1 2Device Interfaces 3 4Introduction 5~~~~~~~~~~~~ 6 7Device interfaces are the logical interfaces of device classes that correlate 8directly to userspace interfaces, like device nodes. 9 10Each device class may have multiple interfaces through which you can 11access the same device. An input device may support the mouse interface, 12the 'evdev' interface, and the touchscreen interface. A SCSI disk would 13support the disk interface, the SCSI generic interface, and possibly a raw 14device interface. 15 16Device interfaces are registered with the class they belong to. As devices 17are added to the class, they are added to each interface registered with 18the class. The interface is responsible for determining whether the device 19supports the interface or not. 20 21 22Programming Interface 23~~~~~~~~~~~~~~~~~~~~~ 24 25struct device_interface { 26 char * name; 27 rwlock_t lock; 28 u32 devnum; 29 struct device_class * devclass; 30 31 struct list_head node; 32 struct driver_dir_entry dir; 33 34 int (*add_device)(struct device *); 35 int (*add_device)(struct intf_data *); 36}; 37 38int interface_register(struct device_interface *); 39void interface_unregister(struct device_interface *); 40 41 42An interface must specify the device class it belongs to. It is added 43to that class's list of interfaces on registration. 44 45 46Interfaces can be added to a device class at any time. Whenever it is 47added, each device in the class is passed to the interface's 48add_device callback. When an interface is removed, each device is 49removed from the interface. 50 51 52Devices 53~~~~~~~ 54Once a device is added to a device class, it is added to each 55interface that is registered with the device class. The class 56is expected to place a class-specific data structure in 57struct device::class_data. The interface can use that (along with 58other fields of struct device) to determine whether or not the driver 59and/or device support that particular interface. 60 61 62Data 63~~~~ 64 65struct intf_data { 66 struct list_head node; 67 struct device_interface * intf; 68 struct device * dev; 69 u32 intf_num; 70}; 71 72int interface_add_data(struct interface_data *); 73 74The interface is responsible for allocating and initializing a struct 75intf_data and calling interface_add_data() to add it to the device's list 76of interfaces it belongs to. This list will be iterated over when the device 77is removed from the class (instead of all possible interfaces for a class). 78This structure should probably be embedded in whatever per-device data 79structure the interface is allocating anyway. 80 81Devices are enumerated within the interface. This happens in interface_add_data() 82and the enumerated value is stored in the struct intf_data for that device. 83 84sysfs 85~~~~~ 86Each interface is given a directory in the directory of the device 87class it belongs to: 88 89Interfaces get a directory in the class's directory as well: 90 91 class/ 92 `-- input 93 |-- devices 94 |-- drivers 95 |-- mouse 96 `-- evdev 97 98When a device is added to the interface, a symlink is created that points 99to the device's directory in the physical hierarchy: 100 101 class/ 102 `-- input 103 |-- devices 104 | `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/ 105 |-- drivers 106 | `-- usb:usb_mouse -> ../../../bus/drivers/usb_mouse/ 107 |-- mouse 108 | `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/ 109 `-- evdev 110 `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/ 111 112 113Future Plans 114~~~~~~~~~~~~ 115A device interface is correlated directly with a userspace interface 116for a device, specifically a device node. For instance, a SCSI disk 117exposes at least two interfaces to userspace: the standard SCSI disk 118interface and the SCSI generic interface. It might also export a raw 119device interface. 120 121Many interfaces have a major number associated with them and each 122device gets a minor number. Or, multiple interfaces might share one 123major number, and each will receive a range of minor numbers (like in 124the case of input devices). 125 126These major and minor numbers could be stored in the interface 127structure. Major and minor allocations could happen when the interface 128is registered with the class, or via a helper function. 129 130