[media] doc-rst: move v4l2-dev doc to a separate file

Move the documentation for video device node creation to
a separate file.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

1 files changed, 343 insertions, 0 deletions

diff --git a/Documentation/media/kapi/v4l2-dev.rst b/Documentation/media/kapi/v4l2-dev.rst
new file mode 100644
index 000000000000..f9b75d211ca0
--- /dev/null
+++ b/Documentation/media/kapi/v4l2-dev.rst
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+Video device creation
+=====================
+
+The actual device nodes in the /dev directory are created using the
+video_device struct (v4l2-dev.h). This struct can either be allocated
+dynamically or embedded in a larger struct.
+
+To allocate it dynamically use:
+
+.. code-block:: none
+
+	struct video_device *vdev = video_device_alloc();
+
+	if (vdev == NULL)
+		return -ENOMEM;
+
+	vdev->release = video_device_release;
+
+If you embed it in a larger struct, then you must set the release()
+callback to your own function:
+
+.. code-block:: none
+
+	struct video_device *vdev = &my_vdev->vdev;
+
+	vdev->release = my_vdev_release;
+
+The release callback must be set and it is called when the last user
+of the video device exits.
+
+The default video_device_release() callback just calls kfree to free the
+allocated memory.
+
+There is also a video_device_release_empty() function that does nothing
+(is empty) and can be used if the struct is embedded and there is nothing
+to do when it is released.
+
+You should also set these fields:
+
+- v4l2_dev: must be set to the v4l2_device parent device.
+
+- name: set to something descriptive and unique.
+
+- vfl_dir: set this to VFL_DIR_RX for capture devices (VFL_DIR_RX has value 0,
+  so this is normally already the default), set to VFL_DIR_TX for output
+  devices and VFL_DIR_M2M for mem2mem (codec) devices.
+
+- fops: set to the v4l2_file_operations struct.
+
+- ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
+  (highly recommended to use this and it might become compulsory in the
+  future!), then set this to your v4l2_ioctl_ops struct. The vfl_type and
+  vfl_dir fields are used to disable ops that do not match the type/dir
+  combination. E.g. VBI ops are disabled for non-VBI nodes, and output ops
+  are disabled for a capture device. This makes it possible to provide
+  just one v4l2_ioctl_ops struct for both vbi and video nodes.
+
+- lock: leave to NULL if you want to do all the locking in the driver.
+  Otherwise you give it a pointer to a struct mutex_lock and before the
+  unlocked_ioctl file operation is called this lock will be taken by the
+  core and released afterwards. See the next section for more details.
+
+- queue: a pointer to the struct vb2_queue associated with this device node.
+  If queue is non-NULL, and queue->lock is non-NULL, then queue->lock is
+  used for the queuing ioctls (VIDIOC_REQBUFS, CREATE_BUFS, QBUF, DQBUF,
+  QUERYBUF, PREPARE_BUF, STREAMON and STREAMOFF) instead of the lock above.
+  That way the vb2 queuing framework does not have to wait for other ioctls.
+  This queue pointer is also used by the vb2 helper functions to check for
+  queuing ownership (i.e. is the filehandle calling it allowed to do the
+  operation).
+
+- prio: keeps track of the priorities. Used to implement VIDIOC_G/S_PRIORITY.
+  If left to NULL, then it will use the struct v4l2_prio_state in v4l2_device.
+  If you want to have a separate priority state per (group of) device node(s),
+  then you can point it to your own struct v4l2_prio_state.
+
+- dev_parent: you only set this if v4l2_device was registered with NULL as
+  the parent device struct. This only happens in cases where one hardware
+  device has multiple PCI devices that all share the same v4l2_device core.
+
+  The cx88 driver is an example of this: one core v4l2_device struct, but
+  it is used by both a raw video PCI device (cx8800) and a MPEG PCI device
+  (cx8802). Since the v4l2_device cannot be associated with two PCI devices
+  at the same time it is setup without a parent device. But when the struct
+  video_device is initialized you *do* know which parent PCI device to use and
+  so you set dev_device to the correct PCI device.
+
+If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2
+in your v4l2_file_operations struct.
+
+Do not use .ioctl! This is deprecated and will go away in the future.
+
+In some cases you want to tell the core that a function you had specified in
+your v4l2_ioctl_ops should be ignored. You can mark such ioctls by calling this
+function before video_device_register is called:
+
+.. code-block:: none
+
+	void v4l2_disable_ioctl(struct video_device *vdev, unsigned int cmd);
+
+This tends to be needed if based on external factors (e.g. which card is
+being used) you want to turns off certain features in v4l2_ioctl_ops without
+having to make a new struct.
+
+The v4l2_file_operations struct is a subset of file_operations. The main
+difference is that the inode argument is omitted since it is never used.
+
+If integration with the media framework is needed, you must initialize the
+media_entity struct embedded in the video_device struct (entity field) by
+calling media_entity_pads_init():
+
+.. code-block:: none
+
+	struct media_pad *pad = &my_vdev->pad;
+	int err;
+
+	err = media_entity_pads_init(&vdev->entity, 1, pad);
+
+The pads array must have been previously initialized. There is no need to
+manually set the struct media_entity type and name fields.
+
+A reference to the entity will be automatically acquired/released when the
+video device is opened/closed.
+
+ioctls and locking
+------------------
+
+The V4L core provides optional locking services. The main service is the
+lock field in struct video_device, which is a pointer to a mutex. If you set
+this pointer, then that will be used by unlocked_ioctl to serialize all ioctls.
+
+If you are using the videobuf2 framework, then there is a second lock that you
+can set: video_device->queue->lock. If set, then this lock will be used instead
+of video_device->lock to serialize all queuing ioctls (see the previous section
+for the full list of those ioctls).
+
+The advantage of using a different lock for the queuing ioctls is that for some
+drivers (particularly USB drivers) certain commands such as setting controls
+can take a long time, so you want to use a separate lock for the buffer queuing
+ioctls. That way your VIDIOC_DQBUF doesn't stall because the driver is busy
+changing the e.g. exposure of the webcam.
+
+Of course, you can always do all the locking yourself by leaving both lock
+pointers at NULL.
+
+If you use the old videobuf then you must pass the video_device lock to the
+videobuf queue initialize function: if videobuf has to wait for a frame to
+arrive, then it will temporarily unlock the lock and relock it afterwards. If
+your driver also waits in the code, then you should do the same to allow other
+processes to access the device node while the first process is waiting for
+something.
+
+In the case of videobuf2 you will need to implement the wait_prepare and
+wait_finish callbacks to unlock/lock if applicable. If you use the queue->lock
+pointer, then you can use the helper functions vb2_ops_wait_prepare/finish.
+
+The implementation of a hotplug disconnect should also take the lock from
+video_device before calling v4l2_device_disconnect. If you are also using
+video_device->queue->lock, then you have to first lock video_device->queue->lock
+followed by video_device->lock. That way you can be sure no ioctl is running
+when you call v4l2_device_disconnect.
+
+video_device registration
+-------------------------
+
+Next you register the video device: this will create the character device
+for you.
+
+.. code-block:: none
+
+	err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
+	if (err) {
+		video_device_release(vdev); /* or kfree(my_vdev); */
+		return err;
+	}
+
+If the v4l2_device parent device has a non-NULL mdev field, the video device
+entity will be automatically registered with the media device.
+
+Which device is registered depends on the type argument. The following
+types exist:
+
+VFL_TYPE_GRABBER: videoX for video input/output devices
+VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
+VFL_TYPE_RADIO: radioX for radio tuners
+VFL_TYPE_SDR: swradioX for Software Defined Radio tuners
+
+The last argument gives you a certain amount of control over the device
+device node number used (i.e. the X in videoX). Normally you will pass -1
+to let the v4l2 framework pick the first free number. But sometimes users
+want to select a specific node number. It is common that drivers allow
+the user to select a specific device node number through a driver module
+option. That number is then passed to this function and video_register_device
+will attempt to select that device node number. If that number was already
+in use, then the next free device node number will be selected and it
+will send a warning to the kernel log.
+
+Another use-case is if a driver creates many devices. In that case it can
+be useful to place different video devices in separate ranges. For example,
+video capture devices start at 0, video output devices start at 16.
+So you can use the last argument to specify a minimum device node number
+and the v4l2 framework will try to pick the first free number that is equal
+or higher to what you passed. If that fails, then it will just pick the
+first free number.
+
+Since in this case you do not care about a warning about not being able
+to select the specified device node number, you can call the function
+video_register_device_no_warn() instead.
+
+Whenever a device node is created some attributes are also created for you.
+If you look in /sys/class/video4linux you see the devices. Go into e.g.
+video0 and you will see 'name', 'dev_debug' and 'index' attributes. The 'name'
+attribute is the 'name' field of the video_device struct. The 'dev_debug' attribute
+can be used to enable core debugging. See the next section for more detailed
+information on this.
+
+The 'index' attribute is the index of the device node: for each call to
+video_register_device() the index is just increased by 1. The first video
+device node you register always starts with index 0.
+
+Users can setup udev rules that utilize the index attribute to make fancy
+device names (e.g. 'mpegX' for MPEG video capture device nodes).
+
+After the device was successfully registered, then you can use these fields:
+
+- vfl_type: the device type passed to video_register_device.
+- minor: the assigned device minor number.
+- num: the device node number (i.e. the X in videoX).
+- index: the device index number.
+
+If the registration failed, then you need to call video_device_release()
+to free the allocated video_device struct, or free your own struct if the
+video_device was embedded in it. The vdev->release() callback will never
+be called if the registration failed, nor should you ever attempt to
+unregister the device if the registration failed.
+
+video device debugging
+----------------------
+
+The 'dev_debug' attribute that is created for each video, vbi, radio or swradio
+device in /sys/class/video4linux/<devX>/ allows you to enable logging of
+file operations.
+
+It is a bitmask and the following bits can be set:
+
+.. code-block:: none
+
+	0x01: Log the ioctl name and error code. VIDIOC_(D)QBUF ioctls are only logged
+	      if bit 0x08 is also set.
+	0x02: Log the ioctl name arguments and error code. VIDIOC_(D)QBUF ioctls are
+	      only logged if bit 0x08 is also set.
+	0x04: Log the file operations open, release, read, write, mmap and
+	      get_unmapped_area. The read and write operations are only logged if
+	      bit 0x08 is also set.
+	0x08: Log the read and write file operations and the VIDIOC_QBUF and
+	      VIDIOC_DQBUF ioctls.
+	0x10: Log the poll file operation.
+
+video_device cleanup
+--------------------
+
+When the video device nodes have to be removed, either during the unload
+of the driver or because the USB device was disconnected, then you should
+unregister them:
+
+.. code-block:: none
+
+	video_unregister_device(vdev);
+
+This will remove the device nodes from sysfs (causing udev to remove them
+from /dev).
+
+After video_unregister_device() returns no new opens can be done. However,
+in the case of USB devices some application might still have one of these
+device nodes open. So after the unregister all file operations (except
+release, of course) will return an error as well.
+
+When the last user of the video device node exits, then the vdev->release()
+callback is called and you can do the final cleanup there.
+
+Don't forget to cleanup the media entity associated with the video device if
+it has been initialized:
+
+.. code-block:: none
+
+	media_entity_cleanup(&vdev->entity);
+
+This can be done from the release callback.
+
+
+video_device helper functions
+-----------------------------
+
+There are a few useful helper functions:
+
+- file/video_device private data
+
+You can set/get driver private data in the video_device struct using:
+
+.. code-block:: none
+
+	void *video_get_drvdata(struct video_device *vdev);
+	void video_set_drvdata(struct video_device *vdev, void *data);
+
+Note that you can safely call video_set_drvdata() before calling
+video_register_device().
+
+And this function:
+
+.. code-block:: none
+
+	struct video_device *video_devdata(struct file *file);
+
+returns the video_device belonging to the file struct.
+
+The video_drvdata function combines video_get_drvdata with video_devdata:
+
+.. code-block:: none
+
+	void *video_drvdata(struct file *file);
+
+You can go from a video_device struct to the v4l2_device struct using:
+
+.. code-block:: none
+
+	struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
+
+- Device node name
+
+The video_device node kernel name can be retrieved using
+
+.. code-block:: none
+
+	const char *video_device_node_name(struct video_device *vdev);
+
+The name is used as a hint by userspace tools such as udev. The function
+should be used where possible instead of accessing the video_device::num and
+video_device::minor fields.
+
+video_device kAPI
+-----------------
+
+.. kernel-doc:: include/media/v4l2-dev.h