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

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

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