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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-06 01:02:30 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-06 01:02:30 +0000 |
commit | 76cb841cb886eef6b3bee341a2266c76578724ad (patch) | |
tree | f5892e5ba6cc11949952a6ce4ecbe6d516d6ce58 /Documentation/media/kapi/v4l2-device.rst | |
parent | Initial commit. (diff) | |
download | linux-c109f8d9e922037b3fa45f46d78384d49db8ad76.tar.xz linux-c109f8d9e922037b3fa45f46d78384d49db8ad76.zip |
Adding upstream version 4.19.249.upstream/4.19.249upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'Documentation/media/kapi/v4l2-device.rst')
-rw-r--r-- | Documentation/media/kapi/v4l2-device.rst | 144 |
1 files changed, 144 insertions, 0 deletions
diff --git a/Documentation/media/kapi/v4l2-device.rst b/Documentation/media/kapi/v4l2-device.rst new file mode 100644 index 000000000..6c58bbbaa --- /dev/null +++ b/Documentation/media/kapi/v4l2-device.rst @@ -0,0 +1,144 @@ +V4L2 device instance +-------------------- + +Each device instance is represented by a struct :c:type:`v4l2_device`. +Very simple devices can just allocate this struct, but most of the time you +would embed this struct inside a larger struct. + +You must register the device instance by calling: + + :c:func:`v4l2_device_register <v4l2_device_register>` + (dev, :c:type:`v4l2_dev <v4l2_device>`). + +Registration will initialize the :c:type:`v4l2_device` struct. If the +dev->driver_data field is ``NULL``, it will be linked to +:c:type:`v4l2_dev <v4l2_device>` argument. + +Drivers that want integration with the media device framework need to set +dev->driver_data manually to point to the driver-specific device structure +that embed the struct :c:type:`v4l2_device` instance. This is achieved by a +``dev_set_drvdata()`` call before registering the V4L2 device instance. +They must also set the struct :c:type:`v4l2_device` mdev field to point to a +properly initialized and registered :c:type:`media_device` instance. + +If :c:type:`v4l2_dev <v4l2_device>`\ ->name is empty then it will be set to a +value derived from dev (driver name followed by the bus_id, to be precise). +If you set it up before calling :c:func:`v4l2_device_register` then it will +be untouched. If dev is ``NULL``, then you **must** setup +:c:type:`v4l2_dev <v4l2_device>`\ ->name before calling +:c:func:`v4l2_device_register`. + +You can use :c:func:`v4l2_device_set_name` to set the name based on a driver +name and a driver-global atomic_t instance. This will generate names like +``ivtv0``, ``ivtv1``, etc. If the name ends with a digit, then it will insert +a dash: ``cx18-0``, ``cx18-1``, etc. This function returns the instance number. + +The first ``dev`` argument is normally the ``struct device`` pointer of a +``pci_dev``, ``usb_interface`` or ``platform_device``. It is rare for dev to +be ``NULL``, but it happens with ISA devices or when one device creates +multiple PCI devices, thus making it impossible to associate +:c:type:`v4l2_dev <v4l2_device>` with a particular parent. + +You can also supply a ``notify()`` callback that can be called by sub-devices +to notify you of events. Whether you need to set this depends on the +sub-device. Any notifications a sub-device supports must be defined in a header +in ``include/media/subdevice.h``. + +V4L2 devices are unregistered by calling: + + :c:func:`v4l2_device_unregister` + (:c:type:`v4l2_dev <v4l2_device>`). + +If the dev->driver_data field points to :c:type:`v4l2_dev <v4l2_device>`, +it will be reset to ``NULL``. Unregistering will also automatically unregister +all subdevs from the device. + +If you have a hotpluggable device (e.g. a USB device), then when a disconnect +happens the parent device becomes invalid. Since :c:type:`v4l2_device` has a +pointer to that parent device it has to be cleared as well to mark that the +parent is gone. To do this call: + + :c:func:`v4l2_device_disconnect` + (:c:type:`v4l2_dev <v4l2_device>`). + +This does *not* unregister the subdevs, so you still need to call the +:c:func:`v4l2_device_unregister` function for that. If your driver is not +hotpluggable, then there is no need to call :c:func:`v4l2_device_disconnect`. + +Sometimes you need to iterate over all devices registered by a specific +driver. This is usually the case if multiple device drivers use the same +hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv +hardware. The same is true for alsa drivers for example. + +You can iterate over all registered devices as follows: + +.. code-block:: c + + static int callback(struct device *dev, void *p) + { + struct v4l2_device *v4l2_dev = dev_get_drvdata(dev); + + /* test if this device was inited */ + if (v4l2_dev == NULL) + return 0; + ... + return 0; + } + + int iterate(void *p) + { + struct device_driver *drv; + int err; + + /* Find driver 'ivtv' on the PCI bus. + pci_bus_type is a global. For USB busses use usb_bus_type. */ + drv = driver_find("ivtv", &pci_bus_type); + /* iterate over all ivtv device instances */ + err = driver_for_each_device(drv, NULL, p, callback); + put_driver(drv); + return err; + } + +Sometimes you need to keep a running counter of the device instance. This is +commonly used to map a device instance to an index of a module option array. + +The recommended approach is as follows: + +.. code-block:: c + + static atomic_t drv_instance = ATOMIC_INIT(0); + + static int drv_probe(struct pci_dev *pdev, const struct pci_device_id *pci_id) + { + ... + state->instance = atomic_inc_return(&drv_instance) - 1; + } + +If you have multiple device nodes then it can be difficult to know when it is +safe to unregister :c:type:`v4l2_device` for hotpluggable devices. For this +purpose :c:type:`v4l2_device` has refcounting support. The refcount is +increased whenever :c:func:`video_register_device` is called and it is +decreased whenever that device node is released. When the refcount reaches +zero, then the :c:type:`v4l2_device` release() callback is called. You can +do your final cleanup there. + +If other device nodes (e.g. ALSA) are created, then you can increase and +decrease the refcount manually as well by calling: + + :c:func:`v4l2_device_get` + (:c:type:`v4l2_dev <v4l2_device>`). + +or: + + :c:func:`v4l2_device_put` + (:c:type:`v4l2_dev <v4l2_device>`). + +Since the initial refcount is 1 you also need to call +:c:func:`v4l2_device_put` in the ``disconnect()`` callback (for USB devices) +or in the ``remove()`` callback (for e.g. PCI devices), otherwise the refcount +will never reach 0. + +v4l2_device functions and data structures +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. kernel-doc:: include/media/v4l2-device.h |