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

[Benson Leung <bleung@google.com>]

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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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