* Re: [PATCH net-next] net: phylink: add phylink_pcs_loopback() method for PCS loopback support
2026-07-24 8:52 [PATCH net-next] net: phylink: add phylink_pcs_loopback() method for PCS loopback support Zxyan Zhu
@ 2026-07-24 9:25 ` Maxime Chevallier
2026-07-24 13:05 ` Andrew Lunn
2026-07-27 9:38 ` [PATCH net-next v2] " Zxyan Zhu
1 sibling, 1 reply; 4+ messages in thread
From: Maxime Chevallier @ 2026-07-24 9:25 UTC (permalink / raw)
To: Zxyan Zhu, netdev, linux-kernel
Cc: linux-stm32, linux-arm-kernel, Andrew Lunn, David S . Miller,
Eric Dumazet, Jakub Kicinski, Paolo Abeni, Russell King,
Heiner Kallweit, Maxime Coquelin, Alexandre Torgue,
Björn Töpel
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
^ permalink raw reply [flat|nested] 4+ messages in thread* [PATCH net-next v2] net: phylink: add phylink_pcs_loopback() method for PCS loopback support
2026-07-24 8:52 [PATCH net-next] net: phylink: add phylink_pcs_loopback() method for PCS loopback support Zxyan Zhu
2026-07-24 9:25 ` Maxime Chevallier
@ 2026-07-27 9:38 ` Zxyan Zhu
1 sibling, 0 replies; 4+ messages in thread
From: Zxyan Zhu @ 2026-07-27 9:38 UTC (permalink / raw)
To: andrew+netdev
Cc: davem, edumazet, kuba, pabeni, mcoquelin.stm32, alexandre.torgue,
linux, hkallweit1, rmk+kernel, maxime.chevallier, netdev,
linux-stm32, linux-arm-kernel, linux-kernel, bjorn, 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
^ permalink raw reply related [flat|nested] 4+ messages in thread