Documentation/auxiliary_bus: Move the text into the code

The code and documentation are more difficult to maintain when kept
separately.  This is further compounded when the standard structure
documentation infrastructure is not used.

Move the documentation into the code, use the standard documentation
infrastructure, add current documented functions, and reference the text
in the rst file.
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20211202044305.4006853-8-ira.weiny@intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/auxiliary.c

@@ -17,6 +17,147 @@
 #include <linux/auxiliary_bus.h>
 #include "base.h"
 
+/**
+ * DOC: PURPOSE
+ *
+ * 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).
+ */
+
+/**
+ * DOC: USAGE
+ *
+ * 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.
+ */
+
+/**
+ * DOC: EXAMPLE
+ *
+ * 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:
+ *
+ * .. code-block:: c
+ *
+ *         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:
+ *
+ * .. code-block:: c
+ *
+ *	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:
+ *
+ * .. code-block:: c
+ *
+ *	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,
+ *	};
+ */
+
 static const struct auxiliary_device_id *auxiliary_match_id(const struct auxiliary_device_id *id,
 							    const struct auxiliary_device *auxdev)
 {

include/linux/auxiliary_bus.h

@@ -11,12 +11,172 @@
 #include <linux/device.h>
 #include <linux/mod_devicetable.h>
 
+/**
+ * DOC: DEVICE_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.
+ */
+
+/**
+ * struct auxiliary_device - auxiliary device object.
+ * @dev: Device,
+ *       The release and parent fields of the device structure must be filled
+ *       in
+ * @name: Match name found by the auxiliary device driver,
+ * @id: 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 auxiliary_device.dev.type.release or auxiliary_device.dev.release 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 auxiliary_device.dev.parent 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.
+ *
+ * .. code-block:: c
+ *
+ *      #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)) {
+ *		auxiliary_device_uninit(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().
+ *
+ * .. code-block:: c
+ *
+ *         auxiliary_device_delete(my_dev->my_aux_dev);
+ *         auxiliary_device_uninit(my_dev->my_aux_dev);
+ */
 struct auxiliary_device {
 	struct device dev;
 	const char *name;
 	u32 id;
 };
 
+/**
+ * struct auxiliary_driver - Definition of an auxiliary bus driver
+ * @probe: Called when a matching device is added to the bus.
+ * @remove: Called when device is removed from the bus.
+ * @shutdown: Called at shut-down time to quiesce the device.
+ * @suspend: Called to put the device to sleep mode. Usually to a power state.
+ * @resume: Called to bring a device from sleep mode.
+ * @name: Driver name.
+ * @driver: Core driver structure.
+ * @id_table: 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.
+ *
+ * .. code-block:: c
+ *
+ *         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
+ *         };
+ */
 struct auxiliary_driver {
 	int (*probe)(struct auxiliary_device *auxdev, const struct auxiliary_device_id *id);
 	void (*remove)(struct auxiliary_device *auxdev);