1 files changed, 150 insertions, 0 deletions
diff --git a/Documentation/userspace-api/media/v4l/dev-osd.rst b/Documentation/userspace-api/media/v4l/dev-osd.rst
new file mode 100644
index 000000000..8e4be9129
--- /dev/null
+++ b/Documentation/userspace-api/media/v4l/dev-osd.rst
@@ -0,0 +1,150 @@
+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+
+.. _osd:
+
+******************************
+Video Output Overlay Interface
+******************************
+
+**Also known as On-Screen Display (OSD)**
+
+Some video output devices can overlay a framebuffer image onto the
+outgoing video signal. Applications can set up such an overlay using
+this interface, which borrows structures and ioctls of the
+:ref:`Video Overlay <overlay>` interface.
+
+The OSD function is accessible through the same character special file
+as the :ref:`Video Output <capture>` function.
+
+.. note::
+
+   The default function of such a ``/dev/video`` device is video
+   capturing or output. The OSD function is only available after calling
+   the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.
+
+
+Querying Capabilities
+=====================
+
+Devices supporting the *Video Output Overlay* interface set the
+``V4L2_CAP_VIDEO_OUTPUT_OVERLAY`` flag in the ``capabilities`` field of
+struct :c:type:`v4l2_capability` returned by the
+:ref:`VIDIOC_QUERYCAP` ioctl.
+
+
+Framebuffer
+===========
+
+Contrary to the *Video Overlay* interface the framebuffer is normally
+implemented on the TV card and not the graphics card. On Linux it is
+accessible as a framebuffer device (``/dev/fbN``). Given a V4L2 device,
+applications can find the corresponding framebuffer device by calling
+the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` ioctl. It returns, amongst
+other information, the physical address of the framebuffer in the
+``base`` field of struct :c:type:`v4l2_framebuffer`.
+The framebuffer device ioctl ``FBIOGET_FSCREENINFO`` returns the same
+address in the ``smem_start`` field of struct
+:c:type:`fb_fix_screeninfo`. The ``FBIOGET_FSCREENINFO``
+ioctl and struct :c:type:`fb_fix_screeninfo` are defined in
+the ``linux/fb.h`` header file.
+
+The width and height of the framebuffer depends on the current video
+standard. A V4L2 driver may reject attempts to change the video standard
+(or any other ioctl which would imply a framebuffer size change) with an
+``EBUSY`` error code until all applications closed the framebuffer device.
+
+Example: Finding a framebuffer device for OSD
+---------------------------------------------
+
+.. code-block:: c
+
+    #include <linux/fb.h>
+
+    struct v4l2_framebuffer fbuf;
+    unsigned int i;
+    int fb_fd;
+
+    if (-1 == ioctl(fd, VIDIOC_G_FBUF, &fbuf)) {
+	perror("VIDIOC_G_FBUF");
+	exit(EXIT_FAILURE);
+    }
+
+    for (i = 0; i < 30; i++) {
+	char dev_name[16];
+	struct fb_fix_screeninfo si;
+
+	snprintf(dev_name, sizeof(dev_name), "/dev/fb%u", i);
+
+	fb_fd = open(dev_name, O_RDWR);
+	if (-1 == fb_fd) {
+	    switch (errno) {
+	    case ENOENT: /* no such file */
+	    case ENXIO:  /* no driver */
+		continue;
+
+	    default:
+		perror("open");
+		exit(EXIT_FAILURE);
+	    }
+	}
+
+	if (0 == ioctl(fb_fd, FBIOGET_FSCREENINFO, &si)) {
+	    if (si.smem_start == (unsigned long)fbuf.base)
+		break;
+	} else {
+	    /* Apparently not a framebuffer device. */
+	}
+
+	close(fb_fd);
+	fb_fd = -1;
+    }
+
+    /* fb_fd is the file descriptor of the framebuffer device
+       for the video output overlay, or -1 if no device was found. */
+
+
+Overlay Window and Scaling
+==========================
+
+The overlay is controlled by source and target rectangles. The source
+rectangle selects a subsection of the framebuffer image to be overlaid,
+the target rectangle an area in the outgoing video signal where the
+image will appear. Drivers may or may not support scaling, and arbitrary
+sizes and positions of these rectangles. Further drivers may support any
+(or none) of the clipping/blending methods defined for the
+:ref:`Video Overlay <overlay>` interface.
+
+A struct :c:type:`v4l2_window` defines the size of the
+source rectangle, its position in the framebuffer and the
+clipping/blending method to be used for the overlay. To get the current
+parameters applications set the ``type`` field of a struct
+:c:type:`v4l2_format` to
+``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY`` and call the
+:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl. The driver fills the
+struct :c:type:`v4l2_window` substructure named ``win``. It is not
+possible to retrieve a previously programmed clipping list or bitmap.
+
+To program the source rectangle applications set the ``type`` field of a
+struct :c:type:`v4l2_format` to
+``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY``, initialize the ``win``
+substructure and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.
+The driver adjusts the parameters against hardware limits and returns
+the actual parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`,
+the :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl can be used to learn
+about driver capabilities without actually changing driver state. Unlike
+:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` this also works after the overlay has been enabled.
+
+A struct :c:type:`v4l2_crop` defines the size and position
+of the target rectangle. The scaling factor of the overlay is implied by
+the width and height given in struct :c:type:`v4l2_window`
+and struct :c:type:`v4l2_crop`. The cropping API applies to
+*Video Output* and *Video Output Overlay* devices in the same way as to
+*Video Capture* and *Video Overlay* devices, merely reversing the
+direction of the data flow. For more information see :ref:`crop`.
+
+
+Enabling Overlay
+================
+
+There is no V4L2 ioctl to enable or disable the overlay, however the
+framebuffer interface of the driver may support the ``FBIOBLANK`` ioctl.