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diff --git a/Documentation/media/uapi/v4l/io.rst b/Documentation/media/uapi/v4l/io.rst new file mode 100644 index 000000000000..94b38a10ee65 --- /dev/null +++ b/Documentation/media/uapi/v4l/io.rst @@ -0,0 +1,51 @@ +.. -*- coding: utf-8; mode: rst -*- + +.. _io: + +############ +Input/Output +############ +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 :ref:`read() <func-read>` and +:ref:`write() <func-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 +:ref:`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 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` +ioctl. For more information see :ref:`overlay`. + +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 :ref:`open`) +and drivers permitting simultaneous video capturing and overlay using +the same file descriptor, for compatibility with V4L and earlier +versions of V4L2. + +:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`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. + + +.. toctree:: + :maxdepth: 1 + + rw + mmap + userp + dmabuf + async + buffer + field-order |