Merge osdl.org:/home/mochel/src/kernel/devel/linux-2.5-virgin

into osdl.org:/home/mochel/src/kernel/devel/linux-2.5-cls

Documentation/driver-model/binding.txt

+
+Driver Binding
+
+Driver binding is the process of associating a device with a device
+driver that can control it. Bus drivers have typically handled this
+because there have been bus-specific structures to represent the
+devices and the drivers. With generic device and device driver
+structures, most of the binding can take place using common code.
+
+
+Bus
+~~~
+
+The bus type structure contains a list of all devices that on that bus
+type in the system. When device_register is called for a device, it is
+inserted into the end of this list. The bus object also contains a
+list of all drivers of that bus type. When driver_register is called
+for a driver, it is inserted into the end of this list. These are the
+two events which trigger driver binding.
+
+
+device_register
+~~~~~~~~~~~~~~~
+
+When a new device is added, the bus's list of drivers is iterated over
+to find one that supports it. In order to determine that, the device
+ID of the device must match one of the device IDs that the driver
+supports. The format and semantics for comparing IDs is bus-specific. 
+Instead of trying to derive a complex state machine and matching
+algorithm, it is up to the bus driver to provide a callback to compare
+a device against the IDs of a driver. The bus returns 1 if a match was
+found; 0 otherwise.
+
+int match(struct device * dev, struct device_driver * drv);
+
+If a match is found, the device's driver field is set to the driver
+and the driver's probe callback is called. This gives the driver a
+chance to verify that it really does support the hardware, and that
+it's in a working state. 
+
+Device Class
+~~~~~~~~~~~~
+
+Upon the successful completion of probe, the device is registered with
+the class to which it belongs. Device drivers belong to one and only
+class, and that is set in the driver's devclass field. 
+devclass_add_device is called to enumerate the device within the class
+and actually register it with the class, which happens with the
+class's register_dev callback.
+
+NOTE: The device class structures and core routines to manipulate them
+are not in the mainline kernel, so the discussion is still a bit
+speculative. 
+
+
+Driver
+~~~~~~
+
+When a driver is attached to a device, the device is inserted into the
+driver's list of devices. 
+
+
+driverfs
+~~~~~~~~
+
+A symlink is created in the bus's 'devices' directory that points to
+the device's directory in the physical hierarchy.
+
+A symlink is created in the driver's 'devices' directory that points
+to the device's directory in the physical hierarchy.
+
+A directory for the device is created in the class's directory. A
+symlink is created in that directory that points to the device's
+physical location in the driverfs tree. 
+
+A symlink can be created (though this isn't done yet) in the device's
+physical directory to either its class directory, or the class's
+top-level directory. One can also be created to point to its driver's
+directory also. 
+
+
+driver_register
+~~~~~~~~~~~~~~~
+
+The process is almost identical for when a new driver is added. 
+The bus's list of devices is iterated over to find a match. Devices
+that already have a driver are skipped. All the devices are iterated
+over, to bind as many devices as possible to the driver.
+
+
+Removal
+~~~~~~~
+
+When a device is removed, the reference count for it will eventually
+go to 0. When it does, the remove callback of the driver is called. It
+is removed from the driver's list of devices and the reference count
+of the driver is decremented. All symlinks between the two are removed.
+
+When a driver is removed, the list of devices that it supports is
+iterated over, and the driver's remove callback is called for each
+one. The device is removed from that list and the symlinks removed. 
+

Documentation/driver-model/bus.txt

+
+Bus Types 
+
+Definition
+~~~~~~~~~~
+
+struct bus_type {
+        char                    * name;
+        rwlock_t                lock;
+        atomic_t                refcount;
+
+        struct list_head        node;
+        struct list_head        devices;
+        struct list_head        drivers;
+
+        struct driver_dir_entry dir;
+        struct driver_dir_entry device_dir;
+        struct driver_dir_entry driver_dir;
+
+        int     (*match)        (struct device * dev, struct device_driver * drv);
+	struct device (*add)	(struct device * parent, char * bus_id);
+};
+
+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 (e.g. the rwlock_t) 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 device 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. 
+
+add(): Adding a child device
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The add callback is available to notify the bus about a child device
+at a particular location. 
+
+The parent parameter is the parent device of the child to be added. If
+parent == NULL, the bus should add the device as a child of a default
+parent device or as a child of the root. This policy decision is up to
+the bus driver.
+
+The format of the bus_id field should be consistent with the format of
+the bus_id field of the rest of the devices on the bus. This requires
+the caller to know the format.
+
+On return, the bus driver should return a pointer to the device that
+was created. If the device was not created, the bus driver should
+return an appropriate error code. Refer to include/linux/err.h for
+helper functions to encode errors. Some sample code:
+
+struct device * pci_bus_add(struct device * parent, char * bus_id)
+{
+	...
+	/* the device already exists */
+	return ERR_PTR(-EEXIST);
+	...
+}
+
+The caller can check the return value using IS_ERR():
+
+    struct device * newdev = pci_bus_type.add(parent,bus_id);
+    if (IS_ERR(newdev)) {
+	...
+    }
+
+
+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, void * data, 
+		     int (*callback)(struct device * dev, void * data));
+int bus_for_each_drv(struct bus_type * bus, void * data,
+		     int (*callback)(struct device_driver * drv, void * data));
+
+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. 
+
+
+driverfs
+~~~~~~~~
+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 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, size_t count, loff_t off);
+        ssize_t (*store)(struct bus_type *, const char * buf, size_t count, loff_t off);
+};
+
+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
+driverfs directory using:
+
+int bus_create_file(struct bus_type *, struct bus_attribute *);
+void bus_remove_file(struct bus_type *, struct bus_attribute *);
+
+

Documentation/driver-model/class.txt

+
+Device Classes
+
+
+Introduction
+~~~~~~~~~~~~
+A device class describes a type of device, like an audio or network
+device. The following device classes have been identified:
+
+<Insert List of Device Classes Here>
+
+
+Each device class defines a set of semantics and a programming interface
+that devices of that class adhere to. Device drivers are the
+implemention of that programming interface for a particular device on
+a particular bus. 
+
+Device classes are agnostic with respect to what bus a device resides
+on. 
+
+
+Programming Interface
+~~~~~~~~~~~~~~~~~~~~~
+The device class structure looks like: 
+
+
+typedef int (*devclass_add)(struct device *);
+typedef void (*devclass_remove)(struct device *);
+
+struct device_class {
+	char			* name;
+	rwlock_t		lock;
+	u32			devnum;
+	struct list_head	node;
+
+	struct list_head	drivers;
+	struct list_head	intf_list;
+
+	struct driver_dir_entry	dir;
+	struct driver_dir_entry	device_dir;
+	struct driver_dir_entry	driver_dir;
+
+	devclass_add		add_device;
+	devclass_remove		remove_device;
+};
+
+A typical device class definition would look like: 
+
+struct device_class input_devclass = {
+        .name		= "input",
+        .add_device	= input_add_device,
+	.remove_device	= input_remove_device,
+};
+
+Each device class structure should be exported in a header file so it
+can be used by drivers, extensions and interfaces.
+
+Device classes are registered and unregistered with the core using: 
+
+int devclass_register(struct device_class * cls);
+void devclass_unregister(struct device_class * cls);
+
+
+Devices
+~~~~~~~
+As devices are bound to drivers, they are added to the device class
+that the driver belongs to. Before the driver model core, this would
+typically happen during the driver's probe() callback, once the device
+has been initialized. It now happens after the probe() callback
+finishes from the core. 
+
+The device is enumerated in the class. Each time a device is added to
+the class, the class's devnum field is incremented and assigned to the
+device. The field is never decremented, so if the device is removed
+from the class and re-added, it will receive a different enumerated
+value. 
+
+The class is allowed to create a class-specific structure for the
+device and store it in the device's class_data pointer. 
+
+There is no list of devices in the device class. Each driver has a
+list of devices that it supports. The device class has a list of
+drivers of that particular class. To access all of the devices in the
+class, iterate over the device lists of each driver in the class.
+
+
+Device Drivers
+~~~~~~~~~~~~~~
+Device drivers are added to device classes when they are registered
+with the core. A driver specifies the class it belongs to by setting
+the struct device_driver::devclass field. 
+
+
+driverfs directory structure
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+There is a top-level driverfs directory named 'class'. 
+
+Each class gets a directory in the class directory, along with two
+default subdirectories:
+
+        class/
+        `-- input
+            |-- devices
+            `-- drivers
+
+
+Drivers registered with the class get a symlink in the drivers/ directory 
+that points the driver's directory (under its bus directory):
+
+   class/
+   `-- input
+       |-- devices
+       `-- drivers
+           `-- usb:usb_mouse -> ../../../bus/drivers/usb_mouse/
+
+
+Each device gets a symlink in the devices/ directory that points to the 
+device's directory in the physical hierarchy:
+
+   class/
+   `-- input
+       |-- devices
+       |   `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/
+       `-- drivers
+
+
+Exporting Attributes
+~~~~~~~~~~~~~~~~~~~~
+struct devclass_attribute {
+        struct attribute        attr;
+        ssize_t (*show)(struct device_class *, char * buf, size_t count, loff_t off);
+        ssize_t (*store)(struct device_class *, const char * buf, size_t count, loff_t off);
+};
+
+Class drivers can export attributes using the DEVCLASS_ATTR macro that works
+similarly to the DEVICE_ATTR macro for devices. For example, a definition 
+like this:
+
+static DEVCLASS_ATTR(debug,0644,show_debug,store_debug);
+
+is equivalent to declaring:
+
+static devclass_attribute devclass_attr_debug;
+
+The bus driver can add and remove the attribute from the class's
+driverfs directory using:
+
+int devclass_create_file(struct device_class *, struct devclass_attribute *);
+void devclass_remove_file(struct device_class *, struct devclass_attribute *);
+
+In the example above, the file will be named 'debug' in placed in the
+class's directory in driverfs. 
+
+
+Interfaces
+~~~~~~~~~~
+There may exist multiple mechanisms for accessing the same device of a
+particular class type. Device interfaces describe these mechanisms. 
+
+When a device is added to a device class, the core attempts to add it
+to every interface that is registered with the device class.
+

Documentation/driver-model/device.txt

+
+The Basic Device Structure
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+struct device {
+        struct list_head g_list;
+        struct list_head node;
+        struct list_head bus_list;
+        struct list_head driver_list;
+        struct list_head intf_list;
+        struct list_head children;
+        struct device   * parent;
+
+        char    name[DEVICE_NAME_SIZE];
+        char    bus_id[BUS_ID_SIZE];
+
+        spinlock_t      lock;
+        atomic_t        refcount;
+
+        struct bus_type * bus;
+        struct driver_dir_entry dir;
+
+	u32		class_num;
+
+        struct device_driver *driver;
+        void            *driver_data;
+        void            *platform_data;
+
+        u32             current_state;
+        unsigned char *saved_state;
+
+        void    (*release)(struct device * dev);
+};
+
+Fields 
+~~~~~~
+g_list:	Node in the global device list.
+
+node:	Node in device's parent's children list.
+
+bus_list: Node in device's bus's devices list.
+
+driver_list:   Node in device's driver's devices list.
+
+intf_list:     List of intf_data. There is one structure allocated for
+	       each interface that the device supports.
+
+children:      List of child devices.
+
+name:	       ASCII description of device. 
+	       Example: " 3Com Corporation 3c905 100BaseTX [Boomerang]"
+
+bus_id:	       ASCII representation of device's bus position. This 
+	       field should a name unique across all devices on the
+	       bus type the device belongs to. 
+
+	       Example: PCI bus_ids are in the form of
+	       <bus number>:<slot number>.<function number> 
+	       This name is unique across all PCI devices in the system.
+
+lock:	       Spinlock for the device. 
+
+refcount:      Reference count on the device.
+
+bus:	       Pointer to struct bus_type that device belongs to.
+
+dir:	       Device's driverfs directory.
+
+driver:	       Pointer to struct device_driver that controls the device.
+
+driver_data:   Driver-specific data.
+
+class_num:     Class-enumerated value of the device.
+
+platform_data: Platform data specific to the device.
+
+current_state: Current power state of the device.
+
+saved_state:   Pointer to saved state of the device. This is usable by
+	       the device driver controlling the device.
+
+release:       Callback to free the device after all references have 
+	       gone away. This should be set by the allocator of the 
+	       device (i.e. the bus driver that discovered the device).
+
+
+Programming Interface
+~~~~~~~~~~~~~~~~~~~~~
+The bus driver that discovers the device uses this to register the
+device with the core:
+
+int device_register(struct device * dev);
+
+The bus should initialize the following fields:
+
+    - parent
+    - name
+    - bus_id
+    - bus
+
+A device is removed from the core when its reference count goes to
+0. The reference count can be adjusted using:
+
+struct device * get_device(struct device * dev);
+void put_device(struct device * dev);
+
+get_device() will return a pointer to the struct device passed to it
+if the reference is not already 0 (if it's in the process of being
+removed already).
+
+A driver can take use the lock in the device structure using: 
+
+void lock_device(struct device * dev);
+void unlock_device(struct device * dev);
+
+
+Attributes
+~~~~~~~~~~
+struct device_attribute {
+        struct attribute        attr;
+        ssize_t (*show)(struct device * dev, char * buf, size_t count, loff_t off);
+        ssize_t (*store)(struct device * dev, const char * buf, size_t count, loff_t off);
+};
+
+Attributes of devices can be exported via drivers using a simple
+procfs-like interface. 
+
+Please see Documentation/filesystems/driverfs.txt for more information
+on how driverfs works.
+
+Attributes are declared using a macro called DEVICE_ATTR:
+
+#define DEVICE_ATTR(name,mode,show,store)
+
+Example:
+
+DEVICE_ATTR(power,0644,show_power,store_power);
+
+This declares a structure of type struct device_attribute named
+'dev_attr_power'. This can then be added and removed to the device's
+directory using:
+
+int device_create_file(struct device *device, struct device_attribute * entry);
+void device_remove_file(struct device * dev, struct device_attribute * attr);
+
+Example:
+
+device_create_file(dev,&dev_attr_power);
+device_remove_file(dev,&dev_attr_power);
+
+The file name will be 'power' with a mode of 0644 (-rw-r--r--).
+

Documentation/driver-model/interface.txt

+
+Device Interfaces
+
+Introduction
+~~~~~~~~~~~~
+
+Device interfaces are the logical interfaces of device classes that correlate
+directly to userspace interfaces, like device nodes. 
+   
+Each device class may have multiple interfaces through which you can 
+access the same device. An input device may support the mouse interface, 
+the 'evdev' interface, and the touchscreen interface. A SCSI disk would 
+support the disk interface, the SCSI generic interface, and possibly a raw 
+device interface. 
+
+Device interfaces are registered with the class they belong to. As devices
+are added to the class, they are added to each interface registered with
+the class. The interface is responsible for determining whether the device
+supports the interface or not. 
+
+
+Programming Interface
+~~~~~~~~~~~~~~~~~~~~~
+
+struct device_interface {
+	char			* name;
+	rwlock_t		lock;
+	u32			devnum;
+	struct device_class	* devclass;
+
+	struct list_head	node;
+	struct driver_dir_entry	dir;
+
+	int (*add_device)(struct device *);
+	int (*add_device)(struct intf_data *);
+};
+
+int interface_register(struct device_interface *);
+void interface_unregister(struct device_interface *);
+
+
+An interface must specify the device class it belongs to. It is added
+to that class's list of interfaces on registration.
+
+
+Interfaces can be added to a device class at any time. Whenever it is
+added, each device in the class is passed to the interface's
+add_device callback. When an interface is removed, each device is
+removed from the interface.
+
+
+Devices
+~~~~~~~
+Once a device is added to a device class, it is added to each
+interface that is registered with the device class. The class
+is expected to place a class-specific data structure in 
+struct device::class_data. The interface can use that (along with
+other fields of struct device) to determine whether or not the driver
+and/or device support that particular interface.
+
+
+Data
+~~~~
+
+struct intf_data {
+	struct list_head	node;
+	struct device_interface	* intf;
+	struct device 		* dev;
+	u32			intf_num;
+};
+
+int interface_add_data(struct interface_data *);
+
+The interface is responsible for allocating and initializing a struct 
+intf_data and calling interface_add_data() to add it to the device's list
+of interfaces it belongs to. This list will be iterated over when the device
+is removed from the class (instead of all possible interfaces for a class).
+This structure should probably be embedded in whatever per-device data 
+structure the interface is allocating anyway.
+   
+Devices are enumerated within the interface. This happens in interface_add_data()
+and the enumerated value is stored in the struct intf_data for that device. 
+
+driverfs
+~~~~~~~~
+Each interface is given a directory in the directory of the device
+class it belongs to:
+
+Interfaces get a directory in the class's directory as well:
+
+   class/
+   `-- input
+       |-- devices
+       |-- drivers
+       |-- mouse
+       `-- evdev
+
+When a device is added to the interface, a symlink is created that points 
+to the device's directory in the physical hierarchy:
+
+   class/
+   `-- input
+       |-- devices
+       |   `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/
+       |-- drivers
+       |   `-- usb:usb_mouse -> ../../../bus/drivers/usb_mouse/
+       |-- mouse
+       |   `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/
+       `-- evdev
+           `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/
+
+
+Future Plans
+~~~~~~~~~~~~
+A device interface is correlated directly with a userspace interface
+for a device, specifically a device node. For instance, a SCSI disk
+exposes at least two interfaces to userspace: the standard SCSI disk
+interface and the SCSI generic interface. It might also export a raw
+device interface. 
+
+Many interfaces have a major number associated with them and each
+device gets a minor number. Or, multiple interfaces might share one
+major number, and each get receive a range of minor numbers (like in
+the case of input devices).
+
+These major and minor numbers could be stored in the interface
+structure. Major and minor allocation could happen when the interface
+is registered with the class, or via a helper function. 
+

Documentation/driver-model/overview.txt

+The Linux Kernel Device Model
+
+Patrick Mochel	<mochel@osdl.org>
+
+26 August 2002
+
+
+Overview
+~~~~~~~~
+
+This driver model is a unification of all the current, disparate driver models
+that are currently in the kernel. It is intended is to augment the
+bus-specific drivers for bridges and devices by consolidating a set of data
+and operations into globally accessible data structures.
+
+Current driver models implement some sort of tree-like structure (sometimes
+just a list) for the devices they control. But, there is no linkage between
+the different bus types.
+
+A common data structure can provide this linkage with little overhead: when a
+bus driver discovers a particular device, it can insert it into the global
+tree as well as its local tree. In fact, the local tree becomes just a subset
+of the global tree.
+
+Common data fields can also be moved out of the local bus models into the
+global model. Some of the manipulation of these fields can also be
+consolidated. Most likely, manipulation functions will become a set
+of helper functions, which the bus drivers wrap around to include any
+bus-specific items.
+
+The common device and bridge interface currently reflects the goals of the
+modern PC: namely the ability to do seamless Plug and Play, power management,
+and hot plug. (The model dictated by Intel and Microsoft (read: ACPI) ensures
+us that any device in the system may fit any of these criteria.)
+
+In reality, not every bus will be able to support such operations. But, most
+buses will support a majority of those operations, and all future buses will.
+In other words, a bus that doesn't support an operation is the exception,
+instead of the other way around.
+
+
+
+Downstream Access
+~~~~~~~~~~~~~~~~~
+
+Common data fields have been moved out of individual bus layers into a common
+data structure. But, these fields must still be accessed by the bus layers,
+and sometimes by the device-specific drivers.
+
+Other bus layers are encouraged to do what has been done for the PCI layer.
+struct pci_dev now looks like this:
+
+struct pci_dev {
+	...
+
+	struct device device;
+};
+
+Note first that it is statically allocated. This means only one allocation on
+device discovery. Note also that it is at the _end_ of struct pci_dev. This is
+to make people think about what they're doing when switching between the bus
+driver and the global driver; and to prevent against mindless casts between
+the two.
+
+The PCI bus layer freely accesses the fields of struct device. It knows about
+the structure of struct pci_dev, and it should know the structure of struct
+device. PCI devices that have been converted generally do not touch the fields
+of struct device. More precisely, device-specific drivers should not touch
+fields of struct device unless there is a strong compelling reason to do so.
+
+This abstraction is prevention of unnecessary pain during transitional phases.
+If the name of the field changes or is removed, then every downstream driver
+will break. On the other hand, if only the bus layer (and not the device
+layer) accesses struct device, it is only those that need to change.
+
+
+User Interface
+~~~~~~~~~~~~~~
+
+By virtue of having a complete hierarchical view of all the devices in the
+system, exporting a complete hierarchical view to userspace becomes relatively
+easy. This has been accomplished by implementing a special purpose virtual
+file system named driverfs. It is hence possible for the user to mount the
+whole driverfs filesystem anywhere in userspace.
+
+This can be done permanently by providing the following entry into the
+/etc/fstab (under the provision that the mount point does exist, of course):
+
+none     	/devices	driverfs    defaults		0	0
+
+Or by hand on the command line:
+
+~: mount -t driverfs none /devices
+
+Whenever a device is inserted into the tree, a directory is created for it.
+This directory may be populated at each layer of discovery - the global layer,
+the bus layer, or the device layer.
+
+The global layer currently creates two files - name and 'power'. The
+former only reports the name of the device. The latter reports the
+current power state of the device. It also be used to set the current
+power state. 
+
+The bus layer may also create files for the devices it finds while probing the
+bus. For example, the PCI layer currently creates 'irq' and 'resource' files
+for each PCI device.
+
+A device-specific driver may also export files in its directory to expose
+device-specific data or tunable interfaces.
+
+More information about the driverfs directory layout can be found in
+the other documents in this directory and in the file 
+Documentation/filesystems/driverfs.txt.
+

include/linux/device.h

@@ -65,7 +65,8 @@ struct bus_type {
 	struct driver_dir_entry	device_dir;
 	struct driver_dir_entry	driver_dir;
 
-	int	(*match)	(struct device * dev, struct device_driver * drv);
+	int		(*match)(struct device * dev, struct device_driver * drv);
+	struct device * (*add)	(struct device * parent, char * bus_id);
 };
 
 
@@ -281,6 +282,8 @@ struct device {
 	void		*driver_data;	/* data private to the driver */
 
 	u32		class_num;	/* class-enumerated value */
+	void		* class_data;	/* class-specific data */
+
 	void		*platform_data;	/* Platform specific data (e.g. ACPI,
 					   BIOS data relevant to device) */