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

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

[Zxyan Zhu]

Add a pcs_loopback callback to struct phylink_pcs_ops to allow PCS
devices to expose loopback capability. This is useful for MAC drivers
running selftests on interfaces that use in-band signalling and have
no external PHY device.

The phylink_pcs_loopback() helper calls the PCS ops callback if
available, and returns -EOPNOTSUPP for PCS devices that do not
support loopback.

In stmmac, use phylink_pcs_loopback() in stmmac_test_phy_loopback()
and stmmac_selftest_run() when no phydev is present but a PCS with
loopback support exists. This enables PHY loopback selftests on
interfaces without an external PHY.

Signed-off-by: Zxyan Zhu <zxyan0222@gmail.com>
---
 .../stmicro/stmmac/stmmac_selftests.c         | 36 ++++++++++++++-----
 drivers/net/phy/phylink.c                     | 19 ++++++++++
 include/linux/phylink.h                       | 16 +++++++++
 3 files changed, 63 insertions(+), 8 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c
index 29e824bd90ca..e6ac51018a65 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c
@@ -377,20 +377,36 @@ static int stmmac_test_mac_loopback(struct stmmac_priv *priv)
 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 = priv->hw->phylink_pcs;
+	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 +2002,8 @@ void stmmac_selftest_run(struct net_device *dev,
 			ret = -EOPNOTSUPP;
 			if (dev->phydev)
 				ret = phy_loopback(dev->phydev, true, 0);
+			else if (priv->hw->phylink_pcs)
+				ret = phylink_pcs_loopback(priv->hw->phylink_pcs, true);
 			if (!ret)
 				break;
 			fallthrough;
@@ -2019,6 +2037,8 @@ void stmmac_selftest_run(struct net_device *dev,
 			ret = -EOPNOTSUPP;
 			if (dev->phydev)
 				ret = phy_loopback(dev->phydev, false, 0);
+			else if (priv->hw->phylink_pcs)
+				ret = phylink_pcs_loopback(priv->hw->phylink_pcs, false);
 			if (!ret)
 				break;
 			fallthrough;
diff --git a/drivers/net/phy/phylink.c b/drivers/net/phy/phylink.c
index 59dfe35afa54..a7f78c82913a 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->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

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

[Maxime Chevallier]

Hi,

+Björn

On 7/24/26 10:52, Zxyan Zhu wrote:
> Add a pcs_loopback callback to struct phylink_pcs_ops to allow PCS
> devices to expose loopback capability. This is useful for MAC drivers
> running selftests on interfaces that use in-band signalling and have
> no external PHY device.
> 
> The phylink_pcs_loopback() helper calls the PCS ops callback if
> available, and returns -EOPNOTSUPP for PCS devices that do not
> support loopback.
> 
> In stmmac, use phylink_pcs_loopback() in stmmac_test_phy_loopback()
> and stmmac_selftest_run() when no phydev is present but a PCS with
> loopback support exists. This enables PHY loopback selftests on
> interfaces without an external PHY.

There have been talks about making a userspace API for loopback :

https://lore.kernel.org/netdev/20260325145022.2607545-4-bjorn@kernel.org/

Even if your patch is a pure internal feature with no uAPI parts, I wonder
if this is the correct API to have.

Maybe the right move would be to ask phylink to configure the loopback,
and let it figure-out where to do so (in the PHY ? in the PCS ? Somewhere
else ?)

With Björn's work we may end-up with an phylink api that may look like:

phylink_set_loopback(struct phylink *pl, bool enable, enum loopback_location loc)

and for cases like this one where we don't care about where the loopback
is set, we may have an enum value 'LOOPBACK_LOC_ANY' ?

Besides that, I don't see and PCS that supports that loopback, I think
it would be nice to also have the PCS code for loopback as well...

Maxime
> 
> Signed-off-by: Zxyan Zhu <zxyan0222@gmail.com>
> ---
>  .../stmicro/stmmac/stmmac_selftests.c         | 36 ++++++++++++++-----
>  drivers/net/phy/phylink.c                     | 19 ++++++++++
>  include/linux/phylink.h                       | 16 +++++++++
>  3 files changed, 63 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c
> index 29e824bd90ca..e6ac51018a65 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c
> @@ -377,20 +377,36 @@ static int stmmac_test_mac_loopback(struct stmmac_priv *priv)
>  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 = priv->hw->phylink_pcs;
> +	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 +2002,8 @@ void stmmac_selftest_run(struct net_device *dev,
>  			ret = -EOPNOTSUPP;
>  			if (dev->phydev)
>  				ret = phy_loopback(dev->phydev, true, 0);
> +			else if (priv->hw->phylink_pcs)
> +				ret = phylink_pcs_loopback(priv->hw->phylink_pcs, true);
>  			if (!ret)
>  				break;
>  			fallthrough;
> @@ -2019,6 +2037,8 @@ void stmmac_selftest_run(struct net_device *dev,
>  			ret = -EOPNOTSUPP;
>  			if (dev->phydev)
>  				ret = phy_loopback(dev->phydev, false, 0);
> +			else if (priv->hw->phylink_pcs)
> +				ret = phylink_pcs_loopback(priv->hw->phylink_pcs, false);
>  			if (!ret)
>  				break;
>  			fallthrough;
> diff --git a/drivers/net/phy/phylink.c b/drivers/net/phy/phylink.c
> index 59dfe35afa54..a7f78c82913a 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->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

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

[Andrew Lunn]

> Besides that, I don't see and PCS that supports that loopback, I think
> it would be nice to also have the PCS code for loopback as well...

That is pretty much required for merging. We want to see both sides of
a new API.

There has also been discussion about in which direction should it
loopback. Towards the media or towards the host. For self test you
want host side, but as an exposed feature for user space to enable,
media side could also be interesting. So you should think about what
the internal API looks like to be able to specify the loop direction.

    Andrew

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

[Zxyan Zhu]

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(-)

Changes in 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.

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

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

[Zxyan Zhu]

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