The V4L2 API defines several different methods to read from or write to a device. All drivers exchanging data with applications must support at least one of them.
The classic I/O method using the read()
and write() function is automatically selected
after opening a V4L2 device. When the driver does not support this
method attempts to read or write will fail at any time.
Other methods must be negotiated. To select the streaming I/O
method with memory mapped or user buffers applications call the
VIDIOC_REQBUFS ioctl. The asynchronous I/O method is not defined
yet.
Video overlay can be considered another I/O method, although
the application does not directly receive the image data. It is
selected by initiating video overlay with the VIDIOC_S_FMT ioctl.
For more information see Section 4.2.
Generally exactly one I/O method, including overlay, is associated with each file descriptor. The only exceptions are applications not exchanging data with a driver ("panel applications", see Section 1.1) and drivers permitting simultaneous video capturing and overlay using the same file descriptor, for compatibility with V4L and earlier versions of V4L2.
VIDIOC_S_FMT and
VIDIOC_REQBUFS would permit this to some degree,
but for simplicity drivers need not support switching the I/O method
(after first switching away from read/write) other than by closing
and reopening the device.
The following sections describe the various I/O methods in more detail.
Input and output devices support the
read() and write() function,
respectively, when the V4L2_CAP_READWRITE flag in
the capabilities field of struct v4l2_capability
returned by the VIDIOC_QUERYCAP ioctl is set.
Drivers may need the CPU to copy the data, but they may also support DMA to or from user memory, so this I/O method is not necessarily less efficient than other methods merely exchanging buffer pointers. It is considered inferior though because no meta-information like frame counters or timestamps are passed. This information is necessary to recognize frame dropping and to synchronize with other data streams. However this is also the simplest I/O method, requiring little or no setup to exchange data. It permits command line stunts like this (the vidctrl tool is fictitious):
> vidctrl /dev/video --input=0 --format=YUYV --size=352x288 > dd if=/dev/video of=myimage.422 bs=202752 count=1
To read from the device applications use the
read() function, to write the write() function.
Drivers must implement one I/O method if they
exchange data with applications, but it need not be this.[1] When reading or writing is supported, the driver
must also support the select() and poll()
function.[2]
| [1] | It would be desirable if applications could depend on drivers supporting all I/O interfaces, but as much as the complex memory mapping I/O can be inadequate for some devices we have no reason to require this interface, which is most useful for simple applications capturing still images. |
| [2] | At the driver level |