xref: /linux/Documentation/driver-api/driver-model/binding.rst (revision fe34c89d)

==============
Driver Binding
==============

Driver binding is the process of associating a device with a device
driver that can control it. Bus drivers have typically handled this
because there have been bus-specific structures to represent the
devices and the drivers. With generic device and device driver
structures, most of the binding can take place using common code.


Bus
~~~

The bus type structure contains a list of all devices that are on that bus
type in the system. When device_register is called for a device, it is
inserted into the end of this list. The bus object also contains a
list of all drivers of that bus type. When driver_register is called
for a driver, it is inserted at the end of this list. These are the
two events which trigger driver binding.


device_register
~~~~~~~~~~~~~~~

When a new device is added, the bus's list of drivers is iterated over
to find one that supports it. In order to determine that, the device
ID of the device must match one of the device IDs that the driver
supports. The format and semantics for comparing IDs is bus-specific.
Instead of trying to derive a complex state machine and matching
algorithm, it is up to the bus driver to provide a callback to compare
a device against the IDs of a driver. The bus returns 1 if a match was
found; 0 otherwise.

int match(struct device * dev, struct device_driver * drv);

If a match is found, the device's driver field is set to the driver
and the driver's probe callback is called. This gives the driver a
chance to verify that it really does support the hardware, and that
it's in a working state.

Device Class
~~~~~~~~~~~~

Upon the successful completion of probe, the device is registered with
the class to which it belongs. Device drivers belong to one and only one
class, and that is set in the driver's devclass field.
devclass_add_device is called to enumerate the device within the class
and actually register it with the class, which happens with the
class's register_dev callback.


Driver
~~~~~~

When a driver is attached to a device, the device is inserted into the
driver's list of devices.


sysfs
~~~~~

A symlink is created in the bus's 'devices' directory that points to
the device's directory in the physical hierarchy.

A symlink is created in the driver's 'devices' directory that points
to the device's directory in the physical hierarchy.

A directory for the device is created in the class's directory. A
symlink is created in that directory that points to the device's
physical location in the sysfs tree.

A symlink can be created (though this isn't done yet) in the device's
physical directory to either its class directory, or the class's
top-level directory. One can also be created to point to its driver's
directory also.


driver_register
~~~~~~~~~~~~~~~

The process is almost identical for when a new driver is added.
The bus's list of devices is iterated over to find a match. Devices
that already have a driver are skipped. All the devices are iterated
over, to bind as many devices as possible to the driver.


Removal
~~~~~~~

When a device is removed, the reference count for it will eventually
go to 0. When it does, the remove callback of the driver is called. It
is removed from the driver's list of devices and the reference count
of the driver is decremented. All symlinks between the two are removed.

When a driver is removed, the list of devices that it supports is
iterated over, and the driver's remove callback is called for each
one. The device is removed from that list and the symlinks removed.