V4L2 device instance
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--------------------
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Each device instance is represented by a struct :c:type:`v4l2_device`.
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Very simple devices can just allocate this struct, but most of the time you
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would embed this struct inside a larger struct.
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You must register the device instance by calling:
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:c:func:`v4l2_device_register <v4l2_device_register>`
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(dev, :c:type:`v4l2_dev <v4l2_device>`).
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Registration will initialize the :c:type:`v4l2_device` struct. If the
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dev->driver_data field is ``NULL``, it will be linked to
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:c:type:`v4l2_dev <v4l2_device>` argument.
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Drivers that want integration with the media device framework need to set
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dev->driver_data manually to point to the driver-specific device structure
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that embed the struct :c:type:`v4l2_device` instance. This is achieved by a
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``dev_set_drvdata()`` call before registering the V4L2 device instance.
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They must also set the struct :c:type:`v4l2_device` mdev field to point to a
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properly initialized and registered :c:type:`media_device` instance.
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If :c:type:`v4l2_dev <v4l2_device>`\ ->name is empty then it will be set to a
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value derived from dev (driver name followed by the bus_id, to be precise).
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If you set it up before calling :c:func:`v4l2_device_register` then it will
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be untouched. If dev is ``NULL``, then you **must** setup
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:c:type:`v4l2_dev <v4l2_device>`\ ->name before calling
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:c:func:`v4l2_device_register`.
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You can use :c:func:`v4l2_device_set_name` to set the name based on a driver
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name and a driver-global atomic_t instance. This will generate names like
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``ivtv0``, ``ivtv1``, etc. If the name ends with a digit, then it will insert
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a dash: ``cx18-0``, ``cx18-1``, etc. This function returns the instance number.
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The first ``dev`` argument is normally the ``struct device`` pointer of a
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``pci_dev``, ``usb_interface`` or ``platform_device``. It is rare for dev to
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be ``NULL``, but it happens with ISA devices or when one device creates
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multiple PCI devices, thus making it impossible to associate
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:c:type:`v4l2_dev <v4l2_device>` with a particular parent.
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You can also supply a ``notify()`` callback that can be called by sub-devices
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to notify you of events. Whether you need to set this depends on the
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sub-device. Any notifications a sub-device supports must be defined in a header
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in ``include/media/subdevice.h``.
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V4L2 devices are unregistered by calling:
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:c:func:`v4l2_device_unregister`
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(:c:type:`v4l2_dev <v4l2_device>`).
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If the dev->driver_data field points to :c:type:`v4l2_dev <v4l2_device>`,
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it will be reset to ``NULL``. Unregistering will also automatically unregister
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all subdevs from the device.
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If you have a hotpluggable device (e.g. a USB device), then when a disconnect
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happens the parent device becomes invalid. Since :c:type:`v4l2_device` has a
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pointer to that parent device it has to be cleared as well to mark that the
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parent is gone. To do this call:
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:c:func:`v4l2_device_disconnect`
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(:c:type:`v4l2_dev <v4l2_device>`).
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This does *not* unregister the subdevs, so you still need to call the
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:c:func:`v4l2_device_unregister` function for that. If your driver is not
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hotpluggable, then there is no need to call :c:func:`v4l2_device_disconnect`.
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Sometimes you need to iterate over all devices registered by a specific
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driver. This is usually the case if multiple device drivers use the same
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hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv
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hardware. The same is true for alsa drivers for example.
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You can iterate over all registered devices as follows:
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.. code-block:: c
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static int callback(struct device *dev, void *p)
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{
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struct v4l2_device *v4l2_dev = dev_get_drvdata(dev);
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/* test if this device was inited */
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if (v4l2_dev == NULL)
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return 0;
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...
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return 0;
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}
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int iterate(void *p)
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{
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struct device_driver *drv;
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int err;
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/* Find driver 'ivtv' on the PCI bus.
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pci_bus_type is a global. For USB busses use usb_bus_type. */
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drv = driver_find("ivtv", &pci_bus_type);
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/* iterate over all ivtv device instances */
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err = driver_for_each_device(drv, NULL, p, callback);
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put_driver(drv);
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return err;
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}
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Sometimes you need to keep a running counter of the device instance. This is
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commonly used to map a device instance to an index of a module option array.
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The recommended approach is as follows:
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.. code-block:: c
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static atomic_t drv_instance = ATOMIC_INIT(0);
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static int drv_probe(struct pci_dev *pdev, const struct pci_device_id *pci_id)
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{
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...
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state->instance = atomic_inc_return(&drv_instance) - 1;
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}
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If you have multiple device nodes then it can be difficult to know when it is
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safe to unregister :c:type:`v4l2_device` for hotpluggable devices. For this
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purpose :c:type:`v4l2_device` has refcounting support. The refcount is
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increased whenever :c:func:`video_register_device` is called and it is
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decreased whenever that device node is released. When the refcount reaches
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zero, then the :c:type:`v4l2_device` release() callback is called. You can
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do your final cleanup there.
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If other device nodes (e.g. ALSA) are created, then you can increase and
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decrease the refcount manually as well by calling:
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:c:func:`v4l2_device_get`
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(:c:type:`v4l2_dev <v4l2_device>`).
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or:
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:c:func:`v4l2_device_put`
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(:c:type:`v4l2_dev <v4l2_device>`).
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Since the initial refcount is 1 you also need to call
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:c:func:`v4l2_device_put` in the ``disconnect()`` callback (for USB devices)
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or in the ``remove()`` callback (for e.g. PCI devices), otherwise the refcount
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will never reach 0.
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v4l2_device functions and data structures
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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.. kernel-doc:: include/media/v4l2-device.h
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