
API for USB Type-C Alternate Mode drivers
=========================================

Introduction
------------

Alternate modes require communication with the partner using Vendor Defined
Messages (VDM) as defined in USB Type-C and USB Power Delivery Specifications.
The communication is SVID (Standard or Vendor ID) specific, i.e. specific for
every alternate mode, so every alternate mode will need a custom driver.

USB Type-C bus allows binding a driver to the discovered partner alternate
modes by using the SVID and the mode number.

:ref:`USB Type-C Connector Class <typec>` provides a device for every alternate
mode a port supports, and separate device for every alternate mode the partner
supports. The drivers for the alternate modes are bound to the partner alternate
mode devices, and the port alternate mode devices must be handled by the port
drivers.

When a new partner alternate mode device is registered, it is linked to the
alternate mode device of the port that the partner is attached to, that has
matching SVID and mode. Communication between the port driver and alternate mode
driver will happen using the same API.

The port alternate mode devices are used as a proxy between the partner and the
alternate mode drivers, so the port drivers are only expected to pass the SVID
specific commands from the alternate mode drivers to the partner, and from the
partners to the alternate mode drivers. No direct SVID specific communication is
needed from the port drivers, but the port drivers need to provide the operation
callbacks for the port alternate mode devices, just like the alternate mode
drivers need to provide them for the partner alternate mode devices.

Usage:
------

General
~~~~~~~

By default, the alternate mode drivers are responsible for entering the mode.
It is also possible to leave the decision about entering the mode to the user
space (See Documentation/ABI/testing/sysfs-class-typec). Port drivers should not
enter any modes on their own.

``->vdm`` is the most important callback in the operation callbacks vector. It
will be used to deliver all the SVID specific commands from the partner to the
alternate mode driver, and vice versa in case of port drivers. The drivers send
the SVID specific commands to each other using :c:func:`typec_altmode_vdm()`.

If the communication with the partner using the SVID specific commands results
in need to reconfigure the pins on the connector, the alternate mode driver
needs to notify the bus using :c:func:`typec_altmode_notify()`. The driver
passes the negotiated SVID specific pin configuration value to the function as
parameter. The bus driver will then configure the mux behind the connector using
that value as the state value for the mux, and also call blocking notification
chain to notify the external drivers about the state of the connector that need
to know it.

NOTE: The SVID specific pin configuration values must always start from
``TYPEC_STATE_MODAL``. USB Type-C specification defines two default states for
the connector: ``TYPEC_STATE_USB`` and ``TYPEC_STATE_SAFE``. These values are
reserved by the bus as the first possible values for the state. When the
alternate mode is entered, the bus will put the connector into
``TYPEC_STATE_SAFE`` before sending Enter or Exit Mode command as defined in USB
Type-C Specification, and also put the connector back to ``TYPEC_STATE_USB``
after the mode has been exited.

An example of working definitions for SVID specific pin configurations would
look like this::

    enum {
        ALTMODEX_CONF_A = TYPEC_STATE_MODAL,
        ALTMODEX_CONF_B,
        ...
    };

Helper macro ``TYPEC_MODAL_STATE()`` can also be used::

#define ALTMODEX_CONF_A = TYPEC_MODAL_STATE(0);
#define ALTMODEX_CONF_B = TYPEC_MODAL_STATE(1);

Notification chain
~~~~~~~~~~~~~~~~~~

The drivers for the components that the alternate modes are designed for need to
get details regarding the results of the negotiation with the partner, and the
pin configuration of the connector. In case of DisplayPort alternate mode for
example, the GPU drivers will need to know those details. In case of
Thunderbolt alternate mode, the thunderbolt drivers will need to know them, and
so on.

The notification chain is designed for this purpose. The drivers can register
notifiers with :c:func:`typec_altmode_register_notifier()`.

Cable plug alternate modes
~~~~~~~~~~~~~~~~~~~~~~~~~~

The alternate mode drivers are not bound to cable plug alternate mode devices,
only to the partner alternate mode devices. If the alternate mode supports, or
requires, a cable that responds to SOP Prime, and optionally SOP Double Prime
messages, the driver for that alternate mode must request handle to the cable
plug alternate modes using :c:func:`typec_altmode_get_plug()`, and take over
their control.

Driver API
----------

Alternate mode driver registering/unregistering
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/usb/typec/bus.c
   :functions: typec_altmode_register_driver typec_altmode_unregister_driver

Alternate mode driver operations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/usb/typec/bus.c
   :functions: typec_altmode_enter typec_altmode_exit typec_altmode_attention typec_altmode_vdm typec_altmode_notify

API for the port drivers
~~~~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/usb/typec/bus.c
   :functions: typec_match_altmode

Cable Plug operations
~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/usb/typec/bus.c
   :functions: typec_altmode_get_plug typec_altmode_put_plug

Notifications
~~~~~~~~~~~~~
.. kernel-doc:: drivers/usb/typec/class.c
   :functions: typec_altmode_register_notifier typec_altmode_unregister_notifier