Bus Types 

Definition
~~~~~~~~~~
See the kerneldoc for the struct bus_type.

int bus_register(struct bus_type * bus);


Declaration
~~~~~~~~~~~

Each bus type in the kernel (PCI, USB, etc) should declare one static
object of this type. They must initialize the name field, and may
optionally initialize the match callback.

struct bus_type pci_bus_type = {
       .name	= "pci",
       .match	= pci_bus_match,
};

The structure should be exported to drivers in a header file:

extern struct bus_type pci_bus_type;


Registration
~~~~~~~~~~~~

When a bus driver is initialized, it calls bus_register. This
initializes the rest of the fields in the bus object and inserts it
into a global list of bus types. Once the bus object is registered, 
the fields in it are usable by the bus driver. 


Callbacks
~~~~~~~~~

match(): Attaching Drivers to Devices
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The format of device ID structures and the semantics for comparing
them are inherently bus-specific. Drivers typically declare an array
of device IDs of devices they support that reside in a bus-specific
driver structure. 

The purpose of the match callback is to give the bus an opportunity to
determine if a particular driver supports a particular device by
comparing the device IDs the driver supports with the device ID of a
particular device, without sacrificing bus-specific functionality or
type-safety. 

When a driver is registered with the bus, the bus's list of devices is
iterated over, and the match callback is called for each device that
does not have a driver associated with it. 



Device and Driver Lists
~~~~~~~~~~~~~~~~~~~~~~~

The lists of devices and drivers are intended to replace the local
lists that many buses keep. They are lists of struct devices and
struct device_drivers, respectively. Bus drivers are free to use the
lists as they please, but conversion to the bus-specific type may be
necessary. 

The LDM core provides helper functions for iterating over each list.

int bus_for_each_dev(struct bus_type * bus, struct device * start, void * data,
		     int (*fn)(struct device *, void *));

int bus_for_each_drv(struct bus_type * bus, struct device_driver * start, 
		     void * data, int (*fn)(struct device_driver *, void *));

These helpers iterate over the respective list, and call the callback
for each device or driver in the list. All list accesses are
synchronized by taking the bus's lock (read currently). The reference
count on each object in the list is incremented before the callback is
called; it is decremented after the next object has been obtained. The
lock is not held when calling the callback. 


sysfs
~~~~~~~~
There is a top-level directory named 'bus'.

Each bus gets a directory in the bus directory, along with two default
directories:

	/sys/bus/pci/
	|-- devices
	`-- drivers

Drivers registered with the bus get a directory in the bus's drivers
directory:

	/sys/bus/pci/
	|-- devices
	`-- drivers
	    |-- Intel ICH
	    |-- Intel ICH Joystick
	    |-- agpgart
	    `-- e100

Each device that is discovered on a bus of that type gets a symlink in
the bus's devices directory to the device's directory in the physical
hierarchy:

	/sys/bus/pci/
	|-- devices
	|   |-- 00:00.0 -> ../../../root/pci0/00:00.0
	|   |-- 00:01.0 -> ../../../root/pci0/00:01.0
	|   `-- 00:02.0 -> ../../../root/pci0/00:02.0
	`-- drivers


Exporting Attributes
~~~~~~~~~~~~~~~~~~~~
struct bus_attribute {
	struct attribute	attr;
	ssize_t (*show)(struct bus_type *, char * buf);
	ssize_t (*store)(struct bus_type *, const char * buf, size_t count);
};

Bus drivers can export attributes using the BUS_ATTR macro that works
similarly to the DEVICE_ATTR macro for devices. For example, a definition 
like this:

static BUS_ATTR(debug,0644,show_debug,store_debug);

is equivalent to declaring:

static bus_attribute bus_attr_debug;

This can then be used to add and remove the attribute from the bus's
sysfs directory using:

int bus_create_file(struct bus_type *, struct bus_attribute *);
void bus_remove_file(struct bus_type *, struct bus_attribute *);