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.

USB Type-C Connector Class 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 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 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.

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);

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 typec_altmode_get_plug(), and take over their control.

Driver API

Alternate mode driver registering/unregistering

Alternate mode driver operations

int typec_altmode_notify(struct typec_altmode *adev, unsigned long conf, void *data)

Communication between the OS and alternate mode driver

Parameters

struct typec_altmode * adev

Handle to the alternate mode

unsigned long conf

Alternate mode specific configuration value

void * data

Alternate mode specific data

Description

The primary purpose for this function is to allow the alternate mode drivers to tell which pin configuration has been negotiated with the partner. That information will then be used for example to configure the muxes. Communication to the other direction is also possible, and low level device drivers can also send notifications to the alternate mode drivers. The actual communication will be specific for every SVID.

int typec_altmode_enter(struct typec_altmode *adev, u32 *vdo)

Enter Mode

Parameters

struct typec_altmode * adev

The alternate mode

u32 * vdo

VDO for the Enter Mode command

Description

The alternate mode drivers use this function to enter mode. The port drivers use this to inform the alternate mode drivers that the partner has initiated Enter Mode command. If the alternate mode does not require VDO, vdo must be NULL.

int typec_altmode_exit(struct typec_altmode *adev)

Exit Mode

Parameters

struct typec_altmode * adev

The alternate mode

Description

The partner of adev has initiated Exit Mode command.

void typec_altmode_attention(struct typec_altmode *adev, u32 vdo)

Attention command

Parameters

struct typec_altmode * adev

The alternate mode

u32 vdo

VDO for the Attention command

Description

Notifies the partner of adev about Attention command.

int typec_altmode_vdm(struct typec_altmode *adev, const u32 header, const u32 *vdo, int count)

Send Vendor Defined Messages (VDM) to the partner

Parameters

struct typec_altmode * adev

Alternate mode handle

const u32 header

VDM Header

const u32 * vdo

Array of Vendor Defined Data Objects

int count

Number of Data Objects

Description

The alternate mode drivers use this function for SVID specific communication with the partner. The port drivers use it to deliver the Structured VDMs received from the partners to the alternate mode drivers.

API for the port drivers

struct typec_altmode *typec_match_altmode(struct typec_altmode **altmodes, size_t n, u16 svid, u8 mode)

Match SVID and mode to an array of alternate modes

Parameters

struct typec_altmode ** altmodes

Array of alternate modes

size_t n

Number of elements in the array, or -1 for NULL terminated arrays

u16 svid

Standard or Vendor ID to match with

u8 mode

Mode to match with

Description

Return pointer to an alternate mode with SVID matching svid, or NULL when no match is found.

Cable Plug operations

struct typec_altmode *typec_altmode_get_plug(struct typec_altmode *adev, enum typec_plug_index index)

Find cable plug alternate mode

Parameters

struct typec_altmode * adev

Handle to partner alternate mode

enum typec_plug_index index

Cable plug index

Description

Increment reference count for cable plug alternate mode device. Returns handle to the cable plug alternate mode, or NULL if none is found.

void typec_altmode_put_plug(struct typec_altmode *plug)

Decrement cable plug alternate mode reference count

Parameters

struct typec_altmode * plug

Handle to the cable plug alternate mode