linux/Documentation/driver-model/driver.txt
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   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
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