typec_bus.rst

  1
  2API for USB Type-C Alternate Mode drivers
  3=========================================
  4
  5Introduction
  6------------
  7
  8Alternate modes require communication with the partner using Vendor Defined
  9Messages (VDM) as defined in USB Type-C and USB Power Delivery Specifications.
 10The communication is SVID (Standard or Vendor ID) specific, i.e. specific for
 11every alternate mode, so every alternate mode will need a custom driver.
 12
 13USB Type-C bus allows binding a driver to the discovered partner alternate
 14modes by using the SVID and the mode number.
 15
 16:ref:`USB Type-C Connector Class <typec>` provides a device for every alternate
 17mode a port supports, and separate device for every alternate mode the partner
 18supports. The drivers for the alternate modes are bound to the partner alternate
 19mode devices, and the port alternate mode devices must be handled by the port
 20drivers.
 21
 22When a new partner alternate mode device is registered, it is linked to the
 23alternate mode device of the port that the partner is attached to, that has
 24matching SVID and mode. Communication between the port driver and alternate mode
 25driver will happen using the same API.
 26
 27The port alternate mode devices are used as a proxy between the partner and the
 28alternate mode drivers, so the port drivers are only expected to pass the SVID
 29specific commands from the alternate mode drivers to the partner, and from the
 30partners to the alternate mode drivers. No direct SVID specific communication is
 31needed from the port drivers, but the port drivers need to provide the operation
 32callbacks for the port alternate mode devices, just like the alternate mode
 33drivers need to provide them for the partner alternate mode devices.
 34
 35Usage:
 36------
 37
 38General
 39~~~~~~~
 40
 41By default, the alternate mode drivers are responsible for entering the mode.
 42It is also possible to leave the decision about entering the mode to the user
 43space (See Documentation/ABI/testing/sysfs-class-typec). Port drivers should not
 44enter any modes on their own.
 45
 46``->vdm`` is the most important callback in the operation callbacks vector. It
 47will be used to deliver all the SVID specific commands from the partner to the
 48alternate mode driver, and vice versa in case of port drivers. The drivers send
 49the SVID specific commands to each other using :c:func:`typec_altmode_vdm()`.
 50
 51If the communication with the partner using the SVID specific commands results
 52in need to reconfigure the pins on the connector, the alternate mode driver
 53needs to notify the bus using :c:func:`typec_altmode_notify()`. The driver
 54passes the negotiated SVID specific pin configuration value to the function as
 55parameter. The bus driver will then configure the mux behind the connector using
 56that value as the state value for the mux.
 57
 58NOTE: The SVID specific pin configuration values must always start from
 59``TYPEC_STATE_MODAL``. USB Type-C specification defines two default states for
 60the connector: ``TYPEC_STATE_USB`` and ``TYPEC_STATE_SAFE``. These values are
 61reserved by the bus as the first possible values for the state. When the
 62alternate mode is entered, the bus will put the connector into
 63``TYPEC_STATE_SAFE`` before sending Enter or Exit Mode command as defined in USB
 64Type-C Specification, and also put the connector back to ``TYPEC_STATE_USB``
 65after the mode has been exited.
 66
 67An example of working definitions for SVID specific pin configurations would
 68look like this::
 69
 70    enum {
 71        ALTMODEX_CONF_A = TYPEC_STATE_MODAL,
 72        ALTMODEX_CONF_B,
 73        ...
 74    };
 75
 76Helper macro ``TYPEC_MODAL_STATE()`` can also be used::
 77
 78#define ALTMODEX_CONF_A = TYPEC_MODAL_STATE(0);
 79#define ALTMODEX_CONF_B = TYPEC_MODAL_STATE(1);
 80
 81Cable plug alternate modes
 82~~~~~~~~~~~~~~~~~~~~~~~~~~
 83
 84The alternate mode drivers are not bound to cable plug alternate mode devices,
 85only to the partner alternate mode devices. If the alternate mode supports, or
 86requires, a cable that responds to SOP Prime, and optionally SOP Double Prime
 87messages, the driver for that alternate mode must request handle to the cable
 88plug alternate modes using :c:func:`typec_altmode_get_plug()`, and take over
 89their control.
 90
 91Driver API
 92----------
 93
 94Alternate mode structs
 95~~~~~~~~~~~~~~~~~~~~~~
 96
 97.. kernel-doc:: include/linux/usb/typec_altmode.h
 98   :functions: typec_altmode_driver typec_altmode_ops
 99
100Alternate mode driver registering/unregistering
101~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
102
103.. kernel-doc:: include/linux/usb/typec_altmode.h
104   :functions: typec_altmode_register_driver typec_altmode_unregister_driver
105
106Alternate mode driver operations
107~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
108
109.. kernel-doc:: drivers/usb/typec/bus.c
110   :functions: typec_altmode_enter typec_altmode_exit typec_altmode_attention typec_altmode_vdm typec_altmode_notify
111
112API for the port drivers
113~~~~~~~~~~~~~~~~~~~~~~~~
114
115.. kernel-doc:: drivers/usb/typec/bus.c
116   :functions: typec_match_altmode
117
118Cable Plug operations
119~~~~~~~~~~~~~~~~~~~~~
120
121.. kernel-doc:: drivers/usb/typec/bus.c
122   :functions: typec_altmode_get_plug typec_altmode_put_plug