usb: typec: Add helper to check cable altmode support

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>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://patch.msgid.link/20260626142702.1941182-2-akuchynski@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 72 insertions, 0 deletions

diff --git a/drivers/usb/typec/class.c b/drivers/usb/typec/class.c
index 0977581ad1b6..e9f34eb14cef 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 d61ec38216fa..10a783b738ef 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);