Auxiliary Bus

In some subsystems, the functionality of the core device (PCI/ACPI/other) is too complex for a single device to be managed by a monolithic driver (e.g. Sound Open Firmware), multiple devices might implement a common intersection of functionality (e.g. NICs + RDMA), or a driver may want to export an interface for another subsystem to drive (e.g. SIOV Physical Function export Virtual Function management). A split of the functionality into child- devices representing sub-domains of functionality makes it possible to compartmentalize, layer, and distribute domain-specific concerns via a Linux device-driver model.

An example for this kind of requirement is the audio subsystem where a single IP is handling multiple entities such as HDMI, Soundwire, local devices such as mics/speakers etc. The split for the core's functionality can be arbitrary or be defined by the DSP firmware topology and include hooks for test/debug. This allows for the audio core device to be minimal and focused on hardware-specific control and communication.

Each auxiliary_device represents a part of its parent functionality. The generic behavior can be extended and specialized as needed by encapsulating an auxiliary_device within other domain-specific structures and the use of .ops callbacks. Devices on the auxiliary bus do not share any structures and the use of a communication channel with the parent is domain-specific.

Note that ops are intended as a way to augment instance behavior within a class of auxiliary devices, it is not the mechanism for exporting common infrastructure from the parent. Consider EXPORT_SYMBOL_NS() to convey infrastructure from the parent module to the auxiliary module(s).

When Should the Auxiliary Bus Be Used

The auxiliary bus is to be used when a driver and one or more kernel modules, who share a common header file with the driver, need a mechanism to connect and provide access to a shared object allocated by the auxiliary_device's registering driver. The registering driver for the auxiliary_device(s) and the kernel module(s) registering auxiliary_drivers can be from the same subsystem, or from multiple subsystems.

The emphasis here is on a common generic interface that keeps subsystem customization out of the bus infrastructure.

One example is a PCI network device that is RDMA-capable and exports a child device to be driven by an auxiliary_driver in the RDMA subsystem. The PCI driver allocates and registers an auxiliary_device for each physical function on the NIC. The RDMA driver registers an auxiliary_driver that claims each of these auxiliary_devices. This conveys data/ops published by the parent PCI device/driver to the RDMA auxiliary_driver.

Another use case is for the PCI device to be split out into multiple sub functions. For each sub function an auxiliary_device is created. A PCI sub function driver binds to such devices that creates its own one or more class devices. A PCI sub function auxiliary device is likely to be contained in a struct with additional attributes such as user defined sub function number and optional attributes such as resources and a link to the parent device. These attributes could be used by systemd/udev; and hence should be initialized before a driver binds to an auxiliary_device.

A key requirement for utilizing the auxiliary bus is that there is no dependency on a physical bus, device, register accesses or regmap support. These individual devices split from the core cannot live on the platform bus as they are not physical devices that are controlled by DT/ACPI. The same argument applies for not using MFD in this scenario as MFD relies on individual function devices being physical devices.

Auxiliary Device Creation

struct auxiliary_device

auxiliary device object.


struct auxiliary_device {
    struct device dev;
    const char *name;
    u32 id;



Device, The release and parent fields of the device structure must be filled in


Match name found by the auxiliary device driver,


unique identitier if multiple devices of the same name are exported,


An auxiliary_device represents a part of its parent device's functionality. It is given a name that, combined with the registering drivers KBUILD_MODNAME, creates a match_name that is used for driver binding, and an id that combined with the match_name provide a unique name to register with the bus subsystem. For example, a driver registering an auxiliary device is named 'foo_mod.ko' and the subdevice is named 'foo_dev'. The match name is therefore 'foo_mod.foo_dev'.

Registering an auxiliary_device is a three-step process.

First, a 'struct auxiliary_device' needs to be defined or allocated for each sub-device desired. The name, id, dev.release, and dev.parent fields of this structure must be filled in as follows.

The 'name' field is to be given a name that is recognized by the auxiliary driver. If two auxiliary_devices with the same match_name, eg "foo_mod.foo_dev", are registered onto the bus, they must have unique id values (e.g. "x" and "y") so that the registered devices names are "foo_mod.foo_dev.x" and "foo_mod.foo_dev.y". If match_name + id are not unique, then the device_add fails and generates an error message.

The or must be populated with a non-NULL pointer to successfully register the auxiliary_device. This release call is where resources associated with the auxiliary device must be free'ed. Because once the device is placed on the bus the parent driver can not tell what other code may have a reference to this data.

The should be set. Typically to the registering drivers device.

Second, call auxiliary_device_init(), which checks several aspects of the auxiliary_device struct and performs a device_initialize(). After this step completes, any error state must have a call to auxiliary_device_uninit() in its resolution path.

The third and final step in registering an auxiliary_device is to perform a call to auxiliary_device_add(), which sets the name of the device and adds the device to the bus.

#define MY_DEVICE_NAME "foo_dev"


struct auxiliary_device *my_aux_dev = my_aux_dev_alloc(xxx);

// Step 1:
my_aux_dev->name = MY_DEVICE_NAME;
my_aux_dev->id = my_unique_id_alloc(xxx);
my_aux_dev->dev.release = my_aux_dev_release;
my_aux_dev->dev.parent = my_dev;

// Step 2:
if (auxiliary_device_init(my_aux_dev))
        goto fail;

// Step 3:
if (auxiliary_device_add(my_aux_dev)) {
        goto fail;


Unregistering an auxiliary_device is a two-step process to mirror the register process. First call auxiliary_device_delete(), then call auxiliary_device_uninit().

int auxiliary_device_init(struct auxiliary_device *auxdev)

check auxiliary_device and initialize


struct auxiliary_device *auxdev

auxiliary device struct


This is the second step in the three-step process to register an auxiliary_device.

When this function returns an error code, then the device_initialize will not have been performed, and the caller will be responsible to free any memory allocated for the auxiliary_device in the error path directly.

It returns 0 on success. On success, the device_initialize has been performed. After this point any error unwinding will need to include a call to auxiliary_device_uninit(). In this post-initialize error scenario, a call to the device's .release callback will be triggered, and all memory clean-up is expected to be handled there.

int __auxiliary_device_add(struct auxiliary_device *auxdev, const char *modname)

add an auxiliary bus device


struct auxiliary_device *auxdev

auxiliary bus device to add to the bus

const char *modname

name of the parent device's driver module


This is the third step in the three-step process to register an auxiliary_device.

This function must be called after a successful call to auxiliary_device_init(), which will perform the device_initialize. This means that if this returns an error code, then a call to auxiliary_device_uninit() must be performed so that the .release callback will be triggered to free the memory associated with the auxiliary_device.

The expectation is that users will call the "auxiliary_device_add" macro so that the caller's KBUILD_MODNAME is automatically inserted for the modname parameter. Only if a user requires a custom name would this version be called directly.

struct auxiliary_device *auxiliary_find_device(struct device *start, const void *data, int (*match)(struct device *dev, const void *data))

auxiliary device iterator for locating a particular device.


struct device *start

Device to begin with

const void *data

Data to pass to match function

int (*match)(struct device *dev, const void *data)

Callback function to check device


This function returns a reference to a device that is 'found' for later use, as determined by the match callback.

The reference returned should be released with put_device().

The callback should return 0 if the device doesn't match and non-zero if it does. If the callback returns non-zero, this function will return to the caller and not iterate over any more devices.

Auxiliary Device Memory Model and Lifespan

The registering driver is the entity that allocates memory for the auxiliary_device and registers it on the auxiliary bus. It is important to note that, as opposed to the platform bus, the registering driver is wholly responsible for the management of the memory used for the device object.

To be clear the memory for the auxiliary_device is freed in the release() callback defined by the registering driver. The registering driver should only call auxiliary_device_delete() and then auxiliary_device_uninit() when it is done with the device. The release() function is then automatically called if and when other code releases their reference to the devices.

A parent object, defined in the shared header file, contains the auxiliary_device. It also contains a pointer to the shared object(s), which also is defined in the shared header. Both the parent object and the shared object(s) are allocated by the registering driver. This layout allows the auxiliary_driver's registering module to perform a container_of() call to go from the pointer to the auxiliary_device, that is passed during the call to the auxiliary_driver's probe function, up to the parent object, and then have access to the shared object(s).

The memory for the shared object(s) must have a lifespan equal to, or greater than, the lifespan of the memory for the auxiliary_device. The auxiliary_driver should only consider that the shared object is valid as long as the auxiliary_device is still registered on the auxiliary bus. It is up to the registering driver to manage (e.g. free or keep available) the memory for the shared object beyond the life of the auxiliary_device.

The registering driver must unregister all auxiliary devices before its own driver.remove() is completed. An easy way to ensure this is to use the devm_add_action_or_reset() call to register a function against the parent device which unregisters the auxiliary device object(s).

Finally, any operations which operate on the auxiliary devices must continue to function (if only to return an error) after the registering driver unregisters the auxiliary device.

Auxiliary Drivers

struct auxiliary_driver

Definition of an auxiliary bus driver


struct auxiliary_driver {
    int (*probe)(struct auxiliary_device *auxdev, const struct auxiliary_device_id *id);
    void (*remove)(struct auxiliary_device *auxdev);
    void (*shutdown)(struct auxiliary_device *auxdev);
    int (*suspend)(struct auxiliary_device *auxdev, pm_message_t state);
    int (*resume)(struct auxiliary_device *auxdev);
    const char *name;
    struct device_driver driver;
    const struct auxiliary_device_id *id_table;



Called when a matching device is added to the bus.


Called when device is removed from the bus.


Called at shut-down time to quiesce the device.


Called to put the device to sleep mode. Usually to a power state.


Called to bring a device from sleep mode.


Driver name.


Core driver structure.


Table of devices this driver should match on the bus.


Auxiliary drivers follow the standard driver model convention, where discovery/enumeration is handled by the core, and drivers provide probe() and remove() methods. They support power management and shutdown notifications using the standard conventions.

Auxiliary drivers register themselves with the bus by calling auxiliary_driver_register(). The id_table contains the match_names of auxiliary devices that a driver can bind with.

static const struct auxiliary_device_id my_auxiliary_id_table[] = {
        { .name = "foo_mod.foo_dev" },

MODULE_DEVICE_TABLE(auxiliary, my_auxiliary_id_table);

struct auxiliary_driver my_drv = {
        .name = "myauxiliarydrv",
        .id_table = my_auxiliary_id_table,
        .probe = my_drv_probe,
        .remove = my_drv_remove

module_auxiliary_driver (__auxiliary_driver)

Helper macro for registering an auxiliary driver



auxiliary driver struct


Helper macro for auxiliary drivers which do not do anything special in module init/exit. This eliminates a lot of boilerplate. Each module may only use this macro once, and calling it replaces module_init() and module_exit()

int __auxiliary_driver_register(struct auxiliary_driver *auxdrv, struct module *owner, const char *modname)

register a driver for auxiliary bus devices


struct auxiliary_driver *auxdrv

auxiliary_driver structure

struct module *owner

owning module/driver

const char *modname

KBUILD_MODNAME for parent driver


The expectation is that users will call the "auxiliary_driver_register" macro so that the caller's KBUILD_MODNAME is automatically inserted for the modname parameter. Only if a user requires a custom name would this version be called directly.

void auxiliary_driver_unregister(struct auxiliary_driver *auxdrv)

unregister a driver


struct auxiliary_driver *auxdrv

auxiliary_driver structure

Example Usage

Auxiliary devices are created and registered by a subsystem-level core device that needs to break up its functionality into smaller fragments. One way to extend the scope of an auxiliary_device is to encapsulate it within a domain- pecific structure defined by the parent device. This structure contains the auxiliary_device and any associated shared data/callbacks needed to establish the connection with the parent.

An example is:

 struct foo {
      struct auxiliary_device auxdev;
      void (*connect)(struct auxiliary_device *auxdev);
      void (*disconnect)(struct auxiliary_device *auxdev);
      void *data;

The parent device then registers the auxiliary_device by calling auxiliary_device_init(), and then auxiliary_device_add(), with the pointer to the auxdev member of the above structure. The parent provides a name for the auxiliary_device that, combined with the parent's KBUILD_MODNAME, creates a match_name that is be used for matching and binding with a driver.

Whenever an auxiliary_driver is registered, based on the match_name, the auxiliary_driver's probe() is invoked for the matching devices. The auxiliary_driver can also be encapsulated inside custom drivers that make the core device's functionality extensible by adding additional domain-specific ops as follows:

struct my_ops {
        void (*send)(struct auxiliary_device *auxdev);
        void (*receive)(struct auxiliary_device *auxdev);

struct my_driver {
        struct auxiliary_driver auxiliary_drv;
        const struct my_ops ops;

An example of this type of usage is:

const struct auxiliary_device_id my_auxiliary_id_table[] = {
        { .name = "foo_mod.foo_dev" },
        { },

const struct my_ops my_custom_ops = {
        .send = my_tx,
        .receive = my_rx,

const struct my_driver my_drv = {
        .auxiliary_drv = {
                .name = "myauxiliarydrv",
                .id_table = my_auxiliary_id_table,
                .probe = my_probe,
                .remove = my_remove,
                .shutdown = my_shutdown,
        .ops = my_custom_ops,