103 lines
3.6 KiB
Text
103 lines
3.6 KiB
Text
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Driver Binding
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Driver binding is the process of associating a device with a device
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driver that can control it. Bus drivers have typically handled this
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because there have been bus-specific structures to represent the
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devices and the drivers. With generic device and device driver
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structures, most of the binding can take place using common code.
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Bus
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~~~
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The bus type structure contains a list of all devices that are on that bus
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type in the system. When device_register is called for a device, it is
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inserted into the end of this list. The bus object also contains a
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list of all drivers of that bus type. When driver_register is called
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for a driver, it is inserted at the end of this list. These are the
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two events which trigger driver binding.
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device_register
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~~~~~~~~~~~~~~~
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When a new device is added, the bus's list of drivers is iterated over
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to find one that supports it. In order to determine that, the device
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ID of the device must match one of the device IDs that the driver
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supports. The format and semantics for comparing IDs is bus-specific.
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Instead of trying to derive a complex state machine and matching
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algorithm, it is up to the bus driver to provide a callback to compare
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a device against the IDs of a driver. The bus returns 1 if a match was
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found; 0 otherwise.
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int match(struct device * dev, struct device_driver * drv);
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If a match is found, the device's driver field is set to the driver
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and the driver's probe callback is called. This gives the driver a
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chance to verify that it really does support the hardware, and that
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it's in a working state.
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Device Class
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~~~~~~~~~~~~
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Upon the successful completion of probe, the device is registered with
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the class to which it belongs. Device drivers belong to one and only one
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class, and that is set in the driver's devclass field.
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devclass_add_device is called to enumerate the device within the class
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and actually register it with the class, which happens with the
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class's register_dev callback.
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NOTE: The device class structures and core routines to manipulate them
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are not in the mainline kernel, so the discussion is still a bit
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speculative.
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Driver
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~~~~~~
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When a driver is attached to a device, the device is inserted into the
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driver's list of devices.
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sysfs
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~~~~~
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A symlink is created in the bus's 'devices' directory that points to
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the device's directory in the physical hierarchy.
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A symlink is created in the driver's 'devices' directory that points
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to the device's directory in the physical hierarchy.
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A directory for the device is created in the class's directory. A
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symlink is created in that directory that points to the device's
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physical location in the sysfs tree.
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A symlink can be created (though this isn't done yet) in the device's
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physical directory to either its class directory, or the class's
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top-level directory. One can also be created to point to its driver's
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directory also.
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driver_register
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~~~~~~~~~~~~~~~
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The process is almost identical for when a new driver is added.
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The bus's list of devices is iterated over to find a match. Devices
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that already have a driver are skipped. All the devices are iterated
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over, to bind as many devices as possible to the driver.
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Removal
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~~~~~~~
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When a device is removed, the reference count for it will eventually
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go to 0. When it does, the remove callback of the driver is called. It
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is removed from the driver's list of devices and the reference count
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of the driver is decremented. All symlinks between the two are removed.
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When a driver is removed, the list of devices that it supports is
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iterated over, and the driver's remove callback is called for each
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one. The device is removed from that list and the symlinks removed.
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