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+
+Bus Types 
+
+Definition
+~~~~~~~~~~
+
+struct bus_type {
+	char			* name;
+
+	struct subsystem	subsys;
+	struct kset		drivers;
+	struct kset		devices;
+
+	struct bus_attribute	* bus_attrs;
+	struct device_attribute	* dev_attrs;
+	struct driver_attribute	* drv_attrs;
+
+	int		(*match)(struct device * dev, struct device_driver * drv);
+	int		(*hotplug) (struct device *dev, char **envp, 
+				    int num_envp, char *buffer, int buffer_size);
+	int		(*suspend)(struct device * dev, u32 state);
+	int		(*resume)(struct device * dev);
+};
+
+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 provide 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 *);
+
+