src/kernel/linux/v4.19/Documentation/driver-model/driver.txt

Device Drivers

See the kerneldoc for the struct device_driver.


Allocation
~~~~~~~~~~

Device drivers are statically allocated structures. Though there may
be multiple devices in a system that a driver supports, struct
device_driver represents the driver as a whole (not a particular
device instance).

Initialization
~~~~~~~~~~~~~~

The driver must initialize at least the name and bus fields. It should
also initialize the devclass field (when it arrives), so it may obtain
the proper linkage internally. It should also initialize as many of
the callbacks as possible, though each is optional.

Declaration
~~~~~~~~~~~

As stated above, struct device_driver objects are statically
allocated. Below is an example declaration of the eepro100
driver. This declaration is hypothetical only; it relies on the driver
being converted completely to the new model. 

static struct device_driver eepro100_driver = {
       .name		= "eepro100",
       .bus		= &pci_bus_type,
       
       .probe		= eepro100_probe,
       .remove		= eepro100_remove,
       .suspend		= eepro100_suspend,
       .resume		= eepro100_resume,
};

Most drivers will not be able to be converted completely to the new
model because the bus they belong to has a bus-specific structure with
bus-specific fields that cannot be generalized. 

The most common example of this are device ID structures. A driver
typically defines an array of device IDs that it supports. The format
of these structures and the semantics for comparing device IDs are
completely bus-specific. Defining them as bus-specific entities would
sacrifice type-safety, so we keep bus-specific structures around. 

Bus-specific drivers should include a generic struct device_driver in
the definition of the bus-specific driver. Like this:

struct pci_driver {
       const struct pci_device_id *id_table;
       struct device_driver	  driver;
};

A definition that included bus-specific fields would look like
(using the eepro100 driver again):

static struct pci_driver eepro100_driver = {
       .id_table       = eepro100_pci_tbl,
       .driver	       = {
		.name		= "eepro100",
		.bus		= &pci_bus_type,
		.probe		= eepro100_probe,
		.remove		= eepro100_remove,
		.suspend	= eepro100_suspend,
		.resume		= eepro100_resume,
       },
};

Some may find the syntax of embedded struct initialization awkward or
even a bit ugly. So far, it's the best way we've found to do what we want...

Registration
~~~~~~~~~~~~

int driver_register(struct device_driver * drv);

The driver registers the structure on startup. For drivers that have
no bus-specific fields (i.e. don't have a bus-specific driver
structure), they would use driver_register and pass a pointer to their
struct device_driver object. 

Most drivers, however, will have a bus-specific structure and will
need to register with the bus using something like pci_driver_register.

It is important that drivers register their driver structure as early as
possible. Registration with the core initializes several fields in the
struct device_driver object, including the reference count and the
lock. These fields are assumed to be valid at all times and may be
used by the device model core or the bus driver.


Transition Bus Drivers
~~~~~~~~~~~~~~~~~~~~~~

By defining wrapper functions, the transition to the new model can be
made easier. Drivers can ignore the generic structure altogether and
let the bus wrapper fill in the fields. For the callbacks, the bus can
define generic callbacks that forward the call to the bus-specific
callbacks of the drivers. 

This solution is intended to be only temporary. In order to get class
information in the driver, the drivers must be modified anyway. Since
converting drivers to the new model should reduce some infrastructural
complexity and code size, it is recommended that they are converted as
class information is added.

Access
~~~~~~

Once the object has been registered, it may access the common fields of
the object, like the lock and the list of devices. 

int driver_for_each_dev(struct device_driver * drv, void * data, 
		        int (*callback)(struct device * dev, void * data));

The devices field is a list of all the devices that have been bound to
the driver. The LDM core provides a helper function to operate on all
the devices a driver controls. This helper locks the driver on each
node access, and does proper reference counting on each device as it
accesses it. 


sysfs
~~~~~

When a driver is registered, a sysfs directory is created in its
bus's directory. In this directory, the driver can export an interface
to userspace to control operation of the driver on a global basis;
e.g. toggling debugging output in the driver.

A future feature of this directory will be a 'devices' directory. This
directory will contain symlinks to the directories of devices it
supports.



Callbacks
~~~~~~~~~

	int	(*probe)	(struct device * dev);

The probe() entry is called in task context, with the bus's rwsem locked
and the driver partially bound to the device.  Drivers commonly use
container_of() to convert "dev" to a bus-specific type, both in probe()
and other routines.  That type often provides device resource data, such
as pci_dev.resource[] or platform_device.resources, which is used in
addition to dev->platform_data to initialize the driver.

This callback holds the driver-specific logic to bind the driver to a
given device.  That includes verifying that the device is present, that
it's a version the driver can handle, that driver data structures can
be allocated and initialized, and that any hardware can be initialized.
Drivers often store a pointer to their state with dev_set_drvdata().
When the driver has successfully bound itself to that device, then probe()
returns zero and the driver model code will finish its part of binding
the driver to that device.

A driver's probe() may return a negative errno value to indicate that
the driver did not bind to this device, in which case it should have
released all resources it allocated.

	void (*sync_state)(struct device *dev);

sync_state is called only once for a device. It's called when all the consumer
devices of the device have successfully probed. The list of consumers of the
device is obtained by looking at the device links connecting that device to its
consumer devices.

The first attempt to call sync_state() is made during late_initcall_sync() to
give firmware and drivers time to link devices to each other. During the first
attempt at calling sync_state(), if all the consumers of the device at that
point in time have already probed successfully, sync_state() is called right
away. If there are no consumers of the device during the first attempt, that
too is considered as "all consumers of the device have probed" and sync_state()
is called right away.

If during the first attempt at calling sync_state() for a device, there are
still consumers that haven't probed successfully, the sync_state() call is
postponed and reattempted in the future only when one or more consumers of the
device probe successfully. If during the reattempt, the driver core finds that
there are one or more consumers of the device that haven't probed yet, then
sync_state() call is postponed again.

A typical use case for sync_state() is to have the kernel cleanly take over
management of devices from the bootloader. For example, if a device is left on
and at a particular hardware configuration by the bootloader, the device's
driver might need to keep the device in the boot configuration until all the
consumers of the device have probed. Once all the consumers of the device have
probed, the device's driver can synchronize the hardware state of the device to
match the aggregated software state requested by all the consumers. Hence the
name sync_state().

While obvious examples of resources that can benefit from sync_state() include
resources such as regulator, sync_state() can also be useful for complex
resources like IOMMUs. For example, IOMMUs with multiple consumers (devices
whose addresses are remapped by the IOMMU) might need to keep their mappings
fixed at (or additive to) the boot configuration until all its consumers have
probed.

While the typical use case for sync_state() is to have the kernel cleanly take
over management of devices from the bootloader, the usage of sync_state() is
not restricted to that. Use it whenever it makes sense to take an action after
all the consumers of a device have probed.

	int 	(*remove)	(struct device * dev);

remove is called to unbind a driver from a device. This may be
called if a device is physically removed from the system, if the
driver module is being unloaded, during a reboot sequence, or
in other cases.

It is up to the driver to determine if the device is present or
not. It should free any resources allocated specifically for the
device; i.e. anything in the device's driver_data field. 

If the device is still present, it should quiesce the device and place
it into a supported low-power state.

	int	(*suspend)	(struct device * dev, pm_message_t state);

suspend is called to put the device in a low power state.

	int	(*resume)	(struct device * dev);

Resume is used to bring a device back from a low power state.


Attributes
~~~~~~~~~~
struct driver_attribute {
        struct attribute        attr;
        ssize_t (*show)(struct device_driver *driver, char *buf);
        ssize_t (*store)(struct device_driver *, const char * buf, size_t count);
};

Device drivers can export attributes via their sysfs directories. 
Drivers can declare attributes using a DRIVER_ATTR_RW and DRIVER_ATTR_RO
macro that works identically to the DEVICE_ATTR_RW and DEVICE_ATTR_RO
macros.

Example:

DRIVER_ATTR_RW(debug);

This is equivalent to declaring:

struct driver_attribute driver_attr_debug;

This can then be used to add and remove the attribute from the
driver's directory using:

int driver_create_file(struct device_driver *, const struct driver_attribute *);
void driver_remove_file(struct device_driver *, const struct driver_attribute *);