V4L2 API 规范详解

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"V4L2 Specification" V4L2（Video for Linux Two）是Linux内核中的一个API，用于支持视频捕获、播放、编码和解码等操作。这个API是V4L（Video for Linux）的升级版本，旨在提供更强大、更灵活的功能，以满足现代多媒体设备的需求。在Linux系统中，V4L2驱动是连接硬件设备（如摄像头、电视卡或数字视频捕获卡）与用户空间应用程序的关键组件。它定义了一系列的系统调用（ioctl）和数据结构，使得开发者能够控制视频设备的参数，如分辨率、帧率、色彩空间等，并进行数据传输。 V4L2规范包含了以下主要内容： 1. **设备模型**：V4L2采用了一种层次化的设备模型，包括视频捕获设备、视频输出设备、音频设备等，每个设备都有自己的文件描述符，可以通过标准的文件操作进行访问。 2. **像素格式**：V4L2支持多种像素格式，例如YUV、RGB、灰度等。在V4L2 Specification的0.24修订版中，新增了pixelformat Y16和SBGGR16，以适应更高精度的图像处理需求。 3. **控制机制**：V4L2提供了控制接口，允许用户查询和设置设备的各种参数，如亮度、对比度、饱和度等。0.24版本还引入了新的控制和相机控制类，增强了对硬件特性的控制能力。 4. **缓冲区管理**：V4L2使用MMAP（内存映射）和USERPTR（用户空间指针）两种方式管理缓冲区，允许用户空间程序直接访问设备内存，提高了数据传输效率。 5. **流式传输**：V4L2支持连续的数据流传输，可以处理实时视频数据。用户可以指定缓冲区数量和大小，设备会按照设定的模式填充或读取缓冲区。 6. **多路复用**：V4L2支持在一个设备上同时打开多个流，这对于处理多路视频输入或输出非常有用。 7. **编码和解码**：虽然V4L2最初设计用于原始视频数据，但随着技术的发展，它也逐渐支持视频编码和解码，如MPEG、JPEG等。 8. **错误处理**：V4L2 API提供了丰富的错误处理机制，包括返回错误代码和设置设备状态，使应用程序能适当地处理各种异常情况。通过V4L2，开发者可以创建各种多媒体应用程序，如视频播放器、视频编辑软件、网络摄像头应用等。同时，由于其开放源码的特性，V4L2也在持续改进和扩展，以适应不断发展的硬件和软件环境。对于学习和开发Linux下的视频驱动程序，理解V4L2规范是至关重要的。

Chapter 1. Common API Elements

printf ("Current input: %s\n", audio.name);

Example 1-4. Switching to the ﬁrst audio input

struct v4l2_audio audio;

memset (&audio, 0, sizeof (audio)); /

clear audio.mode, audio.reserved

audio.index = 0;

if (-1 == ioctl (fd,

VIDIOC_S_AUDIO, &audio)) {

perror ("VIDIOC_S_AUDIO");

exit (EXIT_FAILURE);

}

1.6. Tuners and Modulators

1.6.1. Tuners

Video input devices can have one or more tuners demodulating a RF signal. Each tuner is associated

with one or more video inputs, depending on the number of RF connectors on the tuner. The type

ﬁeld of the respective struct v4l2_input returned by the VIDIOC_ENUMINPUT ioctl is set to

V4L2_INPUT_TYPE_TUNER and its tuner ﬁeld contains the index number of the tuner.

Radio devices have exactly one tuner with index zero, no video inputs.

To query and change tuner properties applications use the VIDIOC_G_TUNER and VIDIOC_S_TUNER

ioctl, respectively. The struct v4l2_tuner returned by VIDIOC_G_TUNER also contains signal status

information applicable when the tuner of the current video input, or a radio tuner is queried. Note

that VIDIOC_S_TUNER does not switch the current tuner, when there is more than one at all. The

tuner is solely determined by the current video input. Drivers must support both ioctls and set the

V4L2_CAP_TUNER ﬂag in the struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl when

the device has one or more tuners.

1.6.2. Modulators

Video output devices can have one or more modulators, uh, modulating a video signal for radiation

or connection to the antenna input of a TV set or video recorder. Each modulator is associated with

one or more video outputs, depending on the number of RF connectors on the modulator. The type

ﬁeld of the respective struct v4l2_output returned by the VIDIOC_ENUMOUTPUT ioctl is set to

V4L2_OUTPUT_TYPE_MODULATOR and its modulator ﬁeld contains the index number of the

modulator. This speciﬁcation does not deﬁne radio output devices.

To query and change modulator properties applications use the VIDIOC_G_MODULATOR and

VIDIOC_S_MODULATOR ioctl. Note that VIDIOC_S_MODULATOR does not switch the current

modulator, when there is more than one at all. The modulator is solely determined by the current

video output. Drivers must support both ioctls and set the V4L2_CAP_TUNER (sic) ﬂag in the

Chapter 1. Common API Elements

struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl when the device has one or more

modulators.

1.6.3. Radio Frequency

To get and set the tuner or modulator radio frequency applications use the VIDIOC_G_FREQUENCY

and VIDIOC_S_FREQUENCY ioctl which both take a pointer to a struct v4l2_frequency. These ioctls

are used for TV and radio devices alike. Drivers must support both ioctls when the tuner or

modulator ioctls are supported, or when the device is a radio device.

1.6.4. Satellite Receivers

To be discussed. See also proposals by Peter Schlaf, [email protected] on 23 Oct 2002,

subject: "Re: [V4L] Re: v4l2 api".

1.7. Video Standards

Video devices typically support one or more different video standards or variations of standards.

Each video input and output may support another set of standards. This set is reported by the std

ﬁeld of struct v4l2_input and struct v4l2_output returned by the VIDIOC_ENUMINPUT and

VIDIOC_ENUMOUTPUT ioctl, respectively.

V4L2 deﬁnes one bit for each analog video standard currently in use worldwide, and sets aside bits

for driver deﬁned standards, e. g. hybrid standards to watch NTSC video tapes on PAL TVs and vice

versa. Applications can use the predeﬁned bits to select a particular standard, although presenting the

user a menu of supported standards is preferred. To enumerate and query the attributes of the

supported standards applications use the VIDIOC_ENUMSTD ioctl.

Many of the deﬁned standards are actually just variations of a few major standards. The hardware

may in fact not distinguish between them, or do so internal and switch automatically. Therefore

enumerated standards also contain sets of one or more standard bits.

Assume a hypothetic tuner capable of demodulating B/PAL, G/PAL and I/PAL signals. The ﬁrst

enumerated standard is a set of B and G/PAL, switched automatically depending on the selected

radio frequency in UHF or VHF band. Enumeration gives a "PAL-B/G" or "PAL-I" choice. Similar a

Composite input may collapse standards, enumerating "PAL-B/G/H/I", "NTSC-M" and

"SECAM-D/K".

To query and select the standard used by the current video input or output applications call the

VIDIOC_G_STD and VIDIOC_S_STD ioctl, respectively. The received standard can be sensed with

the VIDIOC_QUERYSTD ioctl. Note parameter of all these ioctls is a pointer to a v4l2_std_id type (a

standard set), not an index into the standard enumeration.

Drivers must implement all video

standard ioctls when the device has one or more video inputs or outputs.

Special rules apply to USB cameras where the notion of video standards makes little sense. More

generally any capture device, output devices accordingly, which is

• incapable of capturing ﬁelds or frames at the nominal rate of the video standard, or

• where timestamps refer to the instant the ﬁeld or frame was received by the driver, not the capture

time, or

Chapter 1. Common API Elements

1.8. User Controls

Devices typically have a number of user-settable controls such as brightness, saturation and so on,

which would be presented to the user on a graphical user interface. But, different devices will have

different controls available, and furthermore, the range of possible values, and the default value will

vary from device to device. The control ioctls provide the information and a mechanism to create a

nice user interface for these controls that will work correctly with any device.

All controls are accessed using an ID value. V4L2 deﬁnes several IDs for speciﬁc purposes. Drivers

can also implement their own custom controls using V4L2_CID_PRIVATE_BASE and higher values.

The pre-deﬁned control IDs have the preﬁx V4L2_CID_, and are listed in Table 1-1. The ID is used

when querying the attributes of a control, and when getting or setting the current value.

Generally applications should present controls to the user without assumptions about their purpose.

Each control comes with a name string the user is supposed to understand. When the purpose is

non-intuitive the driver writer should provide a user manual, a user interface plug-in or a driver

speciﬁc panel application. Predeﬁned IDs were introduced to change a few controls

programmatically, for example to mute a device during a channel switch.

Drivers may enumerate different controls after switching the current video input or output, tuner or

modulator, or audio input or output. Different in the sense of other bounds, another default and

current value, step size or other menu items. A control with a certain custom ID can also change

name and type.

Control values are stored globally, they do not change when switching except to stay

within the reported bounds. They also do not change e. g. when the device is opened or closed, when

the tuner radio frequency is changed or generally never without application request. Since V4L2

speciﬁes no event mechanism, panel applications intended to cooperate with other panel applications

(be they built into a larger application, as a TV viewer) may need to regularly poll control values to

update their user interface.

Table 1-1. Control IDs

ID Type Description

V4L2_CID_BASE First predeﬁned ID, equal to

V4L2_CID_BRIGHTNESS.

V4L2_CID_USER_BASE Synonym of V4L2_CID_BASE.

V4L2_CID_BRIGHTNESS integer Picture brightness, or more precisely, the black

level.

V4L2_CID_CONTRAST integer Picture contrast or luma gain.

V4L2_CID_SATURATION integer Picture color saturation or chroma gain.

V4L2_CID_HUE integer Hue or color balance.

V4L2_CID_AUDIO_VOLUME integer Overall audio volume. Note some drivers also

provide an OSS or ALSA mixer interface.

V4L2_CID_AUDIO_BALANCE integer Audio stereo balance. Minimum corresponds to

all the way left, maximum to right.

V4L2_CID_AUDIO_BASS integer Audio bass adjustment.

V4L2_CID_AUDIO_TREBLE integer Audio treble adjustment.

V4L2_CID_AUDIO_MUTE boolean Mute audio, i. e. set the volume to zero, however

without affecting V4L2_CID_AUDIO_VOLUME.

Like ALSA drivers, V4L2 drivers must mute at

load time to avoid excessive noise. Actually the

entire device should be reset to a low power

consumption state.

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