[PATCH v3 0/3] Restrict alternate modes based on cable capabilities

[PATCH v3 0/3] Restrict alternate modes based on cable capabilities

[Andrei Kuchynski]

Currently, alternate mode drivers (like DisplayPort and Thunderbolt)
automatically assign operational callbacks (`typec_altmode_set_ops`)
during probing, even if the underlying cable capabilities cannot support
them. This can lead to incorrectly advertised state configurations or
initialization failures later on.

The first patch introduces the core helper function, and the subsequent
patches update the DisplayPort and Thunderbolt altmode drivers to
utilize this check during their respective probe phases.

Changes in v3:
- Extracted core verification logic into a static helper function
- Added an enum to handle supported, unsupported, and unknown cable states
- Patches 2 and 3 remain unchanged from the previous version.

Changes in v2:
- Updated patch 1/3 to use a switch-case statement, added detailed inline
  comments, and separated the non-e-marked cable validation logic.
- Patches 2 and 3 remain unchanged from the previous version.

Andrei Kuchynski (3):
  usb: typec: Add helper to check cable altmode support
  usb: typec: thunderbolt: Check cable altmode support
  usb: typec: displayport: Check cable altmode support

 drivers/usb/typec/altmodes/displayport.c |  5 +-
 drivers/usb/typec/altmodes/thunderbolt.c |  4 ++
 drivers/usb/typec/class.c                | 71 ++++++++++++++++++++++++
 include/linux/usb/typec.h                |  1 +
 4 files changed, 80 insertions(+), 1 deletion(-)

-- 
2.55.0.rc0.799.gd6f94ed593-goog

[PATCH v3 1/3] usb: typec: Add helper to check cable altmode support

[Andrei Kuchynski]

Introduce typec_cable_altmode_unsupported function to evaluate whether an
alternate mode is restricted based on the connected cable's properties.

Implement validation logic that parses the cable's identity to catch
incompatible setups early. Alternate modes are restricted over:
 - cables lacking an identity header
 - passive cables with USB 2.0 speed
 - active cables unless they have corresponding plugs

The function returns false if the cable is not registered or the identifier
is not set.

Suggested-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Andrei Kuchynski <akuchynski@chromium.org>
---
Changes in V3:
- Extracted core verification logic into a static helper function
- Added an enum to handle supported, unsupported, and unknown cable states

 drivers/usb/typec/class.c | 71 +++++++++++++++++++++++++++++++++++++++
 include/linux/usb/typec.h |  1 +
 2 files changed, 72 insertions(+)

diff --git a/drivers/usb/typec/class.c b/drivers/usb/typec/class.c
index 0977581ad1b6e..e9f34eb14ceff 100644
--- a/drivers/usb/typec/class.c
+++ b/drivers/usb/typec/class.c
@@ -1429,6 +1429,77 @@ int typec_cable_is_active(struct typec_cable *cable)
 }
 EXPORT_SYMBOL_GPL(typec_cable_is_active);
 
+enum typec_cable_altmode_support {
+	CABLE_SUPPORT_UNKNOWN,
+	CABLE_SUPPORTED,
+	CABLE_NOT_SUPPORTED,
+};
+
+static enum typec_cable_altmode_support
+typec_cable_check_altmode_support(struct typec_cable *cable,
+				  struct typec_altmode *alt)
+{
+	struct typec_altmode *plug;
+	u32 speed;
+
+	/*
+	 * Check if the cable has an e-marker, supports modal operation, and the
+	 * SOP' altmode nodes are created.
+	 */
+	plug = typec_altmode_get_plug(alt, TYPEC_PLUG_SOP_P);
+	if (plug) {
+		typec_altmode_put_plug(plug);
+		return CABLE_SUPPORTED;
+	}
+
+	/* The identity is not specified */
+	if (!cable->identity)
+		return CABLE_SUPPORT_UNKNOWN;
+
+	/* Non-e-marked cable */
+	if (!cable->identity->id_header)
+		return CABLE_NOT_SUPPORTED;
+
+	switch (PD_IDH_PTYPE(cable->identity->id_header)) {
+	case IDH_PTYPE_PCABLE:
+		speed = VDO_TYPEC_CABLE_SPEED(cable->identity->vdo[0]);
+		if (speed == CABLE_USB2_ONLY)
+			return CABLE_NOT_SUPPORTED;
+		return CABLE_SUPPORTED;
+	case IDH_PTYPE_ACABLE:
+		/*
+		 * Active cables must establish an SOP' communication
+		 * node. Since that check failed at the beginning of
+		 * this function, this active cable does not support
+		 * this specific altmode.
+		 */
+		return CABLE_NOT_SUPPORTED;
+	}
+
+	return CABLE_SUPPORT_UNKNOWN;
+}
+
+/**
+ * typec_cable_altmode_unsupported - Check if a cable restricts altmode
+ * @alt: The Alternate Mode to evaluate
+ *
+ * Returns true if the connected cable is incapable of handling the altmode.
+ */
+bool typec_cable_altmode_unsupported(struct typec_altmode *alt)
+{
+	enum typec_cable_altmode_support support = CABLE_SUPPORT_UNKNOWN;
+	struct typec_cable *cable;
+
+	cable = typec_cable_get(typec_altmode2port(alt));
+	if (cable) {
+		support = typec_cable_check_altmode_support(cable, alt);
+		typec_cable_put(cable);
+	}
+
+	return support == CABLE_NOT_SUPPORTED;
+}
+EXPORT_SYMBOL_GPL(typec_cable_altmode_unsupported);
+
 /**
  * typec_cable_set_identity - Report result from Discover Identity command
  * @cable: The cable updated identity values
diff --git a/include/linux/usb/typec.h b/include/linux/usb/typec.h
index d61ec38216fa9..10a783b738efd 100644
--- a/include/linux/usb/typec.h
+++ b/include/linux/usb/typec.h
@@ -337,6 +337,7 @@ void typec_unregister_cable(struct typec_cable *cable);
 struct typec_cable *typec_cable_get(struct typec_port *port);
 void typec_cable_put(struct typec_cable *cable);
 int typec_cable_is_active(struct typec_cable *cable);
+bool typec_cable_altmode_unsupported(struct typec_altmode *alt);
 
 struct typec_plug *typec_register_plug(struct typec_cable *cable,
 				       struct typec_plug_desc *desc);
-- 
2.55.0.rc0.799.gd6f94ed593-goog

[PATCH v3 2/3] usb: typec: thunderbolt: Check cable altmode support

[Andrei Kuchynski]

Update the probe function to utilize the new
typec_cable_altmode_unsupported() helper. If the cable doesn't support
Thunderbolt altmode, don't initialize altmode_ops and prevent altmode
from being activated.

Signed-off-by: Andrei Kuchynski <akuchynski@chromium.org>
Reviewed-by: Benson Leung <bleung@chromium.org>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
---
 drivers/usb/typec/altmodes/thunderbolt.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/usb/typec/altmodes/thunderbolt.c b/drivers/usb/typec/altmodes/thunderbolt.c
index 32250b94262a9..2eccdddf1b1f4 100644
--- a/drivers/usb/typec/altmodes/thunderbolt.c
+++ b/drivers/usb/typec/altmodes/thunderbolt.c
@@ -284,6 +284,10 @@ static int tbt_altmode_probe(struct typec_altmode *alt)
 
 	alt->desc = "Thunderbolt3";
 	typec_altmode_set_drvdata(alt, tbt);
+
+	if (typec_cable_altmode_unsupported(alt))
+		return 0;
+
 	typec_altmode_set_ops(alt, &tbt_altmode_ops);
 
 	if (!alt->mode_selection && tbt_ready(alt)) {
-- 
2.55.0.rc0.799.gd6f94ed593-goog

[PATCH v3 3/3] usb: typec: displayport: Check cable altmode support

[Andrei Kuchynski]

Update the probe function to utilize the new
typec_cable_altmode_unsupported() helper. If the cable doesn't support
DisplayPort altmode, don't initialize altmode_ops and prevent altmode
from being activated.
A captive cable shouldn't be checked; it may not provide discoverable
capability information, but it is inherently designed to support the
device's requirements.

Signed-off-by: Andrei Kuchynski <akuchynski@chromium.org>
Reviewed-by: Benson Leung <bleung@chromium.org>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
---
 drivers/usb/typec/altmodes/displayport.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/typec/altmodes/displayport.c b/drivers/usb/typec/altmodes/displayport.c
index 263a89c5f3243..5ee33a69b9cf7 100644
--- a/drivers/usb/typec/altmodes/displayport.c
+++ b/drivers/usb/typec/altmodes/displayport.c
@@ -790,7 +790,6 @@ int dp_altmode_probe(struct typec_altmode *alt)
 	dp->alt = alt;
 
 	alt->desc = "DisplayPort";
-	typec_altmode_set_ops(alt, &dp_altmode_ops);
 
 	if (plug) {
 		plug->desc = "Displayport";
@@ -811,6 +810,10 @@ int dp_altmode_probe(struct typec_altmode *alt)
 	if (plug)
 		typec_altmode_set_drvdata(plug, dp);
 
+	if ((alt->vdo & DP_CAP_RECEPTACLE) && typec_cable_altmode_unsupported(alt))
+		return 0;
+
+	typec_altmode_set_ops(alt, &dp_altmode_ops);
 	if (!alt->mode_selection) {
 		dp->state = plug ? DP_STATE_ENTER_PRIME : DP_STATE_ENTER;
 		schedule_work(&dp->work);
-- 
2.55.0.rc0.799.gd6f94ed593-goog

Re: [PATCH v3 1/3] usb: typec: Add helper to check cable altmode support

[Benson Leung]

[-- Attachment #1: Type: text/plain, Size: 4378 bytes --]

On Fri, Jun 26, 2026 at 02:27:00PM +0000, Andrei Kuchynski wrote:
> Introduce typec_cable_altmode_unsupported function to evaluate whether an
> alternate mode is restricted based on the connected cable's properties.
> 
> Implement validation logic that parses the cable's identity to catch
> incompatible setups early. Alternate modes are restricted over:
>  - cables lacking an identity header
>  - passive cables with USB 2.0 speed
>  - active cables unless they have corresponding plugs
> 
> The function returns false if the cable is not registered or the identifier
> is not set.
> 
> Suggested-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> Signed-off-by: Andrei Kuchynski <akuchynski@chromium.org>

Reviewed-by: Benson Leung <bleung@chromium.org>


> ---
> Changes in V3:
> - Extracted core verification logic into a static helper function
> - Added an enum to handle supported, unsupported, and unknown cable states
> 
>  drivers/usb/typec/class.c | 71 +++++++++++++++++++++++++++++++++++++++
>  include/linux/usb/typec.h |  1 +
>  2 files changed, 72 insertions(+)
> 
> diff --git a/drivers/usb/typec/class.c b/drivers/usb/typec/class.c
> index 0977581ad1b6e..e9f34eb14ceff 100644
> --- a/drivers/usb/typec/class.c
> +++ b/drivers/usb/typec/class.c
> @@ -1429,6 +1429,77 @@ int typec_cable_is_active(struct typec_cable *cable)
>  }
>  EXPORT_SYMBOL_GPL(typec_cable_is_active);
>  
> +enum typec_cable_altmode_support {
> +	CABLE_SUPPORT_UNKNOWN,
> +	CABLE_SUPPORTED,
> +	CABLE_NOT_SUPPORTED,
> +};
> +
> +static enum typec_cable_altmode_support
> +typec_cable_check_altmode_support(struct typec_cable *cable,
> +				  struct typec_altmode *alt)
> +{
> +	struct typec_altmode *plug;
> +	u32 speed;
> +
> +	/*
> +	 * Check if the cable has an e-marker, supports modal operation, and the
> +	 * SOP' altmode nodes are created.
> +	 */
> +	plug = typec_altmode_get_plug(alt, TYPEC_PLUG_SOP_P);
> +	if (plug) {
> +		typec_altmode_put_plug(plug);
> +		return CABLE_SUPPORTED;
> +	}
> +
> +	/* The identity is not specified */
> +	if (!cable->identity)
> +		return CABLE_SUPPORT_UNKNOWN;
> +
> +	/* Non-e-marked cable */
> +	if (!cable->identity->id_header)
> +		return CABLE_NOT_SUPPORTED;
> +
> +	switch (PD_IDH_PTYPE(cable->identity->id_header)) {
> +	case IDH_PTYPE_PCABLE:
> +		speed = VDO_TYPEC_CABLE_SPEED(cable->identity->vdo[0]);
> +		if (speed == CABLE_USB2_ONLY)
> +			return CABLE_NOT_SUPPORTED;
> +		return CABLE_SUPPORTED;
> +	case IDH_PTYPE_ACABLE:
> +		/*
> +		 * Active cables must establish an SOP' communication
> +		 * node. Since that check failed at the beginning of
> +		 * this function, this active cable does not support
> +		 * this specific altmode.
> +		 */
> +		return CABLE_NOT_SUPPORTED;
> +	}
> +
> +	return CABLE_SUPPORT_UNKNOWN;
> +}
> +
> +/**
> + * typec_cable_altmode_unsupported - Check if a cable restricts altmode
> + * @alt: The Alternate Mode to evaluate
> + *
> + * Returns true if the connected cable is incapable of handling the altmode.
> + */
> +bool typec_cable_altmode_unsupported(struct typec_altmode *alt)
> +{
> +	enum typec_cable_altmode_support support = CABLE_SUPPORT_UNKNOWN;
> +	struct typec_cable *cable;
> +
> +	cable = typec_cable_get(typec_altmode2port(alt));
> +	if (cable) {
> +		support = typec_cable_check_altmode_support(cable, alt);
> +		typec_cable_put(cable);
> +	}
> +
> +	return support == CABLE_NOT_SUPPORTED;
> +}
> +EXPORT_SYMBOL_GPL(typec_cable_altmode_unsupported);
> +
>  /**
>   * typec_cable_set_identity - Report result from Discover Identity command
>   * @cable: The cable updated identity values
> diff --git a/include/linux/usb/typec.h b/include/linux/usb/typec.h
> index d61ec38216fa9..10a783b738efd 100644
> --- a/include/linux/usb/typec.h
> +++ b/include/linux/usb/typec.h
> @@ -337,6 +337,7 @@ void typec_unregister_cable(struct typec_cable *cable);
>  struct typec_cable *typec_cable_get(struct typec_port *port);
>  void typec_cable_put(struct typec_cable *cable);
>  int typec_cable_is_active(struct typec_cable *cable);
> +bool typec_cable_altmode_unsupported(struct typec_altmode *alt);
>  
>  struct typec_plug *typec_register_plug(struct typec_cable *cable,
>  				       struct typec_plug_desc *desc);
> -- 
> 2.55.0.rc0.799.gd6f94ed593-goog
> 

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