Table of Contents
Programming a V4L2 device consists of these steps:
Opening the device
Changing device properties, selecting a video and audio input, video standard, picture brightness a. o.
Negotiating a data format
Negotiating an input/output method
The actual input/output loop
Closing the device
In practice most steps are optional and can be executed out of order. It depends on the V4L2 device type, you can read about the details in Chapter 4, Interfaces. In this chapter we will discuss the basic concepts applicable to all devices.
V4L2 drivers are implemented as kernel modules, loaded manually by the system administrator or automatically when a device is first discovered. The driver modules plug into the "videodev" kernel module. It provides helper functions and a common application interface specified in this document.
Each driver thus loaded registers one or more device nodes with major number 81 and a minor number between 0 and 255. Minor numbers are allocated dynamically unless the kernel is compiled with the kernel option CONFIG_VIDEO_FIXED_MINOR_RANGES. In that case minor numbers are allocated in ranges depending on the device node type (video, radio, etc.).
Many drivers support "video_nr", "radio_nr" or "vbi_nr" module options to select specific video/radio/vbi node numbers. This allows the user to request that the device node is named e.g. /dev/video5 instead of leaving it to chance. When the driver supports multiple devices of the same type more than one device node number can be assigned, separated by commas:
> modprobe mydriver video_nr=0,1 radio_nr=0,1
In /etc/modules.conf this may be
written as:
options mydriver video_nr=0,1 radio_nr=0,1
When no device node number is given as module option the driver supplies a default.
Normally udev will create the device nodes in /dev automatically for you. If udev is not installed, then you need to enable the CONFIG_VIDEO_FIXED_MINOR_RANGES kernel option in order to be able to correctly relate a minor number to a device node number. I.e., you need to be certain that minor number 5 maps to device node name video5. With this kernel option different device types have different minor number ranges. These ranges are listed in Chapter 4, Interfaces.
The creation of character special files (with mknod) is a privileged operation and devices cannot be opened by major and minor number. That means applications cannot reliable scan for loaded or installed drivers. The user must enter a device name, or the application can try the conventional device names.
Devices can support several functions. For example video capturing, VBI capturing and radio support.
The V4L2 API creates different nodes for each of these functions.
The V4L2 API was designed with the idea that one device node could support all functions. However, in practice this never worked: this 'feature' was never used by applications and many drivers did not support it and if they did it was certainly never tested. In addition, switching a device node between different functions only works when using the streaming I/O API, not with the read()/write() API.
Today each device node supports just one function.
Besides video input or output the hardware may also support audio sampling or playback. If so, these functions are implemented as ALSA PCM devices with optional ALSA audio mixer devices.
One problem with all these devices is that the V4L2 API makes no provisions to find these related devices. Some really complex devices use the Media Controller (see Chapter 18, Media Controller) which can be used for this purpose. But most drivers do not use it, and while some code exists that uses sysfs to discover related devices (see libmedia_dev in the v4l-utils git repository), there is no library yet that can provide a single API towards both Media Controller-based devices and devices that do not use the Media Controller. If you want to work on this please write to the linux-media mailing list: http://www.linuxtv.org/lists.php.
V4L2 devices can be opened more than once.[1] When this is supported by the driver, users can for example start a "panel" application to change controls like brightness or audio volume, while another application captures video and audio. In other words, panel applications are comparable to an ALSA audio mixer application. Just opening a V4L2 device should not change the state of the device.[2]
Once an application has allocated the memory buffers needed for
streaming data (by calling the VIDIOC_REQBUFS or VIDIOC_CREATE_BUFS ioctls,
or implicitly by calling the read() or write() functions) that
application (filehandle) becomes the owner of the device. It is no longer
allowed to make changes that would affect the buffer sizes (e.g. by calling
the VIDIOC_S_FMT ioctl) and other applications are no longer allowed to allocate
buffers or start or stop streaming. The EBUSY error code will be returned instead.
Merely opening a V4L2 device does not grant exclusive access.[3] Initiating data exchange however assigns the right to read or write the requested type of data, and to change related properties, to this file descriptor. Applications can request additional access privileges using the priority mechanism described in the section called “Application Priority”.
V4L2 drivers should not support multiple applications reading or writing the same data stream on a device by copying buffers, time multiplexing or similar means. This is better handled by a proxy application in user space.
[1]
There are still some old and obscure drivers that have not been updated to
allow for multiple opens. This implies that for such drivers open() can
return an EBUSY error code when the device is already in use.
[2] Unfortunately, opening a radio device often switches the state of the device to radio mode in many drivers. This behavior should be fixed eventually as it violates the V4L2 specification.
[3] Drivers could recognize the
O_EXCL open flag. Presently this is not required,
so applications cannot know if it really works.