Manual driver binding and unbinding

One new feature in the 2.6.13-rc3 kernel release, is the ability to bind and unbind drivers from devices manually from user space. Previously, the only way to disconnect a driver from a device was usually to unload the whole driver from memory, using rmmod.

In the sysfs tree, every driver now has bind and unbind files associated with it:

    $ tree /sys/bus/usb/drivers/ub/
    /sys/bus/usb/drivers/ub/
    |-- 1-1:1.0 -> ../../../../devices/pci0000:00/0000:00:1d.7/usb1/1-1/1-1:1.0
    |-- bind
    |-- module -> ../../../../module/ub
    `-- unbind

In order to unbind a device from a driver, simply write the bus id of the device to the unbind file:

    echo -n "1-1:1.0" > /sys/bus/usb/drivers/ub/unbind

and the device will no longer be bound to the driver:

    $ tree /sys/bus/usb/drivers/ub/
    /sys/bus/usb/drivers/ub/
    |-- bind
    |-- module -> ../../../../module/ub
    `-- unbind

To bind a device to a driver, the device must first not be controlled by any other driver. To ensure this, look for the "driver" symlink in the device directory:

    $ tree /sys/bus/usb/devices/1-1:1.0
    /sys/bus/usb/devices/1-1:1.0
    |-- bAlternateSetting
    |-- bInterfaceClass
    |-- bInterfaceNumber
    |-- bInterfaceProtocol
    |-- bInterfaceSubClass
    |-- bNumEndpoints
    |-- bus -> ../../../../../../bus/usb
    |-- modalias
    `-- power
        `-- state

Then, simply write the bus id of the device you wish to bind, into the bind file for that driver:

    echo -n "1-1:1.0" > /sys/bus/usb/drivers/usb-storage/bind

And check that the binding was successful:

    $ tree /sys/bus/usb/devices/1-1:1.0
    /sys/bus/usb/devices/1-1:1.0
    |-- bAlternateSetting
    |-- bInterfaceClass
    |-- bInterfaceNumber
    |-- bInterfaceProtocol
    |-- bInterfaceSubClass
    |-- bNumEndpoints
    |-- bus -> ../../../../../../bus/usb
    |-- driver -> ../../../../../../bus/usb/drivers/usb-storage
    |-- host2
    |   `-- power
    |       `-- state
    |-- modalias
    `-- power
        `-- state

As the example above shows, this capability is very useful for switching devices between drivers which handle the same type of device (both the ub and usb-storage drivers handle USB mass storage devices, like flash drives.)

A number of "enterprise" Linux distributions offer multiple drivers of different version levels in their kernel packages. This manual binding feature will allow configuration tools to pick and choose which devices should be bound to which drivers, allowing users to upgrade only specific devices if they wish to.

In order for a device to bind successfully with a driver, that driver must already support that device. This is why you can not just arbitrarily bind any device to any driver. To help with the issue of adding new devices support to drivers after they are built, the PCI system offers a dynamic_id file in sysfs so that user space can write in new device ids that the driver should bind too. In the future, this ability to add new driver IDs to a running kernel will be moved into the driver core to make it available for all buses.

Index entries for this article
GuestArticles	Kroah-Hartman, Greg

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