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+
+Porting Drivers to the New Driver Model
+
+Patrick Mochel
+
+7 January 2003
+
+
+Overview
+
+Please refer to Documentation/driver-model/*.txt for definitions of
+various driver types and concepts. 
+
+Most of the work of porting devices drivers to the new model happens
+at the bus driver layer. This was intentional, to minimize the
+negative effect on kernel drivers, and to allow a gradual transition
+of bus drivers.
+
+In a nutshell, the driver model consists of a set of objects that can
+be embedded in larger, bus-specific objects. Fields in these generic
+objects can replace fields in the bus-specific objects. 
+
+The generic objects must be registered with the driver model core. By
+doing so, they will exported via the sysfs filesystem. sysfs can be
+mounted by doing 
+
+        # mount -t sysfs sysfs /sys
+
+
+
+The Process
+
+Step 0: Read include/linux/device.h for object and function definitions. 
+
+Step 1: Registering the bus driver. 
+
+
+- Define a struct bus_type for the bus driver.
+
+struct bus_type pci_bus_type = {
+        .name           = "pci",
+};
+
+
+- Register the bus type.
+  This should be done in the initialization function for the bus type,
+  which is usually the module_init(), or equivalent, function. 
+
+static int __init pci_driver_init(void)
+{
+        return bus_register(&pci_bus_type);
+}
+
+subsys_initcall(pci_driver_init);
+
+
+  The bus type may be unregistered (if the bus driver may be compiled
+  as a module) by doing:
+
+     bus_unregister(&pci_bus_type);
+
+
+- Export the bus type for others to use. 
+
+  Other code may wish to reference the bus type, so declare it in a 
+  shared header file and export the symbol.
+
+From include/linux/pci.h:
+
+extern struct bus_type pci_bus_type;
+
+
+From file the above code appears in:
+
+EXPORT_SYMBOL(pci_bus_type);
+
+
+
+- This will cause the bus to show up in /sys/bus/pci/ with two
+  subdirectories: 'devices' and 'drivers'.
+
+# tree -d /sys/bus/pci/
+/sys/bus/pci/
+|-- devices
+`-- drivers
+
+
+
+Step 2: Registering Devices. 
+
+struct device represents a single device. It mainly contains metadata
+describing the relationship the device has to other entities. 
+
+
+- Embedd a struct device in the bus-specific device type. 
+
+
+struct pci_dev {
+       ...
+       struct  device  dev;            /* Generic device interface */
+       ...
+};
+
+  It is recommended that the generic device not be the first item in 
+  the struct to discourage programmers from doing mindless casts
+  between the object types. Instead macros, or inline functions,
+  should be created to convert from the generic object type.
+
+
+#define to_pci_dev(n) container_of(n, struct pci_dev, dev)
+
+or 
+
+static inline struct pci_dev * to_pci_dev(struct kobject * kobj)
+{
+        return container_of(n, struct pci_dev, dev);
+}
+
+  This allows the compiler to verify type-safety of the operations 
+  that are performed (which is Good).
+
+
+- Initialize the device on registration.
+
+  When devices are discovered or registered with the bus type, the 
+  bus driver should initialize the generic device. The most important
+  things to initialize are the bus_id, parent, and bus fields.
+
+  The bus_id is an ASCII string that contains the device's address on
+  the bus. The format of this string is bus-specific. This is
+  necessary for representing devices in sysfs. 
+
+  parent is the physical parent of the device. It is important that
+  the bus driver sets this field correctly. 
+
+  The driver model maintains an ordered list of devices that it uses
+  for power management. This list must be in order to guarantee that
+  devices are shutdown before their physical parents, and vice versa.
+  The order of this list is determined by the parent of registered
+  devices.
+
+  Also, the location of the device's sysfs directory depends on a
+  device's parent. sysfs exports a directory structure that mirrors 
+  the device hierarchy. Accurately setting the parent guarantees that
+  sysfs will accurately represent the hierarchy.
+
+  The device's bus field is a pointer to the bus type the device
+  belongs to. This should be set to the bus_type that was declared
+  and initialized before. 
+
+  Optionally, the bus driver may set the device's name and release
+  fields.
+
+  The name field is an ASCII string describing the device, like
+
+     "ATI Technologies Inc Radeon QD"
+
+  The release field is a callback that the driver model core calls 
+  when the device has been removed, and all references to it have 
+  been released. More on this in a moment.
+
+
+- Register the device. 
+
+  Once the generic device has been initialized, it can be registered
+  with the driver model core by doing:
+
+       device_register(&dev->dev);
+
+  It can later be unregistered by doing: 
+
+       device_unregister(&dev->dev);
+
+  This should happen on buses that support hotpluggable devices. 
+  If a bus driver unregisters a device, it should not immediately free
+  it. It should instead wait for the driver model core to call the 
+  device's release method, then free the bus-specific object. 
+  (There may be other code that is currently referencing the device
+  structure, and it would be rude to free the device while that is 
+  happening).
+
+
+  When the device is registered, a directory in sysfs is created. 
+  The PCI tree in sysfs looks like: 
+
+/sys/devices/pci0/
+|-- 00:00.0
+|-- 00:01.0
+|   `-- 01:00.0
+|-- 00:02.0
+|   `-- 02:1f.0
+|       `-- 03:00.0
+|-- 00:1e.0
+|   `-- 04:04.0
+|-- 00:1f.0
+|-- 00:1f.1
+|   |-- ide0
+|   |   |-- 0.0
+|   |   `-- 0.1
+|   `-- ide1
+|       `-- 1.0
+|-- 00:1f.2
+|-- 00:1f.3
+`-- 00:1f.5
+
+  Also, symlinks are created in the bus's 'devices' directory
+  that point to the device's directory in the physical hierarchy. 
+
+/sys/bus/pci/devices/
+|-- 00:00.0 -> ../../../devices/pci0/00:00.0
+|-- 00:01.0 -> ../../../devices/pci0/00:01.0
+|-- 00:02.0 -> ../../../devices/pci0/00:02.0
+|-- 00:1e.0 -> ../../../devices/pci0/00:1e.0
+|-- 00:1f.0 -> ../../../devices/pci0/00:1f.0
+|-- 00:1f.1 -> ../../../devices/pci0/00:1f.1
+|-- 00:1f.2 -> ../../../devices/pci0/00:1f.2
+|-- 00:1f.3 -> ../../../devices/pci0/00:1f.3
+|-- 00:1f.5 -> ../../../devices/pci0/00:1f.5
+|-- 01:00.0 -> ../../../devices/pci0/00:01.0/01:00.0
+|-- 02:1f.0 -> ../../../devices/pci0/00:02.0/02:1f.0
+|-- 03:00.0 -> ../../../devices/pci0/00:02.0/02:1f.0/03:00.0
+`-- 04:04.0 -> ../../../devices/pci0/00:1e.0/04:04.0
+
+
+
+Step 3: Registering Drivers.
+
+struct device_driver is a simple driver structure that contains a set
+of operations that the driver model core may call. 
+
+
+- Embed a struct device_driver in the bus-specific driver. 
+
+  Just like with devices, do something like:
+
+struct pci_driver {
+       ...
+       struct device_driver    driver;
+};
+
+
+- Initialize the generic driver structure. 
+
+  When the driver registers with the bus (e.g. doing pci_register_driver()),
+  initialize the necessary fields of the driver: the name and bus
+  fields. 
+
+
+- Register the driver.
+
+  After the generic driver has been initialized, call
+
+        driver_register(&drv->driver);
+
+  to register the driver with the core.
+
+  When the driver is unregistered from the bus, unregister it from the
+  core by doing:
+
+        driver_unregister(&drv->driver);
+
+  Note that this will block until all references to the driver have
+  gone away. Normally, there will not be any.
+
+
+- Sysfs representation.
+
+  Drivers are exported via sysfs in their bus's 'driver's directory. 
+  For example:
+
+/sys/bus/pci/drivers/
+|-- 3c59x
+|-- Ensoniq AudioPCI
+|-- agpgart-amdk7
+|-- e100
+`-- serial
+
+
+Step 4: Define Generic Methods for Drivers.
+
+struct device_driver defines a set of operations that the driver model
+core calls. Most of these operations are probably similar to
+operations the bus already defines for drivers, but taking different
+parameters. 
+
+It would be difficult and tedious to force every driver on a bus to
+simultaneously convert their drivers to generic format. Instead, the
+bus driver should define single instances of the generic methods that
+forward call to the bus-specific drivers. For instance: 
+
+
+static int pci_device_remove(struct device * dev)
+{
+        struct pci_dev * pci_dev = to_pci_dev(dev);
+        struct pci_driver * drv = pci_dev->driver;
+
+        if (drv) {
+                if (drv->remove)
+                        drv->remove(pci_dev);
+                pci_dev->driver = NULL;
+        }
+        return 0;
+}
+
+
+The generic driver should be initialized with these methods before it
+is registered. 
+
+        /* initialize common driver fields */
+        drv->driver.name = drv->name;
+        drv->driver.bus = &pci_bus_type;
+        drv->driver.probe = pci_device_probe;
+        drv->driver.resume = pci_device_resume;
+        drv->driver.suspend = pci_device_suspend;
+        drv->driver.remove = pci_device_remove;
+
+        /* register with core */
+        driver_register(&drv->driver);
+
+
+Ideally, the bus should only initialize the fields if they are not
+already set. This allows the drivers to implement their own generic
+methods. 
+
+
+Step 5: Support generic driver binding. 
+
+The model assumes that a device or driver can be dynamically
+registered with the bus at any time. When registration happens,
+devices must be bound to a driver, or drivers must be bound to all
+devices that it supports. 
+
+A driver typically contains a list of device IDs that it supports. The
+bus driver compares these IDs to the IDs of devices registered with it. 
+The format of the device IDs, and the semantics for comparing them are
+bus-specific, so the generic model does attempt to generalize them. 
+
+Instead, a bus may supply a method in struct bus_type that does the
+comparison: 
+
+  int (*match)(struct device * dev, struct device_driver * drv);
+
+match should return '1' if the driver supports the device, and '0'
+otherwise. 
+
+When a device is registered, the bus's list of drivers is iterated
+over. bus->match() is called for each one until a match is found. 
+
+When a driver is registered, the bus's list of devices is iterated
+over. bus->match() is called for each device that is not already
+claimed by a driver. 
+
+When a device is successfully bound to a device, device->driver is
+set, the device is added to a per-driver list of devices, and a
+symlink is created in the driver's sysfs directory that points to the
+device's physical directory:
+
+/sys/bus/pci/drivers/
+|-- 3c59x
+|   `-- 00:0b.0 -> ../../../../devices/pci0/00:0b.0
+|-- Ensoniq AudioPCI
+|-- agpgart-amdk7
+|   `-- 00:00.0 -> ../../../../devices/pci0/00:00.0
+|-- e100
+|   `-- 00:0c.0 -> ../../../../devices/pci0/00:0c.0
+`-- serial
+
+
+This driver binding should replace the existing driver binding
+mechanism the bus currently uses. 
+
+
+Step 6: Supply a hotplug callback.
+
+Whenever a device is registered with the driver model core, the
+userspace program /sbin/hotplug is called to notify userspace. 
+Users can define actions to perform when a device is inserted or
+removed. 
+
+The driver model core passes several arguments to userspace via
+environment variables, including
+
+- ACTION: set to 'add' or 'remove'
+- DEVPATH: set to the device's physical path in sysfs. 
+
+A bus driver may also supply additional parameters for userspace to
+consume. To do this, a bus must implement the 'hotplug' method in
+struct bus_type:
+
+     int (*hotplug) (struct device *dev, char **envp, 
+                     int num_envp, char *buffer, int buffer_size);
+
+This is called immediately before /sbin/hotplug is executed. 
+
+
+Step 7: Cleaning up the bus driver.
+
+The generic bus, device, and driver structures provide several fields
+that can replace those defined privately to the bus driver. 
+
+- Device list.
+
+struct bus_type contains a list of all devices registered with the bus
+type. This includes all devices on all instances of that bus type.
+An internal list that the bus uses may be removed, in favor of using
+this one.
+
+The core provides an iterator to access these devices. 
+
+int bus_for_each_dev(struct bus_type * bus, struct device * start, 
+                     void * data, int (*fn)(struct device *, void *));
+
+
+- Driver list.
+
+struct bus_type also contains a list of all drivers registered with
+it. An internal list of drivers that the bus driver maintains may 
+be removed in favor of using the generic one. 
+
+The drivers may be iterated over, like devices: 
+
+int bus_for_each_drv(struct bus_type * bus, struct device_driver * start,
+                     void * data, int (*fn)(struct device_driver *, void *));
+
+
+Please see drivers/base/bus.c for more information.
+
+
+- rwsem 
+
+struct bus_type contains an rwsem that protects all core accesses to
+the device and driver lists. This can be used by the bus driver
+internally, and should be used when accessing the device or driver
+lists the bus maintains. 
+
+
+- Device and driver fields. 
+
+Some of the fields in struct device and struct device_driver duplicate
+fields in the bus-specific representations of these objects. Feel free
+to remove the bus-specific ones and favor the generic ones. Note
+though, that this will likely mean fixing up all the drivers that
+reference the bus-specific fields (though those should all be 1-line
+changes).
+