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[media] v4l2-dev: add cross-references and improve markup

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Add cross-references for the functions/structs and add
the markup tags to improve its display.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
This commit is contained in:
Mauro Carvalho Chehab 2016-07-21 17:05:35 -03:00
parent 81d866fdcd
commit 243b6935ae
3 changed files with 181 additions and 157 deletions
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334
Documentation/media/kapi/v4l2-dev.rst
View File

@ -1,13 +1,13 @@
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.
The actual device nodes in the ``/dev`` directory are created using the
:c:type:`video_device` struct (``v4l2-dev.h``). This struct can either be
allocated dynamically or embedded in a larger struct.
To allocate it dynamically use:
To allocate it dynamically use :cpp:func:`video_device_alloc`:
.. code-block:: none
.. code-block:: c
struct video_device *vdev = video_device_alloc();
@ -16,100 +16,110 @@ To allocate it dynamically use:
vdev->release = video_device_release;
If you embed it in a larger struct, then you must set the release()
If you embed it in a larger struct, then you must set the ``release()``
callback to your own function:
.. code-block:: none
.. code-block:: c
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
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.
The default :cpp:func:`video_device_release` callback currently
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.
There is also a ::cpp:func:`video_device_release_empty` function that does
nothing (is empty) and should be used if the struct is embedded and there
is nothing to do when it is released.
You should also set these fields:
You should also set these fields of :c:type:`video_device`:
- v4l2_dev: must be set to the v4l2_device parent device.
- :c:type:`video_device`->v4l2_dev: must be set to the :c:type:`v4l2_device`
parent device.
- name: set to something descriptive and unique.
- :c:type:`video_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.
- :c:type:`video_device`->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.
- :c:type:`video_device`->fops: set to the :c:type:`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.
- :c:type:`video_device`->ioctl_ops: if you use the :c:type:`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
:c:type:`v4l2_ioctl_ops` struct. The :c:type:`video_device`->vfl_type and
:c:type:`video_device`->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 :c:type:`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.
- :c:type:`video_device`->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 :c:type:`video_device`->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
- :c:type:`video_device`->queue: a pointer to the struct :c:type:`vb2_queue`
associated with this device node.
If queue is not ``NULL``, and queue->lock is not ``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 :ref:`vb2 <vb2_framework>` queuing framework does not have
to wait for other ioctls. This queue pointer is also used by the
:ref:`vb2 <vb2_framework>` 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.
- :c:type:`video_device`->prio: keeps track of the priorities. Used to
implement ``VIDIOC_G_PRIORITY`` and ``VIDIOC_S_PRIORITY``.
If left to ``NULL``, then it will use the struct :c:type:`v4l2_prio_state`
in :c:type:`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
:c:type:`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.
- :c:type:`video_device`->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 :c:type:`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.
The cx88 driver is an example of this: one core :c:type:`v4l2_device` struct,
but it is used by both a raw video PCI device (cx8800) and a MPEG PCI device
(cx8802). Since the :c:type:`v4l2_device` cannot be associated with two PCI
devices at the same time it is setup without a parent device. But when the
struct :c:type:`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.
If you use :c:type:`v4l2_ioctl_ops`, then you should set
:c:type:`video_device`->unlocked_ioctl to :cpp:func:`video_ioctl2` in your
:c:type:`v4l2_file_operations` struct.
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:
your :c:type:`v4l2_ioctl_ops` should be ignored. You can mark such ioctls by
calling this function before :cpp:func:`video_register_device` is called:
.. code-block:: none
void v4l2_disable_ioctl(struct video_device *vdev, unsigned int cmd);
:cpp:func:`v4l2_disable_ioctl <v4l2_disable_ioctl>`
(:c:type:`vdev <video_device>`, 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.
being used) you want to turns off certain features in :c:type:`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.
The :c:type:`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():
:c:type:`media_entity` struct embedded in the :c:type:`video_device` struct
(entity field) by calling :cpp:func:`media_entity_pads_init`:
.. code-block:: none
.. code-block:: c
struct media_pad *pad = &my_vdev->pad;
int err;
@ -126,47 +136,52 @@ 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.
lock field in struct :c:type:`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
If you are using the :ref:`videobuf2 framework <vb2_framework>`, then there
is a second lock that you can set: :c:type:`video_device`->queue->lock. If
set, then this lock will be used instead of :c:type:`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
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.
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
If you use the old :ref:`videobuf framework <vb_framework>` then you must
pass the :c:type:`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.
In the case of :ref:`videobuf2 <vb2_framework>` 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
:cpp:func:`vb2_ops_wait_prepare` and :cpp:func:`vb2_ops_wait_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.
:c:type:`video_device` before calling v4l2_device_disconnect. If you are also
using :c:type:`video_device`->queue->lock, then you have to first lock
:c:type:`video_device`->queue->lock followed by :c:type:`video_device`->lock.
That way you can be sure no ioctl is running when you call
:c:type:`v4l2_device_disconnect`.
video_device registration
Video device registration
-------------------------
Next you register the video device: this will create the character device
for you.
Next you register the video device with :cpp:func:`video_register_device`.
This will create the character device for you.
.. code-block:: none
.. code-block:: c
err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
if (err) {
@ -174,19 +189,20 @@ for you.
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.
If the :c:type:`v4l2_device` parent device has a not ``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
- ``VFL_TYPE_GRABBER``: ``/dev/videoX`` for video input/output devices
- ``VFL_TYPE_VBI``: ``/dev/vbiX`` for vertical blank data (i.e. closed captions, teletext)
- ``VFL_TYPE_RADIO``: ``/dev/radioX`` for radio tuners
- ``VFL_TYPE_SDR``: ``/dev/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
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
@ -205,85 +221,90 @@ 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.
:cpp:func:`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.
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.
:cpp:func:`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).
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.
- :c:type:`video_device`->vfl_type: the device type passed to
:cpp:func:`video_register_device`.
- :c:type:`video_device`->minor: the assigned device minor number.
- :c:type:`video_device`->num: the device node number (i.e. the X in
``videoX``).
- :c:type:`video_device`->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.
If the registration failed, then you need to call
:cpp:func:`video_device_release` to free the allocated :c:type:`video_device`
struct, or free your own struct if the :c:type:`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
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.
===== ================================================================
Mask Description
===== ================================================================
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
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:
unregister them with:
.. code-block:: none
video_unregister_device(vdev);
:cpp:func:`video_unregister_device`
(:c:type:`vdev <video_device>`);
This will remove the device nodes from sysfs (causing udev to remove them
from /dev).
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
After :cpp:func:`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()
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);
:cpp:func:`media_entity_cleanup <media_entity_cleanup>`
(&vdev->entity);
This can be done from the release callback.
@ -293,45 +314,44 @@ video_device helper functions
There are a few useful helper functions:
- file/video_device private data
- file and :c:type:`video_device` private data
You can set/get driver private data in the video_device struct using:
.. code-block:: none
:cpp:func:`video_get_drvdata <video_get_drvdata>`
(:c:type:`vdev <video_device>`);
void *video_get_drvdata(struct video_device *vdev);
void video_set_drvdata(struct video_device *vdev, void *data);
:cpp:func:`video_set_drvdata <video_set_drvdata>`
(:c:type:`vdev <video_device>`);
Note that you can safely call video_set_drvdata() before calling
video_register_device().
Note that you can safely call :cpp:func:`video_set_drvdata` before calling
:cpp:func:`video_register_device`.
And this function:
.. code-block:: none
struct video_device *video_devdata(struct file *file);
:cpp:func:`video_devdata <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:
The :cpp:func:`video_devdata` function combines :cpp:func:`video_get_drvdata`
with :cpp:func:`video_devdata`:
.. code-block:: none
:cpp:func:`video_drvdata <video_drvdata>`
(struct file \*file);
void *video_drvdata(struct file *file);
You can go from a :c:type:`video_device` struct to the v4l2_device struct using:
You can go from a video_device struct to the v4l2_device struct using:
.. code-block:: none
.. code-block:: c
struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
- Device node name
The video_device node kernel name can be retrieved using
The :c:type:`video_device` node kernel name can be retrieved using:
.. code-block:: none
const char *video_device_node_name(struct video_device *vdev);
:cpp:func:`video_device_node_name <video_device_node_name>`
(:c:type:`vdev <video_device>`);
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

2
Documentation/media/kapi/v4l2-videobuf.rst
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@ -1,3 +1,5 @@
.. _vb_framework:
Videobuf Framework
==================

2
Documentation/media/kapi/v4l2-videobuf2.rst
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@ -1,3 +1,5 @@
.. _vb2_framework:
V4L2 videobuf2 kAPI
^^^^^^^^^^^^^^^^^^^