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