Add auxiliary bus support

Add support for the Auxiliary Bus, auxiliary_device and auxiliary_driver.
It enables drivers to create an auxiliary_device and bind an
auxiliary_driver to it.

The bus supports probe/remove shutdown and suspend/resume callbacks.
Each auxiliary_device has a unique string based id; driver binds to
an auxiliary_device based on this id through the bus.

Co-developed-by: Kiran Patil <kiran.patil@intel.com>
Co-developed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Co-developed-by: Fred Oh <fred.oh@linux.intel.com>
Co-developed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Kiran Patil <kiran.patil@intel.com>
Signed-off-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Signed-off-by: Fred Oh <fred.oh@linux.intel.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Dave Ertman <david.m.ertman@intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Shiraz Saleem <shiraz.saleem@intel.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Martin Habets <mhabets@solarflare.com>
Link: https://lore.kernel.org/r/20201113161859.1775473-2-david.m.ertman@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/160695681289.505290.8978295443574440604.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 235 insertions, 0 deletions

diff --git a/Documentation/driver-api/auxiliary_bus.rst b/Documentation/driver-api/auxiliary_bus.rst
new file mode 100644
index 000000000000..5dd7804631ef
--- /dev/null
+++ b/Documentation/driver-api/auxiliary_bus.rst
@@ -0,0 +1,234 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+
+=============
+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 functinoality 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
+================
+
+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.
+
+Registering an auxiliary_device is a two-step process.  First 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 second step in registering an auxiliary_device is to perform a call to
+auxiliary_device_add(), which sets the name of the device and add the device to
+the bus.
+
+Unregistering an auxiliary_device is also a two-step process to mirror the
+register process.  First call auxiliary_device_delete(), then call
+auxiliary_device_uninit().
+
+.. code-block:: c
+
+	struct auxiliary_device {
+		struct device dev;
+                const char *name;
+		u32 id;
+	};
+
+If two auxiliary_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 are "mod.foo.x" and "mod.foo.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.
+
+The auxiliary_device.dev.parent must also be populated.
+
+Auxiliary Device Memory Model and Lifespan
+------------------------------------------
+
+The registering driver is the entity that allocates memory for the
+auxiliary_device and register 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 for the memory used for the driver object.
+
+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 auxiliary_device is 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 auxiliary_device.  The auxiliary_driver
+should only consider that this 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.
+
+Auxiliary Drivers
+=================
+
+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.
+
+.. code-block:: c
+
+	struct auxiliary_driver {
+		int (*probe)(struct auxiliary_device *,
+                             const struct auxiliary_device_id *id);
+		int (*remove)(struct auxiliary_device *);
+		void (*shutdown)(struct auxiliary_device *);
+		int (*suspend)(struct auxiliary_device *, pm_message_t);
+		int (*resume)(struct auxiliary_device *);
+		struct device_driver driver;
+		const struct auxiliary_device_id *id_table;
+	};
+
+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.
+
+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:
+
+.. 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,
+	};
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index f357f3eb400c..86759a74b7f1 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -72,6 +72,7 @@ available subsections can be seen below.
    thermal/index
    fpga/index
    acpi/index
+   auxiliary_bus
    backlight/lp855x-driver.rst
    connector
    console