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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 10:05:51 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 10:05:51 +0000 |
commit | 5d1646d90e1f2cceb9f0828f4b28318cd0ec7744 (patch) | |
tree | a94efe259b9009378be6d90eb30d2b019d95c194 /Documentation/userspace-api/media/v4l/format.rst | |
parent | Initial commit. (diff) | |
download | linux-upstream/5.10.209.tar.xz linux-upstream/5.10.209.zip |
Adding upstream version 5.10.209.upstream/5.10.209upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to '')
-rw-r--r-- | Documentation/userspace-api/media/v4l/format.rst | 91 |
1 files changed, 91 insertions, 0 deletions
diff --git a/Documentation/userspace-api/media/v4l/format.rst b/Documentation/userspace-api/media/v4l/format.rst new file mode 100644 index 000000000..35bbb2fea --- /dev/null +++ b/Documentation/userspace-api/media/v4l/format.rst @@ -0,0 +1,91 @@ +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later +.. c:namespace:: V4L + +.. _format: + +************ +Data Formats +************ + +Data Format Negotiation +======================= + +Different devices exchange different kinds of data with applications, +for example video images, raw or sliced VBI data, RDS datagrams. Even +within one kind many different formats are possible, in particular there is an +abundance of image formats. Although drivers must provide a default and +the selection persists across closing and reopening a device, +applications should always negotiate a data format before engaging in +data exchange. Negotiation means the application asks for a particular +format and the driver selects and reports the best the hardware can do +to satisfy the request. Of course applications can also just query the +current selection. + +A single mechanism exists to negotiate all data formats using the +aggregate struct :c:type:`v4l2_format` and the +:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and +:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctls. Additionally the +:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl can be used to examine +what the hardware *could* do, without actually selecting a new data +format. The data formats supported by the V4L2 API are covered in the +respective device section in :ref:`devices`. For a closer look at +image formats see :ref:`pixfmt`. + +The :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl is a major turning-point in the +initialization sequence. Prior to this point multiple panel applications +can access the same device concurrently to select the current input, +change controls or modify other properties. The first :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` +assigns a logical stream (video data, VBI data etc.) exclusively to one +file descriptor. + +Exclusive means no other application, more precisely no other file +descriptor, can grab this stream or change device properties +inconsistent with the negotiated parameters. A video standard change for +example, when the new standard uses a different number of scan lines, +can invalidate the selected image format. Therefore only the file +descriptor owning the stream can make invalidating changes. Accordingly +multiple file descriptors which grabbed different logical streams +prevent each other from interfering with their settings. When for +example video overlay is about to start or already in progress, +simultaneous video capturing may be restricted to the same cropping and +image size. + +When applications omit the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl its locking side +effects are implied by the next step, the selection of an I/O method +with the :ref:`VIDIOC_REQBUFS` ioctl or implicit +with the first :c:func:`read()` or +:c:func:`write()` call. + +Generally only one logical stream can be assigned to a file descriptor, +the exception being drivers permitting simultaneous video capturing and +overlay using the same file descriptor for compatibility with V4L and +earlier versions of V4L2. Switching the logical stream or returning into +"panel mode" is possible by closing and reopening the device. Drivers +*may* support a switch using :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`. + +All drivers exchanging data with applications must support the +:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl. Implementation of the +:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is highly recommended but optional. + +Image Format Enumeration +======================== + +Apart of the generic format negotiation functions a special ioctl to +enumerate all image formats supported by video capture, overlay or +output devices is available. [#f1]_ + +The :ref:`VIDIOC_ENUM_FMT` ioctl must be supported +by all drivers exchanging image data with applications. + +.. important:: + + Drivers are not supposed to convert image formats in kernel space. + They must enumerate only formats directly supported by the hardware. + If necessary driver writers should publish an example conversion + routine or library for integration into applications. + +.. [#f1] + Enumerating formats an application has no a-priori knowledge of + (otherwise it could explicitly ask for them and need not enumerate) + seems useless, but there are applications serving as proxy between + drivers and the actual video applications for which this is useful. |