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