
 	.pcs_an_restart = dwmac_integrated_pcs_an_restart,
+	.pcs_loopback = dwmac_integrated_pcs_loopback,
 };
 
 void stmmac_integrated_pcs_irq(struct stmmac_priv *priv, u32 status,
diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c
index 29e824bd90ca..77a457b2fa5c 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c
@@ -19,6 +19,7 @@
 #include <net/udp.h>
 #include <net/tc_act/tc_gact.h>
 #include "stmmac.h"
+#include "stmmac_pcs.h"
 
 struct stmmachdr {
 	__be32 version;
@@ -374,23 +375,51 @@ static int stmmac_test_mac_loopback(struct stmmac_priv *priv)
 	return __stmmac_test_loopback(priv, &attr);
 }
 
+static struct phylink_pcs *stmmac_selftest_get_pcs(struct stmmac_priv *priv)
+{
+	/* Try glue driver's PCS first, then the integrated PCS. */
+	if (priv->hw->phylink_pcs)
+		return priv->hw->phylink_pcs;
+
+	if (priv->integrated_pcs)
+		return &priv->integrated_pcs->pcs;
+
+	return NULL;
+}
+
 static int stmmac_test_phy_loopback(struct stmmac_priv *priv)
 {
 	struct stmmac_packet_attrs attr = { };
+	struct phylink_pcs *pcs;
 	int ret;
 
-	if (!priv->dev->phydev)
-		return -EOPNOTSUPP;
+	if (priv->dev->phydev) {
+		ret = phy_loopback(priv->dev->phydev, true, 0);
+		if (ret)
+			return ret;
 
-	ret = phy_loopback(priv->dev->phydev, true, 0);
-	if (ret)
+		attr.dst = priv->dev->dev_addr;
+		ret = __stmmac_test_loopback(priv, &attr);
+
+		phy_loopback(priv->dev->phydev, false, 0);
 		return ret;
+	}
 
-	attr.dst = priv->dev->dev_addr;
-	ret = __stmmac_test_loopback(priv, &attr);
+	/* Use PCS loopback for interfaces without an external PHY. */
+	pcs = stmmac_selftest_get_pcs(priv);
+	if (pcs) {
+		ret = phylink_pcs_loopback(pcs, true);
+		if (ret)
+			return ret;
 
-	phy_loopback(priv->dev->phydev, false, 0);
-	return ret;
+		attr.dst = priv->dev->dev_addr;
+		ret = __stmmac_test_loopback(priv, &attr);
+
+		phylink_pcs_loopback(pcs, false);
+		return ret;
+	}
+
+	return -EOPNOTSUPP;
 }
 
 static int stmmac_test_mmc(struct stmmac_priv *priv)
@@ -1986,6 +2015,9 @@ void stmmac_selftest_run(struct net_device *dev,
 			ret = -EOPNOTSUPP;
 			if (dev->phydev)
 				ret = phy_loopback(dev->phydev, true, 0);
+			else
+				ret = phylink_pcs_loopback(stmmac_selftest_get_pcs(priv),
+							   true);
 			if (!ret)
 				break;
 			fallthrough;
@@ -2019,6 +2051,9 @@ void stmmac_selftest_run(struct net_device *dev,
 			ret = -EOPNOTSUPP;
 			if (dev->phydev)
 				ret = phy_loopback(dev->phydev, false, 0);
+			else
+				ret = phylink_pcs_loopback(stmmac_selftest_get_pcs(priv),
+							   false);
 			if (!ret)
 				break;
 			fallthrough;
diff --git a/drivers/net/phy/phylink.c b/drivers/net/phy/phylink.c
index 59dfe35afa54..69242caffaef 100644
--- a/drivers/net/phy/phylink.c
+++ b/drivers/net/phy/phylink.c
@@ -997,6 +997,25 @@ int phylink_pcs_pre_init(struct phylink *pl, struct phylink_pcs *pcs)
 }
 EXPORT_SYMBOL_GPL(phylink_pcs_pre_init);
 
+/**
+ * phylink_pcs_loopback() - Enable or disable loopback at the PCS
+ * @pcs: a pointer to a &struct phylink_pcs.
+ * @enable: true to enable loopback, false to disable
+ *
+ * Enable or disable loopback mode at the PCS level. This is used by MAC
+ * drivers for selftest purposes when no external PHY is present.
+ *
+ * Returns 0 on success, negative error code on failure.
+ */
+int phylink_pcs_loopback(struct phylink_pcs *pcs, bool enable)
+{
+	if (pcs && pcs->ops->pcs_loopback)
+		return pcs->ops->pcs_loopback(pcs, enable);
+
+	return -EOPNOTSUPP;
+}
+EXPORT_SYMBOL_GPL(phylink_pcs_loopback);
+
 static void phylink_mac_config(struct phylink *pl,
 			       const struct phylink_link_state *state)
 {
diff --git a/include/linux/phylink.h b/include/linux/phylink.h
index 2bc0db3d52ac..66c7016d4acd 100644
--- a/include/linux/phylink.h
+++ b/include/linux/phylink.h
@@ -518,6 +518,7 @@ struct phylink_pcs {
  *		    the MAC.
  * @pcs_pre_init: configure PCS components necessary for MAC hardware
  *                initialization e.g. RX clock for stmmac.
+ * @pcs_loopback: enable/disable loopback mode at the PCS.
  */
 struct phylink_pcs_ops {
 	int (*pcs_validate)(struct phylink_pcs *pcs, unsigned long *supported,
@@ -542,6 +543,7 @@ struct phylink_pcs_ops {
 	void (*pcs_disable_eee)(struct phylink_pcs *pcs);
 	void (*pcs_enable_eee)(struct phylink_pcs *pcs);
 	int (*pcs_pre_init)(struct phylink_pcs *pcs);
+	int (*pcs_loopback)(struct phylink_pcs *pcs, bool enable);
 };
 
 #if 0 /* For kernel-doc purposes only. */
@@ -717,8 +719,22 @@ void pcs_enable_eee(struct phylink_pcs *pcs);
  */
 int pcs_pre_init(struct phylink_pcs *pcs);
 
+/**
+ * pcs_loopback() - Enable or disable loopback at the PCS
+ * @pcs: a pointer to a &struct phylink_pcs.
+ * @enable: true to enable loopback, false to disable
+ *
+ * Enable or disable loopback mode at the PCS level. This is used by MAC
+ * drivers for selftest purposes when no external PHY is present.
+ *
+ * Returns 0 on success, or a negative error code on failure.
+ */
+int pcs_loopback(struct phylink_pcs *pcs, bool enable);
+
 #endif
 
+int phylink_pcs_loopback(struct phylink_pcs *pcs, bool enable);
+
 struct phylink *phylink_create(struct phylink_config *,
 			       const struct fwnode_handle *,
 			       phy_interface_t,

base-commit: 1df10cef2d1e7f9f2fb7eddb67fc70d3abf101f9
-- 
2.34.1