media: docs: move uAPI book to userspace-api/media
Since 2017, there is an space reserved for userspace API,
created by changeset 1d596dee38 ("docs: Create a user-space API guide").
As the media subsystem was one of the first subsystems to use
Sphinx, until this patch, we were keeping things on a separate
place.
Let's just use the new location, as having all uAPI altogether
will likely make things easier for developers.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
This commit is contained in:
Mauro Carvalho Chehab
157
Documentation/userspace-api/media/v4l/dev-osd.rst
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Documentation/userspace-api/media/v4l/dev-osd.rst
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.. Permission is granted to copy, distribute and/or modify this
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.. document under the terms of the GNU Free Documentation License,
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.. Version 1.1 or any later version published by the Free Software
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.. Foundation, with no Invariant Sections, no Front-Cover Texts
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.. and no Back-Cover Texts. A copy of the license is included at
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.. Documentation/userspace-api/media/fdl-appendix.rst.
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..
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.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
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.. _osd:
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******************************
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Video Output Overlay Interface
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******************************
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**Also known as On-Screen Display (OSD)**
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Some video output devices can overlay a framebuffer image onto the
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outgoing video signal. Applications can set up such an overlay using
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this interface, which borrows structures and ioctls of the
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:ref:`Video Overlay <overlay>` interface.
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The OSD function is accessible through the same character special file
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as the :ref:`Video Output <capture>` function.
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.. note::
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The default function of such a ``/dev/video`` device is video
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capturing or output. The OSD function is only available after calling
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the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.
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Querying Capabilities
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=====================
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Devices supporting the *Video Output Overlay* interface set the
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``V4L2_CAP_VIDEO_OUTPUT_OVERLAY`` flag in the ``capabilities`` field of
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struct :c:type:`v4l2_capability` returned by the
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:ref:`VIDIOC_QUERYCAP` ioctl.
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Framebuffer
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===========
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Contrary to the *Video Overlay* interface the framebuffer is normally
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implemented on the TV card and not the graphics card. On Linux it is
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accessible as a framebuffer device (``/dev/fbN``). Given a V4L2 device,
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applications can find the corresponding framebuffer device by calling
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the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` ioctl. It returns, amongst
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other information, the physical address of the framebuffer in the
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``base`` field of struct :c:type:`v4l2_framebuffer`.
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The framebuffer device ioctl ``FBIOGET_FSCREENINFO`` returns the same
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address in the ``smem_start`` field of struct
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struct :c:type:`fb_fix_screeninfo`. The ``FBIOGET_FSCREENINFO``
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ioctl and struct :c:type:`fb_fix_screeninfo` are defined in
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the ``linux/fb.h`` header file.
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The width and height of the framebuffer depends on the current video
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standard. A V4L2 driver may reject attempts to change the video standard
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(or any other ioctl which would imply a framebuffer size change) with an
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``EBUSY`` error code until all applications closed the framebuffer device.
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Example: Finding a framebuffer device for OSD
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---------------------------------------------
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.. code-block:: c
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#include <linux/fb.h>
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struct v4l2_framebuffer fbuf;
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unsigned int i;
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int fb_fd;
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if (-1 == ioctl(fd, VIDIOC_G_FBUF, &fbuf)) {
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perror("VIDIOC_G_FBUF");
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exit(EXIT_FAILURE);
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}
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for (i = 0; i < 30; i++) {
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char dev_name[16];
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struct fb_fix_screeninfo si;
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snprintf(dev_name, sizeof(dev_name), "/dev/fb%u", i);
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fb_fd = open(dev_name, O_RDWR);
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if (-1 == fb_fd) {
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switch (errno) {
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case ENOENT: /* no such file */
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case ENXIO: /* no driver */
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continue;
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default:
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perror("open");
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exit(EXIT_FAILURE);
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}
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}
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if (0 == ioctl(fb_fd, FBIOGET_FSCREENINFO, &si)) {
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if (si.smem_start == (unsigned long)fbuf.base)
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break;
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} else {
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/* Apparently not a framebuffer device. */
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}
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close(fb_fd);
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fb_fd = -1;
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}
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/* fb_fd is the file descriptor of the framebuffer device
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for the video output overlay, or -1 if no device was found. */
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Overlay Window and Scaling
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==========================
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The overlay is controlled by source and target rectangles. The source
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rectangle selects a subsection of the framebuffer image to be overlaid,
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the target rectangle an area in the outgoing video signal where the
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image will appear. Drivers may or may not support scaling, and arbitrary
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sizes and positions of these rectangles. Further drivers may support any
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(or none) of the clipping/blending methods defined for the
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:ref:`Video Overlay <overlay>` interface.
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A struct :c:type:`v4l2_window` defines the size of the
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source rectangle, its position in the framebuffer and the
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clipping/blending method to be used for the overlay. To get the current
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parameters applications set the ``type`` field of a struct
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:c:type:`v4l2_format` to
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``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY`` and call the
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:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl. The driver fills the
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struct :c:type:`v4l2_window` substructure named ``win``. It is not
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possible to retrieve a previously programmed clipping list or bitmap.
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To program the source rectangle applications set the ``type`` field of a
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struct :c:type:`v4l2_format` to
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``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY``, initialize the ``win``
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substructure and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.
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The driver adjusts the parameters against hardware limits and returns
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the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`,
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the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl can be used to learn
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about driver capabilities without actually changing driver state. Unlike
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:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` this also works after the overlay has been enabled.
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A struct :c:type:`v4l2_crop` defines the size and position
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of the target rectangle. The scaling factor of the overlay is implied by
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the width and height given in struct :c:type:`v4l2_window`
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and struct :c:type:`v4l2_crop`. The cropping API applies to
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*Video Output* and *Video Output Overlay* devices in the same way as to
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*Video Capture* and *Video Overlay* devices, merely reversing the
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direction of the data flow. For more information see :ref:`crop`.
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Enabling Overlay
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================
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There is no V4L2 ioctl to enable or disable the overlay, however the
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framebuffer interface of the driver may support the ``FBIOBLANK`` ioctl.
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