From d0b2f91bede3bd5e3d24dd6803e56eee959c1797 Mon Sep 17 00:00:00 2001 From: André Fabian Silva Delgado Date: Thu, 20 Oct 2016 00:10:27 -0300 Subject: Linux-libre 4.8.2-gnu --- Documentation/media/uapi/v4l/io.rst | 51 +++++++++++++++++++++++++++++++++++++ 1 file changed, 51 insertions(+) create mode 100644 Documentation/media/uapi/v4l/io.rst (limited to 'Documentation/media/uapi/v4l/io.rst') diff --git a/Documentation/media/uapi/v4l/io.rst b/Documentation/media/uapi/v4l/io.rst new file mode 100644 index 000000000..94b38a10e --- /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() ` and +:ref:`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 ` +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 ` 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 -- cgit v1.2.3-54-g00ecf