6.2. Changes of the V4L2 API¶
Soon after the V4L API was added to the kernel it was criticised as too inflexible. In August 1998 Bill Dirks proposed a number of improvements and began to work on documentation, example drivers and applications. With the help of other volunteers this eventually became the V4L2 API, not just an extension but a replacement for the V4L API. However it took another four years and two stable kernel releases until the new API was finally accepted for inclusion into the kernel in its present form.
6.2.1. Early Versions¶
1998-08-20: First version.
1998-08-27: The select() function was introduced.
1998-09-10: New video standard interface.
1998-09-18: The VIDIOC_NONCAP ioctl was replaced by the otherwise
meaningless O_TRUNC open() flag, and the
aliases O_NONCAP and O_NOIO were defined. Applications can set
this flag if they intend to access controls only, as opposed to capture
applications which need exclusive access. The VIDEO_STD_XXX
identifiers are now ordinals instead of flags, and the
video_std_construct() helper function takes id and
transmission arguments.
1998-09-28: Revamped video standard. Made video controls individually enumerable.
1998-10-02: The id field was removed from
struct video_standard and the color subcarrier fields were
renamed. The ioctl VIDIOC_QUERYSTD, VIDIOC_SUBDEV_QUERYSTD ioctl was
renamed to ioctl VIDIOC_ENUMSTD, VIDIOC_SUBDEV_ENUMSTD,
VIDIOC_G_INPUT to
ioctl VIDIOC_ENUMINPUT. A first draft of the
Codec API was released.
1998-11-08: Many minor changes. Most symbols have been renamed. Some
material changes to struct v4l2_capability.
1998-11-12: The read/write direction of some ioctls was misdefined.
1998-11-14: V4L2_PIX_FMT_RGB24 changed to V4L2_PIX_FMT_BGR24,
and V4L2_PIX_FMT_RGB32 changed to V4L2_PIX_FMT_BGR32. Audio
controls are now accessible with the
VIDIOC_G_CTRL and
VIDIOC_S_CTRL ioctls under names starting
with V4L2_CID_AUDIO. The V4L2_MAJOR define was removed from
videodev.h since it was only used once in the videodev kernel
module. The YUV422 and YUV411 planar image formats were added.
1998-11-28: A few ioctl symbols changed. Interfaces for codecs and video output devices were added.
1999-01-14: A raw VBI capture interface was added.
1999-01-19: The VIDIOC_NEXTBUF ioctl was removed.
6.2.2. V4L2 Version 0.16 1999-01-31¶
1999-01-27: There is now one QBUF ioctl, VIDIOC_QWBUF and VIDIOC_QRBUF are gone. VIDIOC_QBUF takes a v4l2_buffer as a parameter. Added digital zoom (cropping) controls.
6.2.3. V4L2 Version 0.18 1999-03-16¶
Added a v4l to V4L2 ioctl compatibility layer to videodev.c. Driver writers, this changes how you implement your ioctl handler. See the Driver Writer’s Guide. Added some more control id codes.
6.2.4. V4L2 Version 0.19 1999-06-05¶
1999-03-18: Fill in the category and catname fields of v4l2_queryctrl objects before passing them to the driver. Required a minor change to the VIDIOC_QUERYCTRL handlers in the sample drivers.
1999-03-31: Better compatibility for v4l memory capture ioctls. Requires changes to drivers to fully support new compatibility features, see Driver Writer’s Guide and v4l2cap.c. Added new control IDs: V4L2_CID_HFLIP, _VFLIP. Changed V4L2_PIX_FMT_YUV422P to _YUV422P, and _YUV411P to _YUV411P.
1999-04-04: Added a few more control IDs.
1999-04-07: Added the button control type.
1999-05-02: Fixed a typo in videodev.h, and added the V4L2_CTRL_FLAG_GRAYED (later V4L2_CTRL_FLAG_GRABBED) flag.
1999-05-20: Definition of VIDIOC_G_CTRL was wrong causing a malfunction of this ioctl.
1999-06-05: Changed the value of V4L2_CID_WHITENESS.
6.2.5. V4L2 Version 0.20 (1999-09-10)¶
Version 0.20 introduced a number of changes which were not backward compatible with 0.19 and earlier versions. Purpose of these changes was to simplify the API, while making it more extensible and following common Linux driver API conventions.
- Some typos in - V4L2_FMT_FLAGsymbols were fixed.- struct v4l2_clipwas changed for compatibility with v4l. (1999-08-30)
- V4L2_TUNER_SUB_LANG1was added. (1999-09-05)
- All ioctl() commands that used an integer argument now take a pointer to an integer. Where it makes sense, ioctls will return the actual new value in the integer pointed to by the argument, a common convention in the V4L2 API. The affected ioctls are: VIDIOC_PREVIEW, VIDIOC_STREAMON, VIDIOC_STREAMOFF, VIDIOC_S_FREQ, VIDIOC_S_INPUT, VIDIOC_S_OUTPUT, VIDIOC_S_EFFECT. For example - err = ioctl (fd, VIDIOC_XXX, V4L2_XXX); - becomes - int a = V4L2_XXX; err = ioctl(fd, VIDIOC_XXX, &a); 
- All the different get- and set-format commands were swept into one VIDIOC_G_FMT and VIDIOC_S_FMT ioctl taking a union and a type field selecting the union member as parameter. Purpose is to simplify the API by eliminating several ioctls and to allow new and driver private data streams without adding new ioctls. - This change obsoletes the following ioctls: - VIDIOC_S_INFMT,- VIDIOC_G_INFMT,- VIDIOC_S_OUTFMT,- VIDIOC_G_OUTFMT,- VIDIOC_S_VBIFMTand- VIDIOC_G_VBIFMT. The image format- struct v4l2_formatwas renamed to- struct v4l2_pix_format, while- struct v4l2_formatis now the enveloping structure for all format negotiations.
- Similar to the changes above, the - VIDIOC_G_PARMand- VIDIOC_S_PARMioctls were merged with- VIDIOC_G_OUTPARMand- VIDIOC_S_OUTPARM. A- typefield in the new- struct v4l2_streamparmselects the respective union member.- This change obsoletes the - VIDIOC_G_OUTPARMand- VIDIOC_S_OUTPARMioctls.
- Control enumeration was simplified, and two new control flags were introduced and one dropped. The - catnamefield was replaced by a- groupfield.- Drivers can now flag unsupported and temporarily unavailable controls with - V4L2_CTRL_FLAG_DISABLEDand- V4L2_CTRL_FLAG_GRABBEDrespectively. The- groupname indicates a possibly narrower classification than the- category. In other words, there may be multiple groups within a category. Controls within a group would typically be drawn within a group box. Controls in different categories might have a greater separation, or may even appear in separate windows.
- The - struct v4l2_buffer- timestampwas changed to a 64 bit integer, containing the sampling or output time of the frame in nanoseconds. Additionally timestamps will be in absolute system time, not starting from zero at the beginning of a stream. The data type name for timestamps is stamp_t, defined as a signed 64-bit integer. Output devices should not send a buffer out until the time in the timestamp field has arrived. I would like to follow SGI’s lead, and adopt a multimedia timestamping system like their UST (Unadjusted System Time). See http://web.archive.org/web/*/http://reality.sgi.com /cpirazzi_engr/lg/time/intro.html. UST uses timestamps that are 64-bit signed integers (not struct timeval’s) and given in nanosecond units. The UST clock starts at zero when the system is booted and runs continuously and uniformly. It takes a little over 292 years for UST to overflow. There is no way to set the UST clock. The regular Linux time-of-day clock can be changed periodically, which would cause errors if it were being used for timestamping a multimedia stream. A real UST style clock will require some support in the kernel that is not there yet. But in anticipation, I will change the timestamp field to a 64-bit integer, and I will change the v4l2_masterclock_gettime() function (used only by drivers) to return a 64-bit integer.
- A - sequencefield was added to- struct v4l2_buffer. The- sequencefield counts captured frames, it is ignored by output devices. When a capture driver drops a frame, the sequence number of that frame is skipped.
6.2.6. V4L2 Version 0.20 incremental changes¶
1999-12-23: In struct v4l2_vbi_format the
reserved1 field became offset. Previously drivers were required
to clear the reserved1 field.
2000-01-13: The V4L2_FMT_FLAG_NOT_INTERLACED flag was added.
2000-07-31: The linux/poll.h header is now included by
videodev.h for compatibility with the original videodev.h file.
2000-11-20: V4L2_TYPE_VBI_OUTPUT and V4L2_PIX_FMT_Y41P were
added.
2000-11-25: V4L2_TYPE_VBI_INPUT was added.
2000-12-04: A couple typos in symbol names were fixed.
2001-01-18: To avoid namespace conflicts the fourcc macro defined in
the videodev.h header file was renamed to v4l2_fourcc.
2001-01-25: A possible driver-level compatibility problem between the
videodev.h file in Linux 2.4.0 and the videodev.h file included
in the videodevX patch was fixed. Users of an earlier version of
videodevX on Linux 2.4.0 should recompile their V4L and V4L2
drivers.
2001-01-26: A possible kernel-level incompatibility between the
videodev.h file in the videodevX patch and the videodev.h
file in Linux 2.2.x with devfs patches applied was fixed.
2001-03-02: Certain V4L ioctls which pass data in both direction although they are defined with read-only parameter, did not work correctly through the backward compatibility layer. [Solution?]
2001-04-13: Big endian 16-bit RGB formats were added.
2001-09-17: New YUV formats and the
VIDIOC_G_FREQUENCY and
VIDIOC_S_FREQUENCY ioctls were added.
(The old VIDIOC_G_FREQ and VIDIOC_S_FREQ ioctls did not take
multiple tuners into account.)
2000-09-18: V4L2_BUF_TYPE_VBI was added. This may break
compatibility as the VIDIOC_G_FMT and
VIDIOC_S_FMT ioctls may fail now if the
struct v4l2_fmt type field does not contain
V4L2_BUF_TYPE_VBI. In the documentation of the struct v4l2_vbi_format`,
the offset field the ambiguous phrase “rising edge” was changed to
“leading edge”.
6.2.7. V4L2 Version 0.20 2000-11-23¶
A number of changes were made to the raw VBI interface.
- Figures clarifying the line numbering scheme were added to the V4L2 API specification. The - start[0] and- start[1] fields no longer count line numbers beginning at zero. Rationale: a) The previous definition was unclear. b) The- start[] values are ordinal numbers. c) There is no point in inventing a new line numbering scheme. We now use line number as defined by ITU-R, period. Compatibility: Add one to the start values. Applications depending on the previous semantics may not function correctly.
- The restriction “count[0] > 0 and count[1] > 0” has been relaxed to “(count[0] + count[1]) > 0”. Rationale: Drivers may allocate resources at scan line granularity and some data services are transmitted only on the first field. The comment that both - countvalues will usually be equal is misleading and pointless and has been removed. This change breaks compatibility with earlier versions: Drivers may return- EINVAL, applications may not function correctly.
- Drivers are again permitted to return negative (unknown) start values as proposed earlier. Why this feature was dropped is unclear. This change may break compatibility with applications depending on the start values being positive. The use of - EBUSYand- EINVALerror codes with the VIDIOC_S_FMT ioctl was clarified. The- EBUSYerror code was finally documented, and the- reserved2field which was previously mentioned only in the- videodev.hheader file.
- New buffer types - V4L2_TYPE_VBI_INPUTand- V4L2_TYPE_VBI_OUTPUTwere added. The former is an alias for the old- V4L2_TYPE_VBI, the latter was missing in the- videodev.hfile.
6.2.8. V4L2 Version 0.20 2002-07-25¶
Added sliced VBI interface proposal.
6.2.9. V4L2 in Linux 2.5.46, 2002-10¶
Around October-November 2002, prior to an announced feature freeze of Linux 2.5, the API was revised, drawing from experience with V4L2 0.20. This unnamed version was finally merged into Linux 2.5.46.
- As specified in Related Devices, drivers must make related device functions available under all minor device numbers. 
- The - open()function requires access mode- O_RDWRregardless of the device type. All V4L2 drivers exchanging data with applications must support the- O_NONBLOCKflag. The- O_NOIOflag, a V4L2 symbol which aliased the meaningless- O_TRUNCto indicate accesses without data exchange (panel applications) was dropped. Drivers must stay in “panel mode” until the application attempts to initiate a data exchange, see Opening and Closing Devices.
- The - struct v4l2_capabilitychanged dramatically. Note that also the size of the structure changed, which is encoded in the ioctl request code, thus older V4L2 devices will respond with an- EINVALerror code to the new ioctl VIDIOC_QUERYCAP ioctl.- There are new fields to identify the driver, a new RDS device function - V4L2_CAP_RDS_CAPTURE, the- V4L2_CAP_AUDIOflag indicates if the device has any audio connectors, another I/O capability V4L2_CAP_ASYNCIO can be flagged. In response to these changes the- typefield became a bit set and was merged into the- flagsfield.- V4L2_FLAG_TUNERwas renamed to- V4L2_CAP_TUNER,- V4L2_CAP_VIDEO_OVERLAYreplaced- V4L2_FLAG_PREVIEWand- V4L2_CAP_VBI_CAPTUREand- V4L2_CAP_VBI_OUTPUTreplaced- V4L2_FLAG_DATA_SERVICE.- V4L2_FLAG_READand- V4L2_FLAG_WRITEwere merged into- V4L2_CAP_READWRITE.- The redundant fields - inputs,- outputsand- audioswere removed. These properties can be determined as described in Video Inputs and Outputs and Audio Inputs and Outputs.- The somewhat volatile and therefore barely useful fields - maxwidth,- maxheight,- minwidth,- minheight,- maxframeratewere removed. This information is available as described in Data Formats and Video Standards.- V4L2_FLAG_SELECTwas removed. We believe the- select()function is important enough to require support of it in all V4L2 drivers exchanging data with applications. The redundant- V4L2_FLAG_MONOCHROMEflag was removed, this information is available as described in Data Formats.
- In - struct v4l2_inputthe- assoc_audiofield and the- capabilityfield and its only flag- V4L2_INPUT_CAP_AUDIOwas replaced by the new- audiosetfield. Instead of linking one video input to one audio input this field reports all audio inputs this video input combines with.- New fields are - tuner(reversing the former link from tuners to video inputs),- stdand- status.- Accordingly - struct v4l2_outputlost its- capabilityand- assoc_audiofields.- audioset,- modulatorand- stdwhere added instead.
- The - struct v4l2_audiofield- audiowas renamed to- index, for consistency with other structures. A new capability flag- V4L2_AUDCAP_STEREOwas added to indicated if the audio input in question supports stereo sound.- V4L2_AUDCAP_EFFECTSand the corresponding- V4L2_AUDMODEflags where removed. This can be easily implemented using controls. (However the same applies to AVL which is still there.)- Again for consistency the - struct v4l2_audiooutfield- audiowas renamed to- index.
- The - struct v4l2_tuner- inputfield was replaced by an- indexfield, permitting devices with multiple tuners. The link between video inputs and tuners is now reversed, inputs point to their tuner. The- stdsubstructure became a simple set (more about this below) and moved into- struct v4l2_input. A- typefield was added.- Accordingly in - struct v4l2_modulatorthe- outputwas replaced by an- indexfield.- In - struct v4l2_frequencythe- portfield was replaced by a- tunerfield containing the respective tuner or modulator index number. A tuner- typefield was added and the- reservedfield became larger for future extensions (satellite tuners in particular).
- The idea of completely transparent video standards was dropped. Experience showed that applications must be able to work with video standards beyond presenting the user a menu. Instead of enumerating supported standards with an ioctl applications can now refer to standards by v4l2_std_id and symbols defined in the - videodev2.hheader file. For details see Video Standards. The VIDIOC_G_STD and VIDIOC_S_STD now take a pointer to this type as argument. ioctl VIDIOC_QUERYSTD, VIDIOC_SUBDEV_QUERYSTD was added to autodetect the received standard, if the hardware has this capability. In- struct v4l2_standardan- indexfield was added for ioctl VIDIOC_ENUMSTD, VIDIOC_SUBDEV_ENUMSTD. A v4l2_std_id field named- idwas added as machine readable identifier, also replacing the- transmissionfield. The misleading- frameratefield was renamed to- frameperiod. The now obsolete- colorstandardinformation, originally needed to distguish between variations of standards, were removed.- Struct - v4l2_enumstdceased to be. ioctl VIDIOC_ENUMSTD, VIDIOC_SUBDEV_ENUMSTD now takes a pointer to a- struct v4l2_standarddirectly. The information which standards are supported by a particular video input or output moved into- struct v4l2_inputand- struct v4l2_outputfields named- std, respectively.
- The struct v4l2_queryctrl fields - categoryand- groupdid not catch on and/or were not implemented as expected and therefore removed.
- The VIDIOC_TRY_FMT ioctl was added to negotiate data formats as with VIDIOC_S_FMT, but without the overhead of programming the hardware and regardless of I/O in progress. - In - struct v4l2_formatthe- fmtunion was extended to contain- struct v4l2_window. All image format negotiations are now possible with- VIDIOC_G_FMT,- VIDIOC_S_FMTand- VIDIOC_TRY_FMT; ioctl. The- VIDIOC_G_WINand- VIDIOC_S_WINioctls to prepare for a video overlay were removed. The- typefield changed to type- enum v4l2_buf_typeand the buffer type names changed as follows.- Old defines - V4L2_BUF_TYPE_CAPTURE- V4L2_BUF_TYPE_VIDEO_CAPTURE- V4L2_BUF_TYPE_CODECIN- Omitted for now - V4L2_BUF_TYPE_CODECOUT- Omitted for now - V4L2_BUF_TYPE_EFFECTSIN- Omitted for now - V4L2_BUF_TYPE_EFFECTSIN2- Omitted for now - V4L2_BUF_TYPE_EFFECTSOUT- Omitted for now - V4L2_BUF_TYPE_VIDEOOUT- V4L2_BUF_TYPE_VIDEO_OUTPUT- -- V4L2_BUF_TYPE_VIDEO_OVERLAY- -- V4L2_BUF_TYPE_VBI_CAPTURE- -- V4L2_BUF_TYPE_VBI_OUTPUT- -- V4L2_BUF_TYPE_SLICED_VBI_CAPTURE- -- V4L2_BUF_TYPE_SLICED_VBI_OUTPUT- V4L2_BUF_TYPE_PRIVATE_BASE- V4L2_BUF_TYPE_PRIVATE(but this is deprecated)
- In - struct v4l2_fmtdesca- enum v4l2_buf_typefield named- typewas added as in- struct v4l2_format. The- VIDIOC_ENUM_FBUFFMTioctl is no longer needed and was removed. These calls can be replaced by ioctl VIDIOC_ENUM_FMT with type- V4L2_BUF_TYPE_VIDEO_OVERLAY.
- In - struct v4l2_pix_formatthe- depthfield was removed, assuming applications which recognize the format by its four-character-code already know the color depth, and others do not care about it. The same rationale lead to the removal of the- V4L2_FMT_FLAG_COMPRESSEDflag. The- V4L2_FMT_FLAG_SWCONVECOMPRESSEDflag was removed because drivers are not supposed to convert images in kernel space. A user library of conversion functions should be provided instead. The- V4L2_FMT_FLAG_BYTESPERLINEflag was redundant. Applications can set the- bytesperlinefield to zero to get a reasonable default. Since the remaining flags were replaced as well, the- flagsfield itself was removed.- The interlace flags were replaced by a - enum v4l2_fieldvalue in a newly added- fieldfield.- Old flag - V4L2_FMT_FLAG_NOT_INTERLACED- ? - V4L2_FMT_FLAG_INTERLACED=- V4L2_FMT_FLAG_COMBINED- V4L2_FIELD_INTERLACED- V4L2_FMT_FLAG_TOPFIELD=- V4L2_FMT_FLAG_ODDFIELD- V4L2_FIELD_TOP- V4L2_FMT_FLAG_BOTFIELD=- V4L2_FMT_FLAG_EVENFIELD- V4L2_FIELD_BOTTOM- -- V4L2_FIELD_SEQ_TB- -- V4L2_FIELD_SEQ_BT- -- V4L2_FIELD_ALTERNATE- The color space flags were replaced by a - enum v4l2_colorspacevalue in a newly added- colorspacefield, where one of- V4L2_COLORSPACE_SMPTE170M,- V4L2_COLORSPACE_BT878,- V4L2_COLORSPACE_470_SYSTEM_Mor- V4L2_COLORSPACE_470_SYSTEM_BGreplaces- V4L2_FMT_CS_601YUV.
- In - struct v4l2_requestbuffersthe- typefield was properly defined as- enum v4l2_buf_type. Buffer types changed as mentioned above. A new- memoryfield of type enum v4l2_memory was added to distinguish between I/O methods using buffers allocated by the driver or the application. See Input/Output for details.
- In - struct v4l2_bufferthe- typefield was properly defined as- enum v4l2_buf_type. Buffer types changed as mentioned above. A- fieldfield of type- enum v4l2_fieldwas added to indicate if a buffer contains a top or bottom field. The old field flags were removed. Since no unadjusted system time clock was added to the kernel as planned, the- timestampfield changed back from type stamp_t, an unsigned 64 bit integer expressing the sample time in nanoseconds, to struct timeval. With the addition of a second memory mapping method the- offsetfield moved into union- m, and a new- memoryfield of type enum v4l2_memory was added to distinguish between I/O methods. See Input/Output for details.- The - V4L2_BUF_REQ_CONTIGflag was used by the V4L compatibility layer, after changes to this code it was no longer needed. The- V4L2_BUF_ATTR_DEVICEMEMflag would indicate if the buffer was indeed allocated in device memory rather than DMA-able system memory. It was barely useful and so was removed.
- In - struct v4l2_framebufferthe- base[3]array anticipating double- and triple-buffering in off-screen video memory, however without defining a synchronization mechanism, was replaced by a single pointer. The- V4L2_FBUF_CAP_SCALEUPand- V4L2_FBUF_CAP_SCALEDOWNflags were removed. Applications can determine this capability more accurately using the new cropping and scaling interface. The- V4L2_FBUF_CAP_CLIPPINGflag was replaced by- V4L2_FBUF_CAP_LIST_CLIPPINGand- V4L2_FBUF_CAP_BITMAP_CLIPPING.
- In - struct v4l2_clipthe- x,- y,- widthand- heightfield moved into a- csubstructure of type- struct v4l2_rect. The- xand- yfields were renamed to- leftand- top, i. e. offsets to a context dependent origin.
- In - struct v4l2_windowthe- x,- y,- widthand- heightfield moved into a- wsubstructure as above. A- fieldfield of type- enum v4l2_fieldwas added to distinguish between field and frame (interlaced) overlay.
- The digital zoom interface, including struct - v4l2_zoomcap, struct- v4l2_zoom,- V4L2_ZOOM_NONCAPand- V4L2_ZOOM_WHILESTREAMINGwas replaced by a new cropping and scaling interface. The previously unused- struct v4l2_cropcapand- struct v4l2_cropwhere redefined for this purpose. See Image Cropping, Insertion and Scaling -- the CROP API for details.
- In - struct v4l2_vbi_formatthe- SAMPLE_FORMATfield now contains a four-character-code as used to identify video image formats and- V4L2_PIX_FMT_GREYreplaces the- V4L2_VBI_SF_UBYTEdefine. The- reservedfield was extended.
- In - struct v4l2_captureparmthe type of the- timeperframefield changed from unsigned long to- struct v4l2_fract. This allows the accurate expression of multiples of the NTSC-M frame rate 30000 / 1001. A new field- readbufferswas added to control the driver behaviour in read I/O mode.- Similar changes were made to - struct v4l2_outputparm.
- The struct - v4l2_performanceand- VIDIOC_G_PERFioctl were dropped. Except when using the read/write I/O method, which is limited anyway, this information is already available to applications.
- The example transformation from RGB to YCbCr color space in the old V4L2 documentation was inaccurate, this has been corrected in Image Formats. 
6.2.10. V4L2 2003-06-19¶
- A new capability flag - V4L2_CAP_RADIOwas added for radio devices. Prior to this change radio devices would identify solely by having exactly one tuner whose type field reads- V4L2_TUNER_RADIO.
- An optional driver access priority mechanism was added, see Application Priority for details. 
- The audio input and output interface was found to be incomplete. - Previously the VIDIOC_G_AUDIO ioctl would enumerate the available audio inputs. An ioctl to determine the current audio input, if more than one combines with the current video input, did not exist. So - VIDIOC_G_AUDIOwas renamed to- VIDIOC_G_AUDIO_OLD, this ioctl was removed on Kernel 2.6.39. The ioctl VIDIOC_ENUMAUDIO ioctl was added to enumerate audio inputs, while VIDIOC_G_AUDIO now reports the current audio input.- The same changes were made to VIDIOC_G_AUDOUT and VIDIOC_ENUMAUDOUT. - Until further the “videodev” module will automatically translate between the old and new ioctls, but drivers and applications must be updated to successfully compile again. 
- The ioctl VIDIOC_OVERLAY ioctl was incorrectly defined with write-read parameter. It was changed to write-only, while the write-read version was renamed to - VIDIOC_OVERLAY_OLD. The old ioctl was removed on Kernel 2.6.39. Until further the “videodev” kernel module will automatically translate to the new version, so drivers must be recompiled, but not applications.
- Video Overlay Interface incorrectly stated that clipping rectangles define regions where the video can be seen. Correct is that clipping rectangles define regions where no video shall be displayed and so the graphics surface can be seen. 
- The VIDIOC_S_PARM and VIDIOC_S_CTRL ioctls were defined with write-only parameter, inconsistent with other ioctls modifying their argument. They were changed to write-read, while a - _OLDsuffix was added to the write-only versions. The old ioctls were removed on Kernel 2.6.39. Drivers and applications assuming a constant parameter need an update.
6.2.11. V4L2 2003-11-05¶
- In RGB Formats the following pixel formats were incorrectly transferred from Bill Dirks’ V4L2 specification. Descriptions below refer to bytes in memory, in ascending address order. - Symbol - In this document prior to revision 0.5 - Corrected - V4L2_PIX_FMT_RGB24- B, G, R - R, G, B - V4L2_PIX_FMT_BGR24- R, G, B - B, G, R - V4L2_PIX_FMT_RGB32- B, G, R, X - R, G, B, X - V4L2_PIX_FMT_BGR32- R, G, B, X - B, G, R, X - The - V4L2_PIX_FMT_BGR24example was always correct.- In Image Properties the mapping of the V4L - VIDEO_PALETTE_RGB24and- VIDEO_PALETTE_RGB32formats to V4L2 pixel formats was accordingly corrected.
- Unrelated to the fixes above, drivers may still interpret some V4L2 RGB pixel formats differently. These issues have yet to be addressed, for details see RGB Formats. 
6.2.12. V4L2 in Linux 2.6.6, 2004-05-09¶
- The ioctl VIDIOC_CROPCAP ioctl was incorrectly defined with read-only parameter. It is now defined as write-read ioctl, while the read-only version was renamed to - VIDIOC_CROPCAP_OLD. The old ioctl was removed on Kernel 2.6.39.
6.2.13. V4L2 in Linux 2.6.8¶
- A new field - input(former- reserved[0]) was added to the- struct v4l2_buffer. Purpose of this field is to alternate between video inputs (e. g. cameras) in step with the video capturing process. This function must be enabled with the new- V4L2_BUF_FLAG_INPUTflag. The- flagsfield is no longer read-only.
6.2.14. V4L2 spec erratum 2004-08-01¶
- The return value of the V4L2 open() function was incorrectly documented. 
- Audio output ioctls end in -AUDOUT, not -AUDIOOUT. 
- In the Current Audio Input example the - VIDIOC_G_AUDIOioctl took the wrong argument.
- The documentation of the ioctl VIDIOC_QBUF, VIDIOC_DQBUF and VIDIOC_DQBUF ioctls did not mention the - struct v4l2_buffer- memoryfield. It was also missing from examples. Also on the- VIDIOC_DQBUFpage the- EIOerror code was not documented.
6.2.15. V4L2 in Linux 2.6.14¶
- A new sliced VBI interface was added. It is documented in Sliced VBI Data Interface and replaces the interface first proposed in V4L2 specification 0.8. 
6.2.16. V4L2 in Linux 2.6.15¶
- The ioctl VIDIOC_LOG_STATUS ioctl was added. 
- New video standards - V4L2_STD_NTSC_443,- V4L2_STD_SECAM_LC,- V4L2_STD_SECAM_DK(a set of SECAM D, K and K1), and- V4L2_STD_ATSC(a set of- V4L2_STD_ATSC_8_VSBand- V4L2_STD_ATSC_16_VSB) were defined. Note the- V4L2_STD_525_60set now includes- V4L2_STD_NTSC_443. See also typedef v4l2_std_id.
- The - VIDIOC_G_COMPand- VIDIOC_S_COMPioctl were renamed to- VIDIOC_G_MPEGCOMPand- VIDIOC_S_MPEGCOMPrespectively. Their argument was replaced by a struct- v4l2_mpeg_compressionpointer. (The- VIDIOC_G_MPEGCOMPand- VIDIOC_S_MPEGCOMPioctls where removed in Linux 2.6.25.)
6.2.17. V4L2 spec erratum 2005-11-27¶
The capture example in Video Capture Example called the VIDIOC_S_CROP ioctl without checking if cropping is supported. In the video standard selection example in Video Standards the VIDIOC_S_STD call used the wrong argument type.
6.2.18. V4L2 spec erratum 2006-01-10¶
- The - V4L2_IN_ST_COLOR_KILLflag in- struct v4l2_inputnot only indicates if the color killer is enabled, but also if it is active. (The color killer disables color decoding when it detects no color in the video signal to improve the image quality.)
- VIDIOC_S_PARM is a write-read ioctl, not write-only as stated on its reference page. The ioctl changed in 2003 as noted above. 
6.2.19. V4L2 spec erratum 2006-02-03¶
- In - struct v4l2_captureparmand- struct v4l2_outputparmthe- timeperframefield gives the time in seconds, not microseconds.
6.2.20. V4L2 spec erratum 2006-02-04¶
- The - clipsfield in- struct v4l2_windowmust point to an array of- struct v4l2_clip, not a linked list, because drivers ignore the- struct v4l2_clip.- nextpointer.
6.2.21. V4L2 in Linux 2.6.17¶
- New video standard macros were added: - V4L2_STD_NTSC_M_KR(NTSC M South Korea), and the sets- V4L2_STD_MN,- V4L2_STD_B,- V4L2_STD_GHand- V4L2_STD_DK. The- V4L2_STD_NTSCand- V4L2_STD_SECAMsets now include- V4L2_STD_NTSC_M_KRand- V4L2_STD_SECAM_LCrespectively.
- A new - V4L2_TUNER_MODE_LANG1_LANG2was defined to record both languages of a bilingual program. The use of- V4L2_TUNER_MODE_STEREOfor this purpose is deprecated now. See the VIDIOC_G_TUNER section for details.
6.2.22. V4L2 spec erratum 2006-09-23 (Draft 0.15)¶
- In various places - V4L2_BUF_TYPE_SLICED_VBI_CAPTUREand- V4L2_BUF_TYPE_SLICED_VBI_OUTPUTof the sliced VBI interface were not mentioned along with other buffer types.
- In VIDIOC_G_AUDIO it was clarified that the - struct v4l2_audio- modefield is a flags field.
- ioctl VIDIOC_QUERYCAP did not mention the sliced VBI and radio capability flags. 
- In VIDIOC_G_FREQUENCY it was clarified that applications must initialize the tuner - typefield of- struct v4l2_frequencybefore calling VIDIOC_S_FREQUENCY.
- The - reservedarray in- struct v4l2_requestbuffershas 2 elements, not 32.
- In Video Output Interface and Raw VBI Data Interface the device file names - /dev/voutwhich never caught on were replaced by- /dev/video.
- With Linux 2.6.15 the possible range for VBI device minor numbers was extended from 224-239 to 224-255. Accordingly device file names - /dev/vbi0to- /dev/vbi31are possible now.
6.2.23. V4L2 in Linux 2.6.18¶
- New ioctls VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS and VIDIOC_TRY_EXT_CTRLS were added, a flag to skip unsupported controls with ioctls VIDIOC_QUERYCTRL, VIDIOC_QUERY_EXT_CTRL and VIDIOC_QUERYMENU, new control types - V4L2_CTRL_TYPE_INTEGER64and- V4L2_CTRL_TYPE_CTRL_CLASS(- enum v4l2_ctrl_type), and new control flags- V4L2_CTRL_FLAG_READ_ONLY,- V4L2_CTRL_FLAG_UPDATE,- V4L2_CTRL_FLAG_INACTIVEand- V4L2_CTRL_FLAG_SLIDER(Control Flags). See Extended Controls API for details.
6.2.24. V4L2 in Linux 2.6.19¶
- In - struct v4l2_sliced_vbi_capa buffer type field was added replacing a reserved field. Note on architectures where the size of enum types differs from int types the size of the structure changed. The VIDIOC_G_SLICED_VBI_CAP ioctl was redefined from being read-only to write-read. Applications must initialize the type field and clear the reserved fields now. These changes may break the compatibility with older drivers and applications.
- The ioctls ioctl VIDIOC_ENUM_FRAMESIZES and ioctl VIDIOC_ENUM_FRAMEINTERVALS were added. 
- A new pixel format - V4L2_PIX_FMT_RGB444(RGB Formats) was added.
6.2.25. V4L2 spec erratum 2006-10-12 (Draft 0.17)¶
- V4L2_PIX_FMT_HM12(Reserved Image Formats) is a YUV 4:2:0, not 4:2:2 format.
6.2.26. V4L2 in Linux 2.6.21¶
- The - videodev2.hheader file is now dual licensed under GNU General Public License version two or later, and under a 3-clause BSD-style license.
6.2.27. V4L2 in Linux 2.6.22¶
- Two new field orders - V4L2_FIELD_INTERLACED_TBand- V4L2_FIELD_INTERLACED_BTwere added. See- enum v4l2_fieldfor details.
- Three new clipping/blending methods with a global or straight or inverted local alpha value were added to the video overlay interface. See the description of the VIDIOC_G_FBUF and VIDIOC_S_FBUF ioctls for details. - A new - global_alphafield was added to- struct v4l2_window, extending the structure. This may break compatibility with applications using a- struct v4l2_windowdirectly. However the VIDIOC_G/S/TRY_FMT ioctls, which take a pointer to a- struct v4l2_formatparent structure with padding bytes at the end, are not affected.
- The format of the - chromakeyfield in- struct v4l2_windowchanged from “host order RGB32” to a pixel value in the same format as the framebuffer. This may break compatibility with existing applications. Drivers supporting the “host order RGB32” format are not known.
6.2.28. V4L2 in Linux 2.6.24¶
- The pixel formats - V4L2_PIX_FMT_PAL8,- V4L2_PIX_FMT_YUV444,- V4L2_PIX_FMT_YUV555,- V4L2_PIX_FMT_YUV565and- V4L2_PIX_FMT_YUV32were added.
6.2.29. V4L2 in Linux 2.6.25¶
- The pixel formats V4L2_PIX_FMT_Y16 and V4L2_PIX_FMT_SBGGR16 were added. 
- New controls - V4L2_CID_POWER_LINE_FREQUENCY,- V4L2_CID_HUE_AUTO,- V4L2_CID_WHITE_BALANCE_TEMPERATURE,- V4L2_CID_SHARPNESSand- V4L2_CID_BACKLIGHT_COMPENSATIONwere added. The controls- V4L2_CID_BLACK_LEVEL,- V4L2_CID_WHITENESS,- V4L2_CID_HCENTERand- V4L2_CID_VCENTERwere deprecated.
- A Camera controls class was added, with the new controls - V4L2_CID_EXPOSURE_AUTO,- V4L2_CID_EXPOSURE_ABSOLUTE,- V4L2_CID_EXPOSURE_AUTO_PRIORITY,- V4L2_CID_PAN_RELATIVE,- V4L2_CID_TILT_RELATIVE,- V4L2_CID_PAN_RESET,- V4L2_CID_TILT_RESET,- V4L2_CID_PAN_ABSOLUTE,- V4L2_CID_TILT_ABSOLUTE,- V4L2_CID_FOCUS_ABSOLUTE,- V4L2_CID_FOCUS_RELATIVEand- V4L2_CID_FOCUS_AUTO.
- The - VIDIOC_G_MPEGCOMPand- VIDIOC_S_MPEGCOMPioctls, which were superseded by the extended controls interface in Linux 2.6.18, where finally removed from the- videodev2.hheader file.
6.2.30. V4L2 in Linux 2.6.26¶
- The pixel formats - V4L2_PIX_FMT_Y16and- V4L2_PIX_FMT_SBGGR16were added.
- Added user controls - V4L2_CID_CHROMA_AGCand- V4L2_CID_COLOR_KILLER.
6.2.31. V4L2 in Linux 2.6.27¶
- The ioctl VIDIOC_S_HW_FREQ_SEEK ioctl and the - V4L2_CAP_HW_FREQ_SEEKcapability were added.
- The pixel formats - V4L2_PIX_FMT_YVYU,- V4L2_PIX_FMT_PCA501,- V4L2_PIX_FMT_PCA505,- V4L2_PIX_FMT_PCA508,- V4L2_PIX_FMT_PCA561,- V4L2_PIX_FMT_SGBRG8,- V4L2_PIX_FMT_PAC207and- V4L2_PIX_FMT_PJPGwere added.
6.2.32. V4L2 in Linux 2.6.28¶
- Added - V4L2_MPEG_AUDIO_ENCODING_AACand- V4L2_MPEG_AUDIO_ENCODING_AC3MPEG audio encodings.
- Added - V4L2_MPEG_VIDEO_ENCODING_MPEG_4_AVCMPEG video encoding.
- The pixel formats - V4L2_PIX_FMT_SGRBG10and- V4L2_PIX_FMT_SGRBG10DPCM8were added.
6.2.33. V4L2 in Linux 2.6.29¶
- The - VIDIOC_G_CHIP_IDENTioctl was renamed to- VIDIOC_G_CHIP_IDENT_OLDand- VIDIOC_DBG_G_CHIP_IDENTwas introduced in its place. The old struct- v4l2_chip_identwas renamed to struct- v4l2_chip_ident_old.
- The pixel formats - V4L2_PIX_FMT_VYUY,- V4L2_PIX_FMT_NV16and- V4L2_PIX_FMT_NV61were added.
- Added camera controls - V4L2_CID_ZOOM_ABSOLUTE,- V4L2_CID_ZOOM_RELATIVE,- V4L2_CID_ZOOM_CONTINUOUSand- V4L2_CID_PRIVACY.
6.2.34. V4L2 in Linux 2.6.30¶
- New control flag - V4L2_CTRL_FLAG_WRITE_ONLYwas added.
- New control - V4L2_CID_COLORFXwas added.
6.2.35. V4L2 in Linux 2.6.32¶
- In order to be easier to compare a V4L2 API and a kernel version, now V4L2 API is numbered using the Linux Kernel version numeration. 
- Finalized the RDS capture API. See RDS Interface for more information. 
- Added new capabilities for modulators and RDS encoders. 
- Add description for libv4l API. 
- Added support for string controls via new type - V4L2_CTRL_TYPE_STRING.
- Added - V4L2_CID_BAND_STOP_FILTERdocumentation.
- Added FM Modulator (FM TX) Extended Control Class: - V4L2_CTRL_CLASS_FM_TXand their Control IDs.
- Added FM Receiver (FM RX) Extended Control Class: - V4L2_CTRL_CLASS_FM_RXand their Control IDs.
- Added Remote Controller chapter, describing the default Remote Controller mapping for media devices. 
6.2.36. V4L2 in Linux 2.6.33¶
- Added support for Digital Video timings in order to support HDTV receivers and transmitters. 
6.2.37. V4L2 in Linux 2.6.34¶
- Added - V4L2_CID_IRIS_ABSOLUTEand- V4L2_CID_IRIS_RELATIVEcontrols to the Camera controls class.
6.2.38. V4L2 in Linux 2.6.37¶
- Remove the vtx (videotext/teletext) API. This API was no longer used and no hardware exists to verify the API. Nor were any userspace applications found that used it. It was originally scheduled for removal in 2.6.35. 
6.2.39. V4L2 in Linux 2.6.39¶
- The old VIDIOC_*_OLD symbols and V4L1 support were removed. 
- Multi-planar API added. Does not affect the compatibility of current drivers and applications. See multi-planar API for details. 
6.2.40. V4L2 in Linux 3.1¶
- VIDIOC_QUERYCAP now returns a per-subsystem version instead of a per-driver one. - Standardize an error code for invalid ioctl. - Added V4L2_CTRL_TYPE_BITMASK. 
6.2.41. V4L2 in Linux 3.2¶
- V4L2_CTRL_FLAG_VOLATILE was added to signal volatile controls to userspace. 
- Add selection API for extended control over cropping and composing. Does not affect the compatibility of current drivers and applications. See selection API for details. 
6.2.42. V4L2 in Linux 3.3¶
- Added - V4L2_CID_ALPHA_COMPONENTcontrol to the User controls class.
- Added the device_caps field to struct v4l2_capabilities and added the new V4L2_CAP_DEVICE_CAPS capability. 
6.2.43. V4L2 in Linux 3.4¶
- Extended the DV Timings API: ioctl VIDIOC_ENUM_DV_TIMINGS, VIDIOC_SUBDEV_ENUM_DV_TIMINGS, ioctl VIDIOC_QUERY_DV_TIMINGS and ioctl VIDIOC_DV_TIMINGS_CAP, VIDIOC_SUBDEV_DV_TIMINGS_CAP. 
6.2.44. V4L2 in Linux 3.5¶
- Added integer menus, the new type will be V4L2_CTRL_TYPE_INTEGER_MENU. 
- Added selection API for V4L2 subdev interface: ioctl VIDIOC_SUBDEV_G_SELECTION, VIDIOC_SUBDEV_S_SELECTION and VIDIOC_SUBDEV_S_SELECTION. 
- Added - V4L2_COLORFX_ANTIQUE,- V4L2_COLORFX_ART_FREEZE,- V4L2_COLORFX_AQUA,- V4L2_COLORFX_SILHOUETTE,- V4L2_COLORFX_SOLARIZATION,- V4L2_COLORFX_VIVIDand- V4L2_COLORFX_ARBITRARY_CBCRmenu items to the- V4L2_CID_COLORFXcontrol.
- Added - V4L2_CID_COLORFX_CBCRcontrol.
- Added camera controls - V4L2_CID_AUTO_EXPOSURE_BIAS,- V4L2_CID_AUTO_N_PRESET_WHITE_BALANCE,- V4L2_CID_IMAGE_STABILIZATION,- V4L2_CID_ISO_SENSITIVITY,- V4L2_CID_ISO_SENSITIVITY_AUTO,- V4L2_CID_EXPOSURE_METERING,- V4L2_CID_SCENE_MODE,- V4L2_CID_3A_LOCK,- V4L2_CID_AUTO_FOCUS_START,- V4L2_CID_AUTO_FOCUS_STOP,- V4L2_CID_AUTO_FOCUS_STATUSand- V4L2_CID_AUTO_FOCUS_RANGE.
6.2.45. V4L2 in Linux 3.6¶
- Replaced - inputin- struct v4l2_bufferby- reserved2and removed- V4L2_BUF_FLAG_INPUT.
- Added V4L2_CAP_VIDEO_M2M and V4L2_CAP_VIDEO_M2M_MPLANE capabilities. 
- Added support for frequency band enumerations: ioctl VIDIOC_ENUM_FREQ_BANDS. 
6.2.46. V4L2 in Linux 3.9¶
- Added timestamp types to - flagsfield in- struct v4l2_buffer. See Buffer Flags.
- Added - V4L2_EVENT_CTRL_CH_RANGEcontrol event changes flag. See Control Changes.
6.2.47. V4L2 in Linux 3.10¶
- Removed obsolete and unused DV_PRESET ioctls VIDIOC_G_DV_PRESET, VIDIOC_S_DV_PRESET, VIDIOC_QUERY_DV_PRESET and VIDIOC_ENUM_DV_PRESET. Remove the related v4l2_input/output capability flags V4L2_IN_CAP_PRESETS and V4L2_OUT_CAP_PRESETS. 
- Added new debugging ioctl ioctl VIDIOC_DBG_G_CHIP_INFO. 
6.2.48. V4L2 in Linux 3.11¶
- Remove obsolete - VIDIOC_DBG_G_CHIP_IDENTioctl.
6.2.49. V4L2 in Linux 3.14¶
- In - struct v4l2_rect, the type of- widthand- heightfields changed from _s32 to _u32.
6.2.50. V4L2 in Linux 3.15¶
- Added Software Defined Radio (SDR) Interface. 
6.2.51. V4L2 in Linux 3.16¶
- Added event V4L2_EVENT_SOURCE_CHANGE. 
6.2.52. V4L2 in Linux 3.17¶
- Extended - struct v4l2_pix_format. Added format flags.
- Added compound control types and VIDIOC_QUERY_EXT_CTRL. 
6.2.53. V4L2 in Linux 3.18¶
- Added - V4L2_CID_PAN_SPEEDand- V4L2_CID_TILT_SPEEDcamera controls.
6.2.54. V4L2 in Linux 3.19¶
- Rewrote Colorspace chapter, added new - enum v4l2_ycbcr_encodingand- enum v4l2_quantizationfields to- struct v4l2_pix_format,- struct v4l2_pix_format_mplaneand- struct v4l2_mbus_framefmt.
6.2.55. V4L2 in Linux 4.4¶
- Renamed - V4L2_TUNER_ADCto- V4L2_TUNER_SDR. The use of- V4L2_TUNER_ADCis deprecated now.
- Added - V4L2_CID_RF_TUNER_RF_GAINRF Tuner control.
- Added transmitter support for Software Defined Radio (SDR) Interface. 
6.2.56. Relation of V4L2 to other Linux multimedia APIs¶
6.2.56.1. X Video Extension¶
The X Video Extension (abbreviated XVideo or just Xv) is an extension of the X Window system, implemented for example by the XFree86 project. Its scope is similar to V4L2, an API to video capture and output devices for X clients. Xv allows applications to display live video in a window, send window contents to a TV output, and capture or output still images in XPixmaps [1]. With their implementation XFree86 makes the extension available across many operating systems and architectures.
Because the driver is embedded into the X server Xv has a number of advantages over the V4L2 video overlay interface. The driver can easily determine the overlay target, i. e. visible graphics memory or off-screen buffers for a destructive overlay. It can program the RAMDAC for a non-destructive overlay, scaling or color-keying, or the clipping functions of the video capture hardware, always in sync with drawing operations or windows moving or changing their stacking order.
To combine the advantages of Xv and V4L a special Xv driver exists in
XFree86 and XOrg, just programming any overlay capable Video4Linux
device it finds. To enable it /etc/X11/XF86Config must contain these
lines:
Section "Module"
    Load "v4l"
EndSection
As of XFree86 4.2 this driver still supports only V4L ioctls, however it should work just fine with all V4L2 devices through the V4L2 backward-compatibility layer. Since V4L2 permits multiple opens it is possible (if supported by the V4L2 driver) to capture video while an X client requested video overlay. Restrictions of simultaneous capturing and overlay are discussed in Video Overlay Interface apply.
Only marginally related to V4L2, XFree86 extended Xv to support hardware YUV to RGB conversion and scaling for faster video playback, and added an interface to MPEG-2 decoding hardware. This API is useful to display images captured with V4L2 devices.
6.2.56.2. Digital Video¶
V4L2 does not support digital terrestrial, cable or satellite broadcast. A separate project aiming at digital receivers exists. You can find its homepage at https://linuxtv.org. The Linux DVB API has no connection to the V4L2 API except that drivers for hybrid hardware may support both.
6.2.56.3. Audio Interfaces¶
[to do - OSS/ALSA]
6.2.57. Experimental API Elements¶
The following V4L2 API elements are currently experimental and may change in the future.
6.2.58. Obsolete API Elements¶
The following V4L2 API elements were superseded by new interfaces and should not be implemented in new drivers.
- VIDIOC_G_MPEGCOMPand- VIDIOC_S_MPEGCOMPioctls. Use Extended Controls, Extended Controls API.
- VIDIOC_G_DV_PRESET, VIDIOC_S_DV_PRESET, VIDIOC_ENUM_DV_PRESETS and VIDIOC_QUERY_DV_PRESET ioctls. Use the DV Timings API (Digital Video (DV) Timings). 
- VIDIOC_SUBDEV_G_CROPand- VIDIOC_SUBDEV_S_CROPioctls. Use- VIDIOC_SUBDEV_G_SELECTIONand- VIDIOC_SUBDEV_S_SELECTION, ioctl VIDIOC_SUBDEV_G_SELECTION, VIDIOC_SUBDEV_S_SELECTION.