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author | Linus Torvalds <torvalds@linux-foundation.org> | 2020-06-04 06:59:38 +0300 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2020-06-04 06:59:38 +0300 |
commit | a98f670e41a99f53acb1fb33cee9c6abbb2e6f23 (patch) | |
tree | f8ae10a4cb91758ad7f9422053753a8c5d0f04dc /Documentation/userspace-api/media/v4l/dev-output.rst | |
parent | ee01c4d72adffb7d424535adf630f2955748fa8b (diff) | |
parent | 938b29db3aa9c293c7c1366b16e55e308f1a1ddd (diff) | |
download | linux-a98f670e41a99f53acb1fb33cee9c6abbb2e6f23.tar.xz |
Merge tag 'media/v5.8-1' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-media
Pull media updates from Mauro Carvalho Chehab:
- Media documentation is now split into admin-guide, driver-api and
userspace-api books (a longstanding request from Jon);
- The media Kconfig was reorganized, in order to make easier to select
drivers and their dependencies;
- The testing drivers now has a separate directory;
- added a new driver for Rockchip Video Decoder IP;
- The atomisp staging driver was resurrected. It is meant to work with
4 generations of cameras on Atom-based laptops, tablets and cell
phones. So, it seems worth investing time to cleanup this driver and
making it in good shape.
- Added some V4L2 core ancillary routines to help with h264 codecs;
- Added an ov2740 image sensor driver;
- The si2157 gained support for Analog TV, which, in turn, added
support for some cx231xx and cx23885 boards to also support analog
standards;
- Added some V4L2 controls (V4L2_CID_CAMERA_ORIENTATION and
V4L2_CID_CAMERA_SENSOR_ROTATION) to help identifying where the camera
is located at the device;
- VIDIOC_ENUM_FMT was extended to support MC-centric devices;
- Lots of drivers improvements and cleanups.
* tag 'media/v5.8-1' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-media: (503 commits)
media: Documentation: media: Refer to mbus format documentation from CSI-2 docs
media: s5k5baf: Replace zero-length array with flexible-array
media: i2c: imx219: Drop <linux/clk-provider.h> and <linux/clkdev.h>
media: i2c: Add ov2740 image sensor driver
media: ov8856: Implement sensor module revision identification
media: ov8856: Add devicetree support
media: dt-bindings: ov8856: Document YAML bindings
media: dvb-usb: Add Cinergy S2 PCIe Dual Port support
media: dvbdev: Fix tuner->demod media controller link
media: dt-bindings: phy: phy-rockchip-dphy-rx0: move rockchip dphy rx0 bindings out of staging
media: staging: dt-bindings: phy-rockchip-dphy-rx0: remove non-used reg property
media: atomisp: unify the version for isp2401 a0 and b0 versions
media: atomisp: update TODO with the current data
media: atomisp: adjust some code at sh_css that could be broken
media: atomisp: don't produce errs for ignored IRQs
media: atomisp: print IRQ when debugging
media: atomisp: isp_mmu: don't use kmem_cache
media: atomisp: add a notice about possible leak resources
media: atomisp: disable the dynamic and reserved pools
media: atomisp: turn on camera before setting it
...
Diffstat (limited to 'Documentation/userspace-api/media/v4l/dev-output.rst')
-rw-r--r-- | Documentation/userspace-api/media/v4l/dev-output.rst | 108 |
1 files changed, 108 insertions, 0 deletions
diff --git a/Documentation/userspace-api/media/v4l/dev-output.rst b/Documentation/userspace-api/media/v4l/dev-output.rst new file mode 100644 index 000000000000..e4f2a1d8b0fc --- /dev/null +++ b/Documentation/userspace-api/media/v4l/dev-output.rst @@ -0,0 +1,108 @@ +.. Permission is granted to copy, distribute and/or modify this +.. document under the terms of the GNU Free Documentation License, +.. Version 1.1 or any later version published by the Free Software +.. Foundation, with no Invariant Sections, no Front-Cover Texts +.. and no Back-Cover Texts. A copy of the license is included at +.. Documentation/userspace-api/media/fdl-appendix.rst. +.. +.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections + +.. _output: + +********************** +Video Output Interface +********************** + +Video output devices encode stills or image sequences as analog video +signal. With this interface applications can control the encoding +process and move images from user space to the driver. + +Conventionally V4L2 video output devices are accessed through character +device special files named ``/dev/video`` and ``/dev/video0`` to +``/dev/video63`` with major number 81 and minor numbers 0 to 63. +``/dev/video`` is typically a symbolic link to the preferred video +device. + +.. note:: The same device file names are used also for video capture devices. + + +Querying Capabilities +===================== + +Devices supporting the video output interface set the +``V4L2_CAP_VIDEO_OUTPUT`` or ``V4L2_CAP_VIDEO_OUTPUT_MPLANE`` flag in +the ``capabilities`` field of struct +:c:type:`v4l2_capability` returned by the +:ref:`VIDIOC_QUERYCAP` ioctl. As secondary device +functions they may also support the :ref:`raw VBI output <raw-vbi>` +(``V4L2_CAP_VBI_OUTPUT``) interface. At least one of the read/write or +streaming I/O methods must be supported. Modulators and audio outputs +are optional. + + +Supplemental Functions +====================== + +Video output devices shall support :ref:`audio output <audio>`, +:ref:`modulator <tuner>`, :ref:`controls <control>`, +:ref:`cropping and scaling <crop>` and +:ref:`streaming parameter <streaming-par>` ioctls as needed. The +:ref:`video output <video>` ioctls must be supported by all video +output devices. + + +Image Format Negotiation +======================== + +The output is determined by cropping and image format parameters. The +former select an area of the video picture where the image will appear, +the latter how images are stored in memory, i. e. in RGB or YUV format, +the number of bits per pixel or width and height. Together they also +define how images are scaled in the process. + +As usual these parameters are *not* reset at :ref:`open() <func-open>` +time to permit Unix tool chains, programming a device and then writing +to it as if it was a plain file. Well written V4L2 applications ensure +they really get what they want, including cropping and scaling. + +Cropping initialization at minimum requires to reset the parameters to +defaults. An example is given in :ref:`crop`. + +To query the current image format applications set the ``type`` field of +a struct :c:type:`v4l2_format` to +``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE`` +and call the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer +to this structure. Drivers fill the struct +:c:type:`v4l2_pix_format` ``pix`` or the struct +:c:type:`v4l2_pix_format_mplane` ``pix_mp`` +member of the ``fmt`` union. + +To request different parameters applications set the ``type`` field of a +struct :c:type:`v4l2_format` as above and initialize all +fields of the struct :c:type:`v4l2_pix_format` +``vbi`` member of the ``fmt`` union, or better just modify the results +of :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, and call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` +ioctl with a pointer to this structure. Drivers may adjust the +parameters and finally return 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 hardware limitations without disabling I/O or +possibly time consuming hardware preparations. + +The contents of struct :c:type:`v4l2_pix_format` and +struct :c:type:`v4l2_pix_format_mplane` are +discussed in :ref:`pixfmt`. See also the specification of the +:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctls for +details. Video output devices must implement both the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` +and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ignores all +requests and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. +:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is optional. + + +Writing Images +============== + +A video output device may support the :ref:`write() function <rw>` +and/or streaming (:ref:`memory mapping <mmap>` or +:ref:`user pointer <userp>`) I/O. See :ref:`io` for details. |