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