Subject: Re: [PATCH v2 1/6] Add ancillary bus support

From: Leon Romanovsky <[email-masked]>
Date: Tue, 6 Oct 2020 10:18:21 +0300

On Mon, Oct 05, 2020 at 11:24:41AM -0700, Dave Ertman wrote:
> Add support for the Ancillary Bus, ancillary_device and ancillary_driver.
> It enables drivers to create an ancillary_device and bind an
> ancillary_driver to it.

I was under impression that this name is going to be changed.

>
> The bus supports probe/remove shutdown and suspend/resume callbacks.
> Each ancillary_device has a unique string based id; driver binds to
> an ancillary_device based on this id through the bus.
>
> Co-developed-by: Kiran Patil <[email-masked]>
> Signed-off-by: Kiran Patil <[email-masked]>
> Co-developed-by: Ranjani Sridharan <[email-masked]>
> Signed-off-by: Ranjani Sridharan <[email-masked]>
> Co-developed-by: Fred Oh <[email-masked]>
> Signed-off-by: Fred Oh <[email-masked]>
> Reviewed-by: Pierre-Louis Bossart <[email-masked]>
> Reviewed-by: Shiraz Saleem <[email-masked]>
> Reviewed-by: Parav Pandit <[email-masked]>
> Reviewed-by: Dan Williams <[email-masked]>
> Signed-off-by: Dave Ertman <[email-masked]>
> ---
>  Documentation/driver-api/ancillary_bus.rst | 229 +++++++++++++++++++++
>  Documentation/driver-api/index.rst         |   1 +
>  drivers/bus/Kconfig                        |   3 +
>  drivers/bus/Makefile                       |   3 +
>  drivers/bus/ancillary.c                    | 225 ++++++++++++++++++++
>  include/linux/ancillary_bus.h              |  69 +++++++
>  include/linux/mod_devicetable.h            |   8 +
>  scripts/mod/devicetable-offsets.c          |   3 +
>  scripts/mod/file2alias.c                   |   8 +
>  9 files changed, 549 insertions(+)
>  create mode 100644 Documentation/driver-api/ancillary_bus.rst
>  create mode 100644 drivers/bus/ancillary.c
>  create mode 100644 include/linux/ancillary_bus.h
>
> diff --git a/Documentation/driver-api/ancillary_bus.rst b/Documentation/driver-api/ancillary_bus.rst
> new file mode 100644
> index 000000000000..66f986e8672f
> --- /dev/null
> +++ b/Documentation/driver-api/ancillary_bus.rst
> @@ -0,0 +1,229 @@
> +.. SPDX-License-Identifier: GPL-2.0-only
> +
> +=============
> +Ancillary Bus
> +=============
> +
> +In some subsystems, the functionality of the core device (PCI/ACPI/other) is
> +too complex for a single device to be managed as a monolithic block or a part of
> +the functionality needs to be exposed to a different subsystem.  Splitting the
> +functionality into smaller orthogonal devices would make it easier to manage
> +data, power management and domain-specific interaction with the hardware. A key
> +requirement for such a split 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.
> +
> +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.
> +
> +The ancillary bus is intended to be minimal, generic and avoid domain-specific
> +assumptions. Each ancillary_device represents a part of its parent
> +functionality. The generic behavior can be extended and specialized as needed
> +by encapsulating an ancillary_device within other domain-specific structures and
> +the use of .ops callbacks. Devices on the ancillary bus do not share any
> +structures and the use of a communication channel with the parent is
> +domain-specific.
> +
> +When Should the Ancillary Bus Be Used
> +=====================================
> +
> +The ancillary 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 ancillary_device's
> +registering driver.  The registering driver for the ancillary_device(s) and the
> +kernel module(s) registering ancillary_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 could be a multi-port PCI network device that is rdma-capable and
> +needs to export this functionality and attach to an rdma driver in another
> +subsystem.  The PCI driver will allocate and register an ancillary_device for
> +each physical function on the NIC.  The rdma driver will register an
> +ancillary_driver that will be matched with and probed for each of these
> +ancillary_devices.  This will give the rdma driver access to the shared data/ops
> +in the PCI drivers shared object to establish a connection with the PCI driver.
> +
> +Another use case is for the PCI device to be split out into multiple sub
> +functions.  For each sub function an ancillary_device will be created.  A PCI
> +sub function driver will bind to such devices that will create its own one or
> +more class devices.  A PCI sub function ancillary device will likely 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 ancillary_device.
> +
> +Ancillary Device
> +================
> +
> +An ancillary_device is created and registered to represent 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.
> +
> +Registering an ancillary_device is a two-step process.  First you must call
> +ancillary_device_initialize(), which will check several aspects of the
> +ancillary_device struct and perform a device_initialize().  After this step
> +completes, any error state must have a call to put_device() in its resolution
> +path.  The second step in registering an ancillary_device is to perform a call
> +to ancillary_device_add(), which will set the name of the device and add the
> +device to the bus.
> +
> +To unregister an ancillary_device, just a call to ancillary_device_unregister()
> +is used.  This will perform both a device_del() and a put_device().
> +
> +.. code-block:: c
> +
> +	struct ancillary_device {
> +		struct device dev;
> +                const char *name;
> +		u32 id;
> +	};
> +
> +If two ancillary_devices both with a match_name "mod.foo" are registered onto
> +the bus, they must have unique id values (e.g. "x" and "y") so that the
> +registered devices names will be "mod.foo.x" and "mod.foo.y".  If match_name +
> +id are not unique, then the device_add will fail and generate an error message.
> +
> +The ancillary_device.dev.type.release or ancillary_device.dev.release must be
> +populated with a non-NULL pointer to successfully register the ancillary_device.
> +
> +The ancillary_device.dev.parent must also be populated.
> +
> +Ancillary Device Memory Model and Lifespan
> +------------------------------------------
> +
> +When a kernel driver registers an ancillary_device on the ancillary bus, we will
> +use the nomenclature to refer to this kernel driver as a registering driver.  It
> +is the entity that will allocate memory for the ancillary_device and register it
> +on the ancillary bus.  It is important to note that, as opposed to the platform
> +bus, the registering driver is wholly responsible for the management for the
> +memory used for the driver object.
> +
> +A parent object, defined in the shared header file, will contain the
> +ancillary_device.  It will also contain a pointer to the shared object(s), which
> +will also be defined in the shared header.  Both the parent object and the
> +shared object(s) will be allocated by the registering driver.  This layout
> +allows the ancillary_driver's registering module to perform a container_of()
> +call to go from the pointer to the ancillary_device, that is passed during the
> +call to the ancillary_driver's probe function, up to the parent object, and then
> +have access to the shared object(s).
> +
> +The memory for the ancillary_device will be freed only in its release()
> +callback flow as defined by its registering driver.
> +
> +The memory for the shared object(s) must have a lifespan equal to, or greater
> +than, the lifespan of the memory for the ancillary_device.  The ancillary_driver
> +should only consider that this shared object is valid as long as the
> +ancillary_device is still registered on the ancillary 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 ancillary_device.
> +
> +Registering driver must unregister all ancillary devices before its registering
> +parent device's remove() is completed.
> +
> +Ancillary Drivers
> +=================
> +
> +Ancillary 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.
> +
> +.. code-block:: c
> +
> +	struct ancillary_driver {
> +		int (*probe)(struct ancillary_device *,
> +                             const struct ancillary_device_id *id);
> +		int (*remove)(struct ancillary_device *);
> +		void (*shutdown)(struct ancillary_device *);
> +		int (*suspend)(struct ancillary_device *, pm_message_t);
> +		int (*resume)(struct ancillary_device *);
> +		struct device_driver driver;
> +		const struct ancillary_device_id *id_table;
> +	};
> +
> +Ancillary drivers register themselves with the bus by calling
> +ancillary_driver_register(). The id_table contains the match_names of ancillary
> +devices that a driver can bind with.
> +
> +Example Usage
> +=============
> +
> +Ancillary 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 ancillary_device would be to encapsulate it within a
> +domain-specific structure defined by the parent device. This structure contains
> +the ancillary_device and any associated shared data/callbacks needed to
> +establish the connection with the parent.
> +
> +An example would be:
> +
> +.. code-block:: c
> +
> +        struct foo {
> +		struct ancillary_device ancildev;
> +		void (*connect)(struct ancillary_device *ancildev);
> +		void (*disconnect)(struct ancillary_device *ancildev);
> +		void *data;
> +        };
> +
> +The parent device would then register the ancillary_device by calling
> +ancillary_device_initialize(), and then ancillary_device_add(), with the pointer
> +to the ancildev member of the above structure. The parent would provide a name
> +for the ancillary_device that, combined with the parent's KBUILD_MODNAME, will
> +create a match_name that will be used for matching and binding with a driver.
> +
> +Whenever an ancillary_driver is registered, based on the match_name, the
> +ancillary_driver's probe() is invoked for the matching devices.  The
> +ancillary_driver can also be encapsulated inside custom drivers that make the
> +core device's functionality extensible by adding additional domain-specific ops
> +as follows:
> +
> +.. code-block:: c
> +
> +	struct my_ops {
> +		void (*send)(struct ancillary_device *ancildev);
> +		void (*receive)(struct ancillary_device *ancildev);
> +	};
> +
> +
> +	struct my_driver {
> +		struct ancillary_driver ancillary_drv;
> +		const struct my_ops ops;
> +	};
> +
> +An example of this type of usage would be:
> +
> +.. code-block:: c
> +
> +	const struct ancillary_device_id my_ancillary_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 = {
> +		.ancillary_drv = {
> +			.driver = {
> +				.name = "myancillarydrv",

Why do we need to give control over driver name to the driver authors?
It can be problematic if author puts name that already exists.

> +			},
> +			.id_table = my_ancillary_id_table,
> +			.probe = my_probe,
> +			.remove = my_remove,
> +			.shutdown = my_shutdown,
> +		},
> +		.ops = my_custom_ops,
> +	};
> diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
> index 5ef2cfe3a16b..9584ac2ed1f5 100644
> --- a/Documentation/driver-api/index.rst
> +++ b/Documentation/driver-api/index.rst
> @@ -74,6 +74,7 @@ available subsections can be seen below.
>     thermal/index
>     fpga/index
>     acpi/index
> +   ancillary_bus
>     backlight/lp855x-driver.rst
>     connector
>     console
> diff --git a/drivers/bus/Kconfig b/drivers/bus/Kconfig
> index 0c262c2aeaf2..ba82a045b847 100644
> --- a/drivers/bus/Kconfig
> +++ b/drivers/bus/Kconfig
> @@ -5,6 +5,9 @@
>
>  menu "Bus devices"
>
> +config ANCILLARY_BUS
> +       tristate
> +
>  config ARM_CCI
>  	bool
>
> diff --git a/drivers/bus/Makefile b/drivers/bus/Makefile
> index 397e35392bff..7c217eb1dbb7 100644
> --- a/drivers/bus/Makefile
> +++ b/drivers/bus/Makefile
> @@ -3,6 +3,9 @@
>  # Makefile for the bus drivers.
>  #
>
> +# Ancillary bus driver
> +obj-$(CONFIG_ANCILLARY_BUS)	+= ancillary.o
> +
>  # Interconnect bus drivers for ARM platforms
>  obj-$(CONFIG_ARM_CCI)		+= arm-cci.o
>  obj-$(CONFIG_ARM_INTEGRATOR_LM)	+= arm-integrator-lm.o
> diff --git a/drivers/bus/ancillary.c b/drivers/bus/ancillary.c
> new file mode 100644
> index 000000000000..93888ca36fb1
> --- /dev/null
> +++ b/drivers/bus/ancillary.c
> @@ -0,0 +1,225 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Software based bus for Ancillary devices
> + *
> + * Copyright (c) 2019-2020 Intel Corporation
> + *
> + * Please see Documentation/driver-api/ancillary_bus.rst for more information.
> + */
> +
> +#define pr_fmt(fmt) "%s:%s: " fmt, KBUILD_MODNAME, __func__
> +
> +#include <linux/device.h>
> +#include <linux/init.h>
> +#include <linux/module.h>
> +#include <linux/pm_domain.h>
> +#include <linux/pm_runtime.h>
> +#include <linux/string.h>
> +#include <linux/ancillary_bus.h>
> +
> +static const struct ancillary_device_id *ancillary_match_id(const struct ancillary_device_id *id,
> +							    const struct ancillary_device *ancildev)
> +{
> +	while (id->name[0]) {
> +		const char *p = strrchr(dev_name(&ancildev->dev), '.');
> +		int match_size;
> +
> +		if (!p) {
> +			id++;
> +			continue;
> +		}
> +		match_size = p - dev_name(&ancildev->dev);
> +
> +		/* use dev_name(&ancildev->dev) prefix before last '.' char to match to */
> +		if (!strncmp(dev_name(&ancildev->dev), id->name, match_size))
> +			return id;
> +		id++;
> +	}
> +	return NULL;
> +}
> +
> +static int ancillary_match(struct device *dev, struct device_driver *drv)
> +{
> +	struct ancillary_device *ancildev = to_ancillary_dev(dev);
> +	struct ancillary_driver *ancildrv = to_ancillary_drv(drv);
> +
> +	return !!ancillary_match_id(ancildrv->id_table, ancildev);
> +}
> +
> +static int ancillary_uevent(struct device *dev, struct kobj_uevent_env *env)
> +{
> +	const char *name, *p;
> +
> +	name = dev_name(dev);
> +	p = strrchr(name, '.');
> +
> +	return add_uevent_var(env, "MODALIAS=%s%.*s", ANCILLARY_MODULE_PREFIX, (int)(p - name),
> +			      name);
> +}
> +
> +static const struct dev_pm_ops ancillary_dev_pm_ops = {
> +	SET_RUNTIME_PM_OPS(pm_generic_runtime_suspend, pm_generic_runtime_resume, NULL)
> +	SET_SYSTEM_SLEEP_PM_OPS(pm_generic_suspend, pm_generic_resume)
> +};
> +
> +struct bus_type ancillary_bus_type = {
> +	.name = "ancillary",
> +	.match = ancillary_match,
> +	.uevent = ancillary_uevent,
> +	.pm = &ancillary_dev_pm_ops,
> +};
> +
> +/**
> + * ancillary_device_initialize - check ancillary_device and initialize
> + * @ancildev: ancillary device struct
> + *
> + * This is the first step in the two-step process to register an ancillary_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
> + * ancillary_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 put_device().
> + * In this post-initialize error scenario, a call to the device's .release callback will be
> + * triggered by put_device(), and all memory clean-up is expected to be handled there.
> + */
> +int ancillary_device_initialize(struct ancillary_device *ancildev)
> +{
> +	struct device *dev = &ancildev->dev;
> +
> +	dev->bus = &ancillary_bus_type;
> +
> +	if (!dev->parent) {
> +		pr_err("ancillary_device has a NULL dev->parent\n");
> +		return -EINVAL;
> +	}
> +
> +	if (!ancildev->name) {
> +		pr_err("acillary_device has a NULL name\n");
> +		return -EINVAL;
> +	}
> +
> +	if (!(dev->type && dev->type->release) && !dev->release) {
> +		pr_err("ancillary_device does not have a release callback defined\n");
> +		return -EINVAL;
> +	}
> +
> +	device_initialize(&ancildev->dev);
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(ancillary_device_initialize);
> +
> +/**
> + * __ancillary_device_add - add an ancillary bus device
> + * @ancildev: ancillary bus device to add to the bus
> + * @modname: name of the parent device's driver module
> + *
> + * This is the second step in the two-step process to register an ancillary_device.
> + *
> + * This function must be called after a successful call to ancillary_device_initialize(), which
> + * will perform the device_initialize.  This means that if this returns an error code, then a
> + * put_device must be performed so that the .release callback will be triggered to free the
> + * memory associated with the ancillary_device.
> + */
> +int __ancillary_device_add(struct ancillary_device *ancildev, const char *modname)
> +{
> +	struct device *dev = &ancildev->dev;
> +	int ret;
> +
> +	if (!modname) {
> +		pr_err("ancillary device modname is NULL\n");
> +		return -EINVAL;
> +	}
> +
> +	ret = dev_set_name(dev, "%s.%s.%d", modname, ancildev->name, ancildev->id);
> +	if (ret) {
> +		pr_err("ancillary device dev_set_name failed: %d\n", ret);
> +		return ret;
> +	}
> +
> +	ret = device_add(dev);
> +	if (ret)
> +		dev_err(dev, "adding ancillary device failed!: %d\n", ret);
> +
> +	return ret;
> +}

Sorry, but this is very strange API that requires users to put
internal call to "dev" that is buried inside "struct ancillary_device".

For example in your next patch, you write this "put_device(&cdev->ancildev.dev);"

I'm pretty sure that the amount of bugs in error unwind will be
astonishing, so if you are doing wrappers over core code, better do not
pass complexity to the users.

> +EXPORT_SYMBOL_GPL(__ancillary_device_add);
> +
> +static int ancillary_probe_driver(struct device *dev)
> +{
> +	struct ancillary_driver *ancildrv = to_ancillary_drv(dev->driver);
> +	struct ancillary_device *ancildev = to_ancillary_dev(dev);
> +	int ret;
> +
> +	ret = dev_pm_domain_attach(dev, true);
> +	if (ret) {
> +		dev_warn(dev, "Failed to attach to PM Domain : %d\n", ret);
> +		return ret;
> +	}
> +
> +	ret = ancildrv->probe(ancildev, ancillary_match_id(ancildrv->id_table, ancildev));

I don't think that you need to call ->probe() if ancillary_match_id()
returned NULL and probably that check should be done before
dev_pm_domain_attach().

> +	if (ret)
> +		dev_pm_domain_detach(dev, true);
> +
> +	return ret;
> +}
> +
> +static int ancillary_remove_driver(struct device *dev)
> +{
> +	struct ancillary_driver *ancildrv = to_ancillary_drv(dev->driver);
> +	struct ancillary_device *ancildev = to_ancillary_dev(dev);
> +	int ret;
> +
> +	ret = ancildrv->remove(ancildev);
> +	dev_pm_domain_detach(dev, true);
> +
> +	return ret;

You returned an error to user and detached from PM, what will user do
with this information? Should user ignore it? retry?

> +}
> +
> +static void ancillary_shutdown_driver(struct device *dev)
> +{
> +	struct ancillary_driver *ancildrv = to_ancillary_drv(dev->driver);
> +	struct ancillary_device *ancildev = to_ancillary_dev(dev);
> +
> +	ancildrv->shutdown(ancildev);
> +}
> +
> +/**
> + * __ancillary_driver_register - register a driver for ancillary bus devices
> + * @ancildrv: ancillary_driver structure
> + * @owner: owning module/driver
> + */
> +int __ancillary_driver_register(struct ancillary_driver *ancildrv, struct module *owner)
> +{
> +	if (WARN_ON(!ancildrv->probe) || WARN_ON(!ancildrv->remove) ||
> +	    WARN_ON(!ancildrv->shutdown) || WARN_ON(!ancildrv->id_table))
> +		return -EINVAL;
> +
> +	ancildrv->driver.owner = owner;
> +	ancildrv->driver.bus = &ancillary_bus_type;
> +	ancildrv->driver.probe = ancillary_probe_driver;
> +	ancildrv->driver.remove = ancillary_remove_driver;
> +	ancildrv->driver.shutdown = ancillary_shutdown_driver;
> +
> +	return driver_register(&ancildrv->driver);
> +}
> +EXPORT_SYMBOL_GPL(__ancillary_driver_register);
> +
> +static int __init ancillary_bus_init(void)
> +{
> +	return bus_register(&ancillary_bus_type);
> +}
> +
> +static void __exit ancillary_bus_exit(void)
> +{
> +	bus_unregister(&ancillary_bus_type);
> +}
> +
> +module_init(ancillary_bus_init);
> +module_exit(ancillary_bus_exit);
> +
> +MODULE_LICENSE("GPL v2");
> +MODULE_DESCRIPTION("Ancillary Bus");
> +MODULE_AUTHOR("David Ertman <[email-masked]>");
> +MODULE_AUTHOR("Kiran Patil <[email-masked]>");
> diff --git a/include/linux/ancillary_bus.h b/include/linux/ancillary_bus.h
> new file mode 100644
> index 000000000000..72169c8a5dfe
> --- /dev/null
> +++ b/include/linux/ancillary_bus.h
> @@ -0,0 +1,69 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (c) 2019-2020 Intel Corporation
> + *
> + * Please see Documentation/driver-api/ancillary_bus.rst for more information.
> + */
> +
> +#ifndef _ANCILLARY_BUS_H_
> +#define _ANCILLARY_BUS_H_
> +
> +#include <linux/device.h>
> +#include <linux/mod_devicetable.h>
> +#include <linux/slab.h>
> +
> +struct ancillary_device {
> +	struct device dev;
> +	const char *name;
> +	u32 id;
> +};
> +
> +struct ancillary_driver {
> +	int (*probe)(struct ancillary_device *ancildev, const struct ancillary_device_id *id);
> +	int (*remove)(struct ancillary_device *ancildev);
> +	void (*shutdown)(struct ancillary_device *ancildev);
> +	int (*suspend)(struct ancillary_device *ancildev, pm_message_t state);
> +	int (*resume)(struct ancillary_device *ancildev);
> +	struct device_driver driver;
> +	const struct ancillary_device_id *id_table;
> +};
> +
> +static inline struct ancillary_device *to_ancillary_dev(struct device *dev)
> +{
> +	return container_of(dev, struct ancillary_device, dev);
> +}
> +
> +static inline struct ancillary_driver *to_ancillary_drv(struct device_driver *drv)
> +{
> +	return container_of(drv, struct ancillary_driver, driver);
> +}
> +
> +int ancillary_device_initialize(struct ancillary_device *ancildev);
> +int __ancillary_device_add(struct ancillary_device *ancildev, const char *modname);
> +#define ancillary_device_add(ancildev) __ancillary_device_add(ancildev, KBUILD_MODNAME)
> +
> +static inline void ancillary_device_unregister(struct ancillary_device *ancildev)
> +{
> +	device_unregister(&ancildev->dev);
> +}
> +
> +int __ancillary_driver_register(struct ancillary_driver *ancildrv, struct module *owner);
> +#define ancillary_driver_register(ancildrv) __ancillary_driver_register(ancildrv, THIS_MODULE)
> +
> +static inline void ancillary_driver_unregister(struct ancillary_driver *ancildrv)
> +{
> +	driver_unregister(&ancildrv->driver);
> +}
> +
> +/**
> + * module_ancillary_driver() - Helper macro for registering an ancillary driver
> + * @__ancillary_driver: ancillary driver struct
> + *
> + * Helper macro for ancillary 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()
> + */
> +#define module_ancillary_driver(__ancillary_driver) \
> +	module_driver(__ancillary_driver, ancillary_driver_register, ancillary_driver_unregister)
> +
> +#endif /* _ANCILLARY_BUS_H_ */
> diff --git a/include/linux/mod_devicetable.h b/include/linux/mod_devicetable.h
> index 5b08a473cdba..7d596dc30833 100644
> --- a/include/linux/mod_devicetable.h
> +++ b/include/linux/mod_devicetable.h
> @@ -838,4 +838,12 @@ struct mhi_device_id {
>  	kernel_ulong_t driver_data;
>  };
>
> +#define ANCILLARY_NAME_SIZE 32
> +#define ANCILLARY_MODULE_PREFIX "ancillary:"
> +
> +struct ancillary_device_id {
> +	char name[ANCILLARY_NAME_SIZE];

I hope that this be enough.

> +	kernel_ulong_t driver_data;
> +};
> +
>  #endif /* LINUX_MOD_DEVICETABLE_H */
> diff --git a/scripts/mod/devicetable-offsets.c b/scripts/mod/devicetable-offsets.c
> index 27007c18e754..79e37c4c25b3 100644
> --- a/scripts/mod/devicetable-offsets.c
> +++ b/scripts/mod/devicetable-offsets.c
> @@ -243,5 +243,8 @@ int main(void)
>  	DEVID(mhi_device_id);
>  	DEVID_FIELD(mhi_device_id, chan);
>
> +	DEVID(ancillary_device_id);
> +	DEVID_FIELD(ancillary_device_id, name);
> +
>  	return 0;
>  }
> diff --git a/scripts/mod/file2alias.c b/scripts/mod/file2alias.c
> index 2417dd1dee33..99c4fcd82bf3 100644
> --- a/scripts/mod/file2alias.c
> +++ b/scripts/mod/file2alias.c
> @@ -1364,6 +1364,13 @@ static int do_mhi_entry(const char *filename, void *symval, char *alias)
>  {
>  	DEF_FIELD_ADDR(symval, mhi_device_id, chan);
>  	sprintf(alias, MHI_DEVICE_MODALIAS_FMT, *chan);
> +	return 1;
> +}
> +
> +static int do_ancillary_entry(const char *filename, void *symval, char *alias)
> +{
> +	DEF_FIELD_ADDR(symval, ancillary_device_id, name);
> +	sprintf(alias, ANCILLARY_MODULE_PREFIX "%s", *name);
>
>  	return 1;
>  }
> @@ -1442,6 +1449,7 @@ static const struct devtable devtable[] = {
>  	{"tee", SIZE_tee_client_device_id, do_tee_entry},
>  	{"wmi", SIZE_wmi_device_id, do_wmi_entry},
>  	{"mhi", SIZE_mhi_device_id, do_mhi_entry},
> +	{"ancillary", SIZE_ancillary_device_id, do_ancillary_entry},
>  };
>
>  /* Create MODULE_ALIAS() statements.
> --
> 2.26.2
>