path: root/Documentation/userspace-api/media/v4l/vidioc-g-fbuf.rst

.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later

.. _VIDIOC_G_FBUF:

**********************************
ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF
**********************************

Name
====

VIDIOC_G_FBUF - VIDIOC_S_FBUF - Get or set frame buffer overlay parameters


Synopsis
========

.. c:function:: int ioctl( int fd, VIDIOC_G_FBUF, struct v4l2_framebuffer *argp )
    :name: VIDIOC_G_FBUF

.. c:function:: int ioctl( int fd, VIDIOC_S_FBUF, const struct v4l2_framebuffer *argp )
    :name: VIDIOC_S_FBUF


Arguments
=========

``fd``
    File descriptor returned by :ref:`open() <func-open>`.

``argp``
    Pointer to struct :c:type:`v4l2_framebuffer`.


Description
===========

Applications can use the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` and :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` ioctl
to get and set the framebuffer parameters for a
:ref:`Video Overlay <overlay>` or :ref:`Video Output Overlay <osd>`
(OSD). The type of overlay is implied by the device type (capture or
output device) and can be determined with the
:ref:`VIDIOC_QUERYCAP` ioctl. One ``/dev/videoN``
device must not support both kinds of overlay.

The V4L2 API distinguishes destructive and non-destructive overlays. A
destructive overlay copies captured video images into the video memory
of a graphics card. A non-destructive overlay blends video images into a
VGA signal or graphics into a video signal. *Video Output Overlays* are
always non-destructive.

To get the current parameters applications call the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>`
ioctl with a pointer to a struct :c:type:`v4l2_framebuffer`
structure. The driver fills all fields of the structure or returns an
EINVAL error code when overlays are not supported.

To set the parameters for a *Video Output Overlay*, applications must
initialize the ``flags`` field of a struct
:c:type:`v4l2_framebuffer`. Since the framebuffer is
implemented on the TV card all other parameters are determined by the
driver. When an application calls :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` with a pointer to
this structure, the driver prepares for the overlay and returns the
framebuffer parameters as :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` does, or it returns an error
code.

To set the parameters for a *non-destructive Video Overlay*,
applications must initialize the ``flags`` field, the ``fmt``
substructure, and call :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>`. Again the driver prepares for
the overlay and returns the framebuffer parameters as :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>`
does, or it returns an error code.

For a *destructive Video Overlay* applications must additionally provide
a ``base`` address. Setting up a DMA to a random memory location can
jeopardize the system security, its stability or even damage the
hardware, therefore only the superuser can set the parameters for a
destructive video overlay.


.. tabularcolumns:: |p{3.5cm}|p{3.5cm}|p{3.5cm}|p{7.0cm}|

.. c:type:: v4l2_framebuffer

.. cssclass:: longtable

.. flat-table:: struct v4l2_framebuffer
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 1 2

    * - __u32
      - ``capability``
      -
      - Overlay capability flags set by the driver, see
	:ref:`framebuffer-cap`.
    * - __u32
      - ``flags``
      -
      - Overlay control flags set by application and driver, see
	:ref:`framebuffer-flags`
    * - void *
      - ``base``
      -
      - Physical base address of the framebuffer, that is the address of
	the pixel in the top left corner of the framebuffer. [#f1]_
    * -
      -
      -
      - This field is irrelevant to *non-destructive Video Overlays*. For
	*destructive Video Overlays* applications must provide a base
	address. The driver may accept only base addresses which are a
	multiple of two, four or eight bytes. For *Video Output Overlays*
	the driver must return a valid base address, so applications can
	find the corresponding Linux framebuffer device (see
	:ref:`osd`).
    * - struct
      - ``fmt``
      -
      - Layout of the frame buffer.
    * -
      - __u32
      - ``width``
      - Width of the frame buffer in pixels.
    * -
      - __u32
      - ``height``
      - Height of the frame buffer in pixels.
    * -
      - __u32
      - ``pixelformat``
      - The pixel format of the framebuffer.
    * -
      -
      -
      - For *non-destructive Video Overlays* this field only defines a
	format for the struct :c:type:`v4l2_window`
	``chromakey`` field.
    * -
      -
      -
      - For *destructive Video Overlays* applications must initialize this
	field. For *Video Output Overlays* the driver must return a valid
	format.
    * -
      -
      -
      - Usually this is an RGB format (for example
	:ref:`V4L2_PIX_FMT_RGB565 <V4L2-PIX-FMT-RGB565>`) but YUV
	formats (only packed YUV formats when chroma keying is used, not
	including ``V4L2_PIX_FMT_YUYV`` and ``V4L2_PIX_FMT_UYVY``) and the
	``V4L2_PIX_FMT_PAL8`` format are also permitted. The behavior of
	the driver when an application requests a compressed format is
	undefined. See :ref:`pixfmt` for information on pixel formats.
    * -
      - enum :c:type:`v4l2_field`
      - ``field``
      - Drivers and applications shall ignore this field. If applicable,
	the field order is selected with the
	:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, using the ``field``
	field of struct :c:type:`v4l2_window`.
    * -
      - __u32
      - ``bytesperline``
      - Distance in bytes between the leftmost pixels in two adjacent
	lines.
    * - :cspan:`3`

	This field is irrelevant to *non-destructive Video Overlays*.

	For *destructive Video Overlays* both applications and drivers can
	set this field to request padding bytes at the end of each line.
	Drivers however may ignore the requested value, returning
	``width`` times bytes-per-pixel or a larger value required by the
	hardware. That implies applications can just set this field to
	zero to get a reasonable default.

	For *Video Output Overlays* the driver must return a valid value.

	Video hardware may access padding bytes, therefore they must
	reside in accessible memory. Consider for example the case where
	padding bytes after the last line of an image cross a system page
	boundary. Capture devices may write padding bytes, the value is
	undefined. Output devices ignore the contents of padding bytes.

	When the image format is planar the ``bytesperline`` value applies
	to the first plane and is divided by the same factor as the
	``width`` field for the other planes. For example the Cb and Cr
	planes of a YUV 4:2:0 image have half as many padding bytes
	following each line as the Y plane. To avoid ambiguities drivers
	must return a ``bytesperline`` value rounded up to a multiple of
	the scale factor.
    * -
      - __u32
      - ``sizeimage``
      - This field is irrelevant to *non-destructive Video Overlays*. For
	*destructive Video Overlays* applications must initialize this
	field. For *Video Output Overlays* the driver must return a valid
	format.

	Together with ``base`` it defines the framebuffer memory
	accessible by the driver.
    * -
      - enum :c:type:`v4l2_colorspace`
      - ``colorspace``
      - This information supplements the ``pixelformat`` and must be set
	by the driver, see :ref:`colorspaces`.
    * -
      - __u32
      - ``priv``
      - Reserved. Drivers and applications must set this field to zero.


.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|

.. _framebuffer-cap:

.. flat-table:: Frame Buffer Capability Flags
    :header-rows:  0
    :stub-columns: 0
    :widths:       3 1 4

    * - ``V4L2_FBUF_CAP_EXTERNOVERLAY``
      - 0x0001
      - The device is capable of non-destructive overlays. When the driver
	clears this flag, only destructive overlays are supported. There
	are no drivers yet which support both destructive and
	non-destructive overlays. Video Output Overlays are in practice
	always non-destructive.
    * - ``V4L2_FBUF_CAP_CHROMAKEY``
      - 0x0002
      - The device supports clipping by chroma-keying the images. That is,
	image pixels replace pixels in the VGA or video signal only where
	the latter assume a certain color. Chroma-keying makes no sense
	for destructive overlays.
    * - ``V4L2_FBUF_CAP_LIST_CLIPPING``
      - 0x0004
      - The device supports clipping using a list of clip rectangles.
    * - ``V4L2_FBUF_CAP_BITMAP_CLIPPING``
      - 0x0008
      - The device supports clipping using a bit mask.
    * - ``V4L2_FBUF_CAP_LOCAL_ALPHA``
      - 0x0010
      - The device supports clipping/blending using the alpha channel of
	the framebuffer or VGA signal. Alpha blending makes no sense for
	destructive overlays.
    * - ``V4L2_FBUF_CAP_GLOBAL_ALPHA``
      - 0x0020
      - The device supports alpha blending using a global alpha value.
	Alpha blending makes no sense for destructive overlays.
    * - ``V4L2_FBUF_CAP_LOCAL_INV_ALPHA``
      - 0x0040
      - The device supports clipping/blending using the inverted alpha
	channel of the framebuffer or VGA signal. Alpha blending makes no
	sense for destructive overlays.
    * - ``V4L2_FBUF_CAP_SRC_CHROMAKEY``
      - 0x0080
      - The device supports Source Chroma-keying. Video pixels with the
	chroma-key colors are replaced by framebuffer pixels, which is
	exactly opposite of ``V4L2_FBUF_CAP_CHROMAKEY``


.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|

.. _framebuffer-flags:

.. cssclass:: longtable

.. flat-table:: Frame Buffer Flags
    :header-rows:  0
    :stub-columns: 0
    :widths:       3 1 4

    * - ``V4L2_FBUF_FLAG_PRIMARY``
      - 0x0001
      - The framebuffer is the primary graphics surface. In other words,
	the overlay is destructive. This flag is typically set by any
	driver that doesn't have the ``V4L2_FBUF_CAP_EXTERNOVERLAY``
	capability and it is cleared otherwise.
    * - ``V4L2_FBUF_FLAG_OVERLAY``
      - 0x0002
      - If this flag is set for a video capture device, then the driver
	will set the initial overlay size to cover the full framebuffer
	size, otherwise the existing overlay size (as set by
	:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`) will be used. Only one
	video capture driver (bttv) supports this flag. The use of this
	flag for capture devices is deprecated. There is no way to detect
	which drivers support this flag, so the only reliable method of
	setting the overlay size is through
	:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`. If this flag is set for a
	video output device, then the video output overlay window is
	relative to the top-left corner of the framebuffer and restricted
	to the size of the framebuffer. If it is cleared, then the video
	output overlay window is relative to the video output display.
    * - ``V4L2_FBUF_FLAG_CHROMAKEY``
      - 0x0004
      - Use chroma-keying. The chroma-key color is determined by the
	``chromakey`` field of struct :c:type:`v4l2_window`
	and negotiated with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
	ioctl, see :ref:`overlay` and :ref:`osd`.
    * - :cspan:`2` There are no flags to enable clipping using a list of
	clip rectangles or a bitmap. These methods are negotiated with the
	:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
	and :ref:`osd`.
    * - ``V4L2_FBUF_FLAG_LOCAL_ALPHA``
      - 0x0008
      - Use the alpha channel of the framebuffer to clip or blend
	framebuffer pixels with video images. The blend function is:
	output = framebuffer pixel * alpha + video pixel * (1 - alpha).
	The actual alpha depth depends on the framebuffer pixel format.
    * - ``V4L2_FBUF_FLAG_GLOBAL_ALPHA``
      - 0x0010
      - Use a global alpha value to blend the framebuffer with video
	images. The blend function is: output = (framebuffer pixel * alpha
	+ video pixel * (255 - alpha)) / 255. The alpha value is
	determined by the ``global_alpha`` field of struct
	:c:type:`v4l2_window` and negotiated with the
	:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
	and :ref:`osd`.
    * - ``V4L2_FBUF_FLAG_LOCAL_INV_ALPHA``
      - 0x0020
      - Like ``V4L2_FBUF_FLAG_LOCAL_ALPHA``, use the alpha channel of the
	framebuffer to clip or blend framebuffer pixels with video images,
	but with an inverted alpha value. The blend function is: output =
	framebuffer pixel * (1 - alpha) + video pixel * alpha. The actual
	alpha depth depends on the framebuffer pixel format.
    * - ``V4L2_FBUF_FLAG_SRC_CHROMAKEY``
      - 0x0040
      - Use source chroma-keying. The source chroma-key color is
	determined by the ``chromakey`` field of struct
	:c:type:`v4l2_window` and negotiated with the
	:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
	and :ref:`osd`. Both chroma-keying are mutual exclusive to each
	other, so same ``chromakey`` field of struct
	:c:type:`v4l2_window` is being used.


Return Value
============

On success 0 is returned, on error -1 and the ``errno`` variable is set
appropriately. The generic error codes are described at the
:ref:`Generic Error Codes <gen-errors>` chapter.

EPERM
    :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` can only be called by a privileged user to
    negotiate the parameters for a destructive overlay.

EINVAL
    The :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` parameters are unsuitable.

.. [#f1]
   A physical base address may not suit all platforms. GK notes in
   theory we should pass something like PCI device + memory region +
   offset instead. If you encounter problems please discuss on the
   linux-media mailing list:
   `https://linuxtv.org/lists.php <https://linuxtv.org/lists.php>`__.