1 2Device Drivers 3 4See the kerneldoc for the struct device_driver. 5 6 7Allocation 8~~~~~~~~~~ 9 10Device drivers are statically allocated structures. Though there may 11be multiple devices in a system that a driver supports, struct 12device_driver represents the driver as a whole (not a particular 13device instance). 14 15Initialization 16~~~~~~~~~~~~~~ 17 18The driver must initialize at least the name and bus fields. It should 19also initialize the devclass field (when it arrives), so it may obtain 20the proper linkage internally. It should also initialize as many of 21the callbacks as possible, though each is optional. 22 23Declaration 24~~~~~~~~~~~ 25 26As stated above, struct device_driver objects are statically 27allocated. Below is an example declaration of the eepro100 28driver. This declaration is hypothetical only; it relies on the driver 29being converted completely to the new model. 30 31static struct device_driver eepro100_driver = { 32 .name = "eepro100", 33 .bus = &pci_bus_type, 34 35 .probe = eepro100_probe, 36 .remove = eepro100_remove, 37 .suspend = eepro100_suspend, 38 .resume = eepro100_resume, 39}; 40 41Most drivers will not be able to be converted completely to the new 42model because the bus they belong to has a bus-specific structure with 43bus-specific fields that cannot be generalized. 44 45The most common example of this are device ID structures. A driver 46typically defines an array of device IDs that it supports. The format 47of these structures and the semantics for comparing device IDs are 48completely bus-specific. Defining them as bus-specific entities would 49sacrifice type-safety, so we keep bus-specific structures around. 50 51Bus-specific drivers should include a generic struct device_driver in 52the definition of the bus-specific driver. Like this: 53 54struct pci_driver { 55 const struct pci_device_id *id_table; 56 struct device_driver driver; 57}; 58 59A definition that included bus-specific fields would look like 60(using the eepro100 driver again): 61 62static struct pci_driver eepro100_driver = { 63 .id_table = eepro100_pci_tbl, 64 .driver = { 65 .name = "eepro100", 66 .bus = &pci_bus_type, 67 .probe = eepro100_probe, 68 .remove = eepro100_remove, 69 .suspend = eepro100_suspend, 70 .resume = eepro100_resume, 71 }, 72}; 73 74Some may find the syntax of embedded struct initialization awkward or 75even a bit ugly. So far, it's the best way we've found to do what we want... 76 77Registration 78~~~~~~~~~~~~ 79 80int driver_register(struct device_driver * drv); 81 82The driver registers the structure on startup. For drivers that have 83no bus-specific fields (i.e. don't have a bus-specific driver 84structure), they would use driver_register and pass a pointer to their 85struct device_driver object. 86 87Most drivers, however, will have a bus-specific structure and will 88need to register with the bus using something like pci_driver_register. 89 90It is important that drivers register their driver structure as early as 91possible. Registration with the core initializes several fields in the 92struct device_driver object, including the reference count and the 93lock. These fields are assumed to be valid at all times and may be 94used by the device model core or the bus driver. 95 96 97Transition Bus Drivers 98~~~~~~~~~~~~~~~~~~~~~~ 99 100By defining wrapper functions, the transition to the new model can be 101made easier. Drivers can ignore the generic structure altogether and 102let the bus wrapper fill in the fields. For the callbacks, the bus can 103define generic callbacks that forward the call to the bus-specific 104callbacks of the drivers. 105 106This solution is intended to be only temporary. In order to get class 107information in the driver, the drivers must be modified anyway. Since 108converting drivers to the new model should reduce some infrastructural 109complexity and code size, it is recommended that they are converted as 110class information is added. 111 112Access 113~~~~~~ 114 115Once the object has been registered, it may access the common fields of 116the object, like the lock and the list of devices. 117 118int driver_for_each_dev(struct device_driver * drv, void * data, 119 int (*callback)(struct device * dev, void * data)); 120 121The devices field is a list of all the devices that have been bound to 122the driver. The LDM core provides a helper function to operate on all 123the devices a driver controls. This helper locks the driver on each 124node access, and does proper reference counting on each device as it 125accesses it. 126 127 128sysfs 129~~~~~ 130 131When a driver is registered, a sysfs directory is created in its 132bus's directory. In this directory, the driver can export an interface 133to userspace to control operation of the driver on a global basis; 134e.g. toggling debugging output in the driver. 135 136A future feature of this directory will be a 'devices' directory. This 137directory will contain symlinks to the directories of devices it 138supports. 139 140 141 142Callbacks 143~~~~~~~~~ 144 145 int (*probe) (struct device * dev); 146 147The probe() entry is called in task context, with the bus's rwsem locked 148and the driver partially bound to the device. Drivers commonly use 149container_of() to convert "dev" to a bus-specific type, both in probe() 150and other routines. That type often provides device resource data, such 151as pci_dev.resource[] or platform_device.resources, which is used in 152addition to dev->platform_data to initialize the driver. 153 154This callback holds the driver-specific logic to bind the driver to a 155given device. That includes verifying that the device is present, that 156it's a version the driver can handle, that driver data structures can 157be allocated and initialized, and that any hardware can be initialized. 158Drivers often store a pointer to their state with dev_set_drvdata(). 159When the driver has successfully bound itself to that device, then probe() 160returns zero and the driver model code will finish its part of binding 161the driver to that device. 162 163A driver's probe() may return a negative errno value to indicate that 164the driver did not bind to this device, in which case it should have 165released all resources it allocated. 166 167 int (*remove) (struct device * dev); 168 169remove is called to unbind a driver from a device. This may be 170called if a device is physically removed from the system, if the 171driver module is being unloaded, during a reboot sequence, or 172in other cases. 173 174It is up to the driver to determine if the device is present or 175not. It should free any resources allocated specifically for the 176device; i.e. anything in the device's driver_data field. 177 178If the device is still present, it should quiesce the device and place 179it into a supported low-power state. 180 181 int (*suspend) (struct device * dev, pm_message_t state); 182 183suspend is called to put the device in a low power state. 184 185 int (*resume) (struct device * dev); 186 187Resume is used to bring a device back from a low power state. 188 189 190Attributes 191~~~~~~~~~~ 192struct driver_attribute { 193 struct attribute attr; 194 ssize_t (*show)(struct device_driver *driver, char *buf); 195 ssize_t (*store)(struct device_driver *, const char * buf, size_t count); 196}; 197 198Device drivers can export attributes via their sysfs directories. 199Drivers can declare attributes using a DRIVER_ATTR macro that works 200identically to the DEVICE_ATTR macro. 201 202Example: 203 204DRIVER_ATTR(debug,0644,show_debug,store_debug); 205 206This is equivalent to declaring: 207 208struct driver_attribute driver_attr_debug; 209 210This can then be used to add and remove the attribute from the 211driver's directory using: 212 213int driver_create_file(struct device_driver *, const struct driver_attribute *); 214void driver_remove_file(struct device_driver *, const struct driver_attribute *); 215