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Date:   Fri, 20 Nov 2020 21:40:23 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Helen Koike <helen.koike@...labora.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Sakari Ailus <sakari.ailus@....fi>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Hirokazu Honda <hiroh@...omium.org>,
        Nicolas Dufresne <nicolas@...fresne.ca>,
        Brian Starkey <Brian.Starkey@....com>, kernel@...labora.com,
        Boris Brezillon <boris.brezillon@...labora.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Fritz Koenig <frkoenig@...omium.org>,
        Maxime Jourdan <mjourdan@...libre.com>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>
Subject: Re: [PATCH v5 7/7] media: docs: add documentation for the Extended API

On Fri, Nov 20, 2020 at 9:24 PM Hans Verkuil <hverkuil@...all.nl> wrote:
>
> On 20/11/2020 12:06, Tomasz Figa wrote:z
> > Hi Helen,
> >
> > On Tue, Aug 04, 2020 at 04:29:39PM -0300, Helen Koike wrote:
> >> Add documentation and update references in current documentation for the
> >> Extended API.
> >>
> >
> > Thank you for the patch. Please see my comments inline.
> >
> >> Signed-off-by: Helen Koike <helen.koike@...labora.com>
> >> ---
> >> Changes in v5:
> >> - new patch
> >>
> >>  .../userspace-api/media/v4l/buffer.rst        |   5 +
> >>  .../userspace-api/media/v4l/common.rst        |   1 +
> >>  .../userspace-api/media/v4l/dev-capture.rst   |   5 +
> >>  .../userspace-api/media/v4l/dev-output.rst    |   5 +
> >>  .../userspace-api/media/v4l/ext-api.rst       | 107 +++++++++
> >>  .../userspace-api/media/v4l/format.rst        |  16 +-
> >>  .../userspace-api/media/v4l/user-func.rst     |   5 +
> >>  .../media/v4l/vidioc-ext-create-bufs.rst      |  95 ++++++++
> >>  .../media/v4l/vidioc-ext-prepare-buf.rst      |  62 ++++++
> >>  .../media/v4l/vidioc-ext-qbuf.rst             | 204 ++++++++++++++++++
> >>  .../media/v4l/vidioc-ext-querybuf.rst         |  79 +++++++
> >>  .../media/v4l/vidioc-g-ext-pix-fmt.rst        | 117 ++++++++++
> >>  12 files changed, 697 insertions(+), 4 deletions(-)
> >>  create mode 100644 Documentation/userspace-api/media/v4l/ext-api.rst
> >>  create mode 100644 Documentation/userspace-api/media/v4l/vidioc-ext-create-bufs.rst
> >>  create mode 100644 Documentation/userspace-api/media/v4l/vidioc-ext-prepare-buf.rst
> >>  create mode 100644 Documentation/userspace-api/media/v4l/vidioc-ext-qbuf.rst
> >>  create mode 100644 Documentation/userspace-api/media/v4l/vidioc-ext-querybuf.rst
> >>  create mode 100644 Documentation/userspace-api/media/v4l/vidioc-g-ext-pix-fmt.rst
> >>
> >> diff --git a/Documentation/userspace-api/media/v4l/buffer.rst b/Documentation/userspace-api/media/v4l/buffer.rst
> >> index 57e752aaf414a..c832bedd64e4c 100644
> >> --- a/Documentation/userspace-api/media/v4l/buffer.rst
> >> +++ b/Documentation/userspace-api/media/v4l/buffer.rst
> >> @@ -27,6 +27,11 @@ such as pointers and sizes for each plane, are stored in
> >>  struct :c:type:`v4l2_plane` instead. In that case,
> >>  struct :c:type:`v4l2_buffer` contains an array of plane structures.
> >>
> >> +.. note::
> >> +
> >> +    The :ref:`ext_api` version can also be used, and it is
> >> +    preferable when applicable.
> >
> > Would rephrasing this as below making a bit more definitive?
> >
> >       For modern applications, these operations are replaced by their
> >       :ref:`ext_api` counterparts, which should be used instead.
>
> You can't say that, since especially in the beginning userspace will be running
> on older kernels that do not support this.
>
> This will work: "should be used instead, if supported by the driver."
>

With the wrappers that the patches provide, all drivers would support
the new API, so this boils down to the kernel version only, not
specific drivers.

Agreed, though, that application developers must be made aware that
the new API is only available in new kernels and old API must be used
for compatibility with old kernels.

> >
> >> +
> >>  Dequeued video buffers come with timestamps. The driver decides at which
> >>  part of the frame and with which clock the timestamp is taken. Please
> >>  see flags in the masks ``V4L2_BUF_FLAG_TIMESTAMP_MASK`` and
> >> diff --git a/Documentation/userspace-api/media/v4l/common.rst b/Documentation/userspace-api/media/v4l/common.rst
> >> index 7d81c58a13cd7..3430e0bdad667 100644
> >> --- a/Documentation/userspace-api/media/v4l/common.rst
> >> +++ b/Documentation/userspace-api/media/v4l/common.rst
> >> @@ -59,6 +59,7 @@ applicable to all devices.
> >>      ext-ctrls-detect
> >>      fourcc
> >>      format
> >> +    ext-api
> >>      planar-apis
> >>      selection-api
> >>      crop
> >> diff --git a/Documentation/userspace-api/media/v4l/dev-capture.rst b/Documentation/userspace-api/media/v4l/dev-capture.rst
> >> index 44d3094093ab6..5077639787d92 100644
> >> --- a/Documentation/userspace-api/media/v4l/dev-capture.rst
> >> +++ b/Documentation/userspace-api/media/v4l/dev-capture.rst
> >> @@ -102,6 +102,11 @@ and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC
> >>  requests and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does.
> >>  :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is optional.
> >>
> >> +.. note::
> >> +
> >> +    The :ref:`ext_api` version can also be used, and it is
> >> +    preferable when applicable.
> >> +
> >>
> >>  Reading Images
> >>  ==============
> >> diff --git a/Documentation/userspace-api/media/v4l/dev-output.rst b/Documentation/userspace-api/media/v4l/dev-output.rst
> >> index e4f2a1d8b0fc7..f8f40c708e49f 100644
> >> --- a/Documentation/userspace-api/media/v4l/dev-output.rst
> >> +++ b/Documentation/userspace-api/media/v4l/dev-output.rst
> >> @@ -99,6 +99,11 @@ and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, even if :ref:`VIDIOC_S_FMT <VIDIOC
> >>  requests and always returns default parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does.
> >>  :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is optional.
> >>
> >> +.. note::
> >> +
> >> +    The :ref:`ext_api` version can also be used, and it is
> >> +    preferable when applicable.
> >> +
> >>
> >>  Writing Images
> >>  ==============
> >> diff --git a/Documentation/userspace-api/media/v4l/ext-api.rst b/Documentation/userspace-api/media/v4l/ext-api.rst
> >> new file mode 100644
> >> index 0000000000000..da2a82960d22f
> >> --- /dev/null
> >> +++ b/Documentation/userspace-api/media/v4l/ext-api.rst
> >> @@ -0,0 +1,107 @@
> >> +.. 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
> >> +
> >> +.. _ext_api:
> >> +
> >> +*************
> >> +Extendend API
> >> +*************
> >> +
> >> +Introduction
> >> +============
> >> +
> >> +The Extended Format API was conceived to extend V4L2 capabilities and
> >> +to simplify certain mechanisms.
> >> +It unifies single- vs multi- planar handling,
> >> +brings the concept of pixelformat + modifiers, allows planes to be placed
> >> +in any offset inside a buffer and let userspace to change colorspace
> >> +attributes if supported by the driver.
> >
> > How about simplifying this paragraph a bit:
> >
> > The Extended Format API unifies single- and multi- planar format handling
> > and provides a base for further functional extensions, such as format
> > modifiers, arbitrary plane offsets, colorspace configuration.
>
> Actually, I prefer Helen's version.
>

To me it's a lot of text to read to get the same information, but I
won't insist.

> >
> >> +
> >> +Data format negotiation and buffer handling works very similar to the classical
> >> +version, thus in this document we'll just mention the main differences.
> >
> > nit: "we will" or maybe even just "we mention only the main differences".
> >
> >> +
> >> +Data Format Negotiation
> >> +=======================
> >> +
> >> +The API replaces the classical ioctls:
> >> +
> >> +:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>`, :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`,
> >> +:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>`
> >> +(with :c:type:`v4l2_format` as the main parameter).
> >> +
> >> +By the extended versions:
> >> +
> >> +:ref:`VIDIOC_G_EXT_PIX_FMT <VIDIOC_G_EXT_PIX_FMT>`,
> >> +:ref:`VIDIOC_S_EXT_PIX_FMT <VIDIOC_G_EXT_PIX_FMT>`,
> >> +:ref:`VIDIOC_TRY_EXT_PIX_FMT <VIDIOC_G_EXT_PIX_FMT>`
> >> +(with :c:type:`v4l2_ext_pix_format` as the main parameter).
> >> +
> >> +For CAPTURE and OUTPUT queues only.
> >> +
> >> +The ``type`` field of struct :c:type:`v4l2_ext_pix_format` only accepts
> >> +``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or ``V4L2_BUF_TYPE_VIDEO_OUTPUT``, and it
> >> +supports multiplanar format through a combination of ``pixelfomat`` and
> >> +``modifier`` fields.
> >> +
> >> +Only the single-planar variants of the pixel formats are valid in the
> >> +``pixelformat`` field.
> >
> > Some formats have only multi-planar variants, like NV12MT. What should we
> > do about them? Since we have to keep those defines to support the legacy
> > API anyway, maybe we can just accept both variants and treat them equally?
>
> Helen mentioned that as well in a reply to patch 1/7. I'll have a think about
> that myself and reply to that post because this is a good question.
>

I'm thinking about it as well. :)

> >
> >> +To support multi-planar, a modifier should be set in the ``modifier`` field to
> >> +provide more information regarding the memory layout of pixels and number of
> >> +planes.
> >> +
> >> +The ``plane_fmt`` field is an array which holds information by plane using
> >> +the :c:type:`v4l2_plane_ext_pix_format`. When this struct is filled, its
> >> +``sizeimage`` field should be non-zero for all valid planes for a given
> >> +``pixelformat`` + ``modifier`` combination, and zeroed for the invalid ones.
> >> +
> >> +Colospace attributes are not read-only as in the classical version, i.e, they
> >> +can be set by application and drivers will adjust accordingly depending on what
> >> +is supported by the underlying hardware.
> >> +
> >> +Buffers
> >> +=======
> >> +
> >> +The API replaces the classical ioctls:
> >> +
> >> +:ref:`VIDIOC_CREATE_BUFS <VIDIOC_CREATE_BUFS>`
> >> +(with :c:type:`v4l2_create_buffers` as the main parameter),
> >> +:ref:`VIDIOC_QUERYBUF <VIDIOC_QUERYBUF>`, :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`,
> >> +:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`, :ref:`VIDIOC_PREPARE_BUF <VIDIOC_PREPARE_BUF>`
> >> +(with :c:type:`v4l2_buffer` as the main parameter)
> >> +
> >> +By the extended versions:
> >> +
> >> +:ref:`VIDIOC_EXT_CREATE_BUFS <VIDIOC_EXT_CREATE_BUFS>`
> >> +(with :c:type:`v4l2_ext_create_buffers` as the main parameter),
> >> +:ref:`VIDIOC_EXT_QUERYBUF <VIDIOC_EXT_QUERYBUF>`,
> >> +:ref:`VIDIOC_EXT_QBUF <VIDIOC_EXT_QBUF>`,
> >> +:ref:`VIDIOC_EXT_DQBUF <VIDIOC_EXT_QBUF>`,
> >> +:ref:`VIDIOC_EXT_PREPARE_BUF <VIDIOC_EXT_PREPARE_BUF>`
> >> +(with :c:type:`v4l2_ext_buffer` as the main parameter)
> >> +
> >> +The basic difference between :c:type:`v4l2_create_buffers` and
> >> +:c:type:`v4l2_ext_create_buffers` is that the later have a
> >> +:c:type:`v4l2_ext_pix_format` as the type of the ``format`` field.
> >> +
> >> +Comparing :c:type:`v4l2_ext_buffer` with :c:type:`v4l2_buffer`, in the
> >> +extended version there is a unification of the single-/multi- planar handling
> >> +through the ``planes`` field of type :c:type:`v4l2_ext_plane`.
> >> +
> >> +The :c:type:`v4l2_ext_plane also allows planes to be placed in a given offset
> >> +inside a buffer.
> >> +Planes can be placed in different locations inside the same buffer, or in
> >> +different buffers.
> >> +
> >> +
> >> +.. kernel-doc:: include/uapi/linux/videodev2.h
> >> +   :functions: v4l2_ext_plane
> >> +
> >> +
> >> +.. kernel-doc:: include/uapi/linux/videodev2.h
> >> +   :functions: v4l2_ext_buffer
> >> diff --git a/Documentation/userspace-api/media/v4l/format.rst b/Documentation/userspace-api/media/v4l/format.rst
> >> index e47fc0505727c..b96d26f26793c 100644
> >> --- a/Documentation/userspace-api/media/v4l/format.rst
> >> +++ b/Documentation/userspace-api/media/v4l/format.rst
> >> @@ -28,13 +28,19 @@ 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
> >> +There are two mechanism to negociate data formats:
> >
> > negotiate
> >
> >> +
> >> +The first one is 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
> >> +format.
> >> +
> >> +The second is through the :ref:`ext_api`, please refer to its documentation
> >> +for more information.
> >> +
> >
> > Should we reverse the order to prefer the new API?
> >
> >> +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`.
> >>
> >> @@ -71,7 +77,9 @@ earlier versions of V4L2. Switching the logical stream or returning into
> >>  *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_G_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl
> >> +or the Extended respective versions (TODO: link).
> >
> > TODO? :)
> >
> >> +Implementation of the
> >>  :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is highly recommended but optional.
> >>
> >>
> >> diff --git a/Documentation/userspace-api/media/v4l/user-func.rst b/Documentation/userspace-api/media/v4l/user-func.rst
> >> index bf77c842718e5..67b254812791c 100644
> >> --- a/Documentation/userspace-api/media/v4l/user-func.rst
> >> +++ b/Documentation/userspace-api/media/v4l/user-func.rst
> >> @@ -20,6 +20,7 @@ Function Reference
> >>      func-close
> >>      func-ioctl
> >>      vidioc-create-bufs
> >> +    vidioc-ext-create-bufs
> >>      vidioc-cropcap
> >>      vidioc-dbg-g-chip-info
> >>      vidioc-dbg-g-register
> >> @@ -48,6 +49,7 @@ Function Reference
> >>      vidioc-g-ext-ctrls
> >>      vidioc-g-fbuf
> >>      vidioc-g-fmt
> >> +    vidioc-g-ext-pix-fmt
> >>      vidioc-g-frequency
> >>      vidioc-g-input
> >>      vidioc-g-jpegcomp
> >> @@ -62,8 +64,11 @@ Function Reference
> >>      vidioc-log-status
> >>      vidioc-overlay
> >>      vidioc-prepare-buf
> >> +    vidioc-ext-prepare-buf
> >>      vidioc-qbuf
> >> +    vidioc-ext-qbuf
> >>      vidioc-querybuf
> >> +    vidioc-ext-querybuf
> >>      vidioc-querycap
> >>      vidioc-queryctrl
> >>      vidioc-query-dv-timings
> >> diff --git a/Documentation/userspace-api/media/v4l/vidioc-ext-create-bufs.rst b/Documentation/userspace-api/media/v4l/vidioc-ext-create-bufs.rst
> >> new file mode 100644
> >> index 0000000000000..67f2c541e4d79
> >> --- /dev/null
> >> +++ b/Documentation/userspace-api/media/v4l/vidioc-ext-create-bufs.rst
> >> @@ -0,0 +1,95 @@
> >> +.. 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
> >> +
> >
> > TODO? :)
> >
> >> +.. _VIDIOC_EXT_CREATE_BUFS:
> >> +
> >> +****************************
> >> +ioctl VIDIOC_EXT_CREATE_BUFS
> >> +****************************
> >> +
> >> +Name
> >> +====
> >> +
> >> +VIDIOC_EXT_CREATE_BUFS - Create buffers for Memory Mapped or User Pointer or DMA Buffer I/O
> >> +
> >> +
> >> +Synopsis
> >> +========
> >> +
> >> +.. c:function:: int ioctl( int fd, VIDIOC_EXT_CREATE_BUFS, struct v4l2_ext_create_buffers *argp )
> >> +    :name: VIDIOC_EXT_CREATE_BUFS
> >> +
> >> +
> >> +Arguments
> >> +=========
> >> +
> >> +``fd``
> >> +    File descriptor returned by :ref:`open() <func-open>`.
> >> +
> >> +``argp``
> >> +    Pointer to struct :c:type:`v4l2_ext_create_buffers`.
> >> +
> >> +
> >> +Description
> >> +===========
> >> +
> >> +This ioctl is used to create buffers for :ref:`memory mapped <mmap>`
> >> +or :ref:`user pointer <userp>` or :ref:`DMA buffer <dmabuf>` I/O.
> >> +This ioctl can be called multiple times to
> >> +create buffers of different sizes.
> >> +
> >> +To allocate the device buffers applications must initialize the relevant
> >> +fields of the struct :c:type:`v4l2_ext_create_buffers` structure. The
> >> +``count`` field must be set to the number of requested buffers, the
> >> +``memory`` field specifies the requested I/O method and the ``reserved``
> >> +array must be zeroed.
> >> +
> >> +The ``format`` field specifies the image format that the buffers must be
> >> +able to handle. The application has to fill in this struct
> >> +:c:type:`v4l2_format`. Usually this will be done using the
> >> +:ref:`VIDIOC_TRY_EXT_PIX_FMT <VIDIOC_G_EXT_PIX_FMT>` or
> >> +:ref:`VIDIOC_G_EXT_PIX_FMT <VIDIOC_G_EXT_PIX_FMT>` ioctls to ensure that the
> >> +requested format is supported by the driver.
> >> +The driver may return an error if the size(s) are not supported by the
> >> +hardware (usually because they are too small).
> >> +
> >> +The driver can create a memory buffer per plane, or a single memory buffer
> >> +containing all the planes, with a minimum size according to the size
> >> +defined by the ``format.plane_fmt[i].sizeimage`` field.
> >> +Usually if the ``format.plane_fmt[i].sizeimage``
> >> +field is less than the minimum required for the given format, then an
> >> +error will be returned since drivers will typically not allow this. If
> >> +it is larger, then the value will be used as-is. In other words, the
> >> +driver may reject the requested size, but if it is accepted the driver
> >> +will use it unchanged.
> >> +
> >> +When the ioctl is called with a pointer to this structure the driver
> >> +will attempt to allocate up to the requested number of buffers and store
> >> +the actual number allocated and the starting index in the ``count`` and
> >> +the ``index`` fields respectively. On return ``count`` can be smaller
> >> +than the number requested.
> >
> > The 'capabilities' and 'flags' fields seem to go undocumented.
> >
> >> +
> >> +
> >> +.. kernel-doc:: include/uapi/linux/videodev2.h
> >> +   :functions: v4l2_ext_create_buffers
> >> +
> >> +
> >> +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.
> >> +
> >> +ENOMEM
> >> +    No memory to allocate buffers for :ref:`memory mapped <mmap>` I/O.
> >> +
> >> +EINVAL
> >> +    The buffer type (``format.type`` field), requested I/O method
> >> +    (``memory``) or format (``format`` field) is not valid.
> >> diff --git a/Documentation/userspace-api/media/v4l/vidioc-ext-prepare-buf.rst b/Documentation/userspace-api/media/v4l/vidioc-ext-prepare-buf.rst
> >> new file mode 100644
> >> index 0000000000000..88e4b57121254
> >> --- /dev/null
> >> +++ b/Documentation/userspace-api/media/v4l/vidioc-ext-prepare-buf.rst
> >> @@ -0,0 +1,62 @@
> >> +.. 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
> >> +
> >> +.. _VIDIOC_EXT_PREPARE_BUF:
> >> +
> >> +****************************
> >> +ioctl VIDIOC_EXT_PREPARE_BUF
> >> +****************************
> >> +
> >> +Name
> >> +====
> >> +
> >> +VIDIOC_EXT_PREPARE_BUF - Prepare a buffer for I/O
> >> +
> >> +
> >> +Synopsis
> >> +========
> >> +
> >> +.. c:function:: int ioctl( int fd, VIDIOC_EXT_PREPARE_BUF, struct v4l2_ext_buffer *argp )
> >> +    :name: VIDIOC_EXT_PREPARE_BUF
> >> +
> >> +
> >> +Arguments
> >> +=========
> >> +
> >> +``fd``
> >> +    File descriptor returned by :ref:`open() <func-open>`.
> >> +
> >> +``argp``
> >> +    Pointer to struct :c:type:`v4l2_ext_buffer`.
> >> +
> >> +
> >> +Description
> >> +===========
> >> +
> >> +Applications can optionally call the :ref:`VIDIOC_EXT_PREPARE_BUF` ioctl to
> >> +pass ownership of the buffer to the driver before actually enqueuing it,
> >> +using the :ref:`VIDIOC_EXT_QBUF <VIDIOC_EXT_QBUF>` ioctl, and to prepare it for future I/O. Such
> >> +preparations may include cache invalidation or cleaning. Performing them
> >> +in advance saves time during the actual I/O.
> >> +
> >> +
> >> +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.
> >> +
> >> +EBUSY
> >> +    File I/O is in progress.
> >> +
> >> +EINVAL
> >> +    The buffer ``type`` is not supported, or the ``index`` is out of
> >> +    bounds, or no buffers have been allocated yet, or the ``userptr`` or
> >> +    ``length`` are invalid.
> >> diff --git a/Documentation/userspace-api/media/v4l/vidioc-ext-qbuf.rst b/Documentation/userspace-api/media/v4l/vidioc-ext-qbuf.rst
> >> new file mode 100644
> >> index 0000000000000..083e106cf6bad
> >> --- /dev/null
> >> +++ b/Documentation/userspace-api/media/v4l/vidioc-ext-qbuf.rst
> >> @@ -0,0 +1,204 @@
> >> +.. 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
> >> +
> >> +.. _VIDIOC_EXT_QBUF:
> >> +
> >> +***************************************
> >> +ioctl VIDIOC_EXT_QBUF, VIDIOC_EXT_DQBUF
> >> +***************************************
> >> +
> >> +Name
> >> +====
> >> +
> >> +VIDIOC_EXT_QBUF - VIDIOC_EXT_DQBUF - Exchange a buffer with the driver
> >> +
> >> +
> >> +Synopsis
> >> +========
> >> +
> >> +.. c:function:: int ioctl( int fd, VIDIOC_EXT_QBUF, struct v4l2_ext_buffer *argp )
> >> +    :name: VIDIOC_EXT_QBUF
> >> +
> >> +.. c:function:: int ioctl( int fd, VIDIOC_EXT_DQBUF, struct v4l2_ext_buffer *argp )
> >> +    :name: VIDIOC_EXT_DQBUF
> >> +
> >> +
> >> +Arguments
> >> +=========
> >> +
> >> +``fd``
> >> +    File descriptor returned by :ref:`open() <func-open>`.
> >> +
> >> +``argp``
> >> +    Pointer to struct :c:type:`v4l2_ext_buffer`.
> >> +
> >> +
> >> +Description
> >> +===========
> >> +
> >> +Applications call the ``VIDIOC_EXT_QBUF`` ioctl to enqueue an empty
> >> +(capturing) or filled (output) buffer in the driver's incoming queue.
> >> +The semantics depend on the selected I/O method.
> >> +
> >> +To enqueue a buffer applications set the ``type`` field of a struct
> >> +:c:type:`v4l2_ext_buffer` to the same buffer type as was
> >> +previously used with struct :c:type:`v4l2_ext_pix_format` ``type``.
> >> +Applications must also set the ``index`` field. Valid index numbers
> >> +range from zero to the number of buffers allocated with
> >> +:ref:`VIDIOC_EXT_CREATE_BUFS` (struct
> >> +:c:type:`v4l2_ext_create_buffers` ``count``) minus
> >> +one. The contents of the struct :c:type:`v4l2_ext_buffer` returned
> >
> > I think this might not be entirely accurate. One can call
> > VIDIOC_EXT_CREATE_BUFS multiple times. Moreover, I don't think we've
> > prohibited VIDIOC_REQBUFS for the ext API, or have we?
> >
> >> +by a :ref:`VIDIOC_EXT_QUERYBUF` ioctl will do as well.
> >> +When the buffer is intended for output (``type`` is
> >> +``V4L2_BUF_TYPE_VIDEO_OUTPUT``) applications must also initialize the
> >> +``timestamp`` fields and the ``planes[i].plane_length`` for each valid plane,
> >> +and invalid ones must be set as zero.
> >> +see :ref:`buffer` for details. Applications must also set ``flags`` to 0. The
> >> +``reserved`` field must be set to 0.
> >> +
> >> +To enqueue a :ref:`memory mapped <mmap>` buffer applications set the
> >> +``planes[i].memory`` field to ``V4L2_MEMORY_MMAP`` in all the valid planes,
> >> +and invalid ones must be set as zero.
> >> +When ``VIDIOC_EXT_QBUF`` is called
> >> +with a pointer to this structure the driver sets the
> >> +``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_QUEUED`` flags and clears
> >> +the ``V4L2_BUF_FLAG_DONE`` flag in the ``flags`` field, or it returns an
> >> +``EINVAL`` error code.
> >> +
> >> +To enqueue a :ref:`user pointer <userp>` buffer applications set the
> >> +``planes[i].memory`` field to ``V4L2_MEMORY_USERPTR`` in all the valid planes,
> >> +and invalid ones must be set as zero, the ``planes[i].m.userptr`` field to the
> >> +address of the buffer,``planes[i].buffer_length`` to the size of the memory
> >> +buffer, ``planes[i].plane_length`` to the size that should be used by the plane,
> >> +and ``planes[i].offset`` of the plane in the memory buffer.
> >
> > Does offset make sense here if an arbitrary pointer can be given for the
> > start of the plane?
> >
> >> +
> >> +When ``VIDIOC_EXT_QBUF`` is called with a pointer to this structure
> >> +the driver sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the
> >> +``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the
> >> +``flags`` field, or it returns an error code. This ioctl locks the
> >
> > This seems to be a copy of the text few lines above.
> >
> >> +memory pages of the buffer in physical memory, they cannot be swapped
> >> +out to disk. Buffers remain locked until dequeued, until the
> >> +:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or
> >> +:ref:`VIDIOC_EXT_CREATE_BUFS` ioctl is called, or until the
> >> +device is closed.
> >
> > Why would CREATE_BUFS affect existing buffers?
> >
> >> +
> >> +To enqueue a :ref:`DMABUF <dmabuf>` buffer applications set the
> >> +``planes[i].memory`` field to ``V4L2_MEMORY_DMABUF`` in all the valid planes,
> >> +and invalid ones must be set as zero, the ``planes[i].m.fd`` field to a
> >> +file descriptor associated with a DMABUF buffer.,``planes[i].buffer_length`` to
> >> +the size of the memory buffer, ``planes[i].plane_length`` to the size that
> >
> > What is the buffer_length field needed for? DMA-bufs by definition are
> > kernel objects and the kernel should already know their sizes.
>
> See this discussion:
>
> https://patchwork.linuxtv.org/project/linux-media/patch/gh5kef5bkeel3o6b2dkgc2dfagu9klj4c0@4ax.com/
>

I briefly read through it and I don't see what the conclusion was.
Generally I'd still prefer if the need to manually specify the buffer
size was eliminated, because that imposes unnecessary burden on the
userspace for the majority of cases. It could still remain valid for
userspace that wants to restrict the buffer size as it was mentioned
in that thread, though.

> >
> >> +should be used by the plane, and ``planes[i].offset`` of the plane in the memory buffer.
> >> +When ``VIDIOC_EXT_QBUF`` is called with a pointer to this structure the driver
> >> +sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the
> >> +``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the
> >> +``flags`` field, or it returns an error code. This ioctl locks the
> >
> > Ditto.
> >
> >> +buffer. Locking a buffer means passing it to a driver for a hardware
> >> +access (usually DMA). If an application accesses (reads/writes) a locked
> >> +buffer then the result is undefined. Buffers remain locked until
> >> +dequeued, until the :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or
> >> +:ref:`VIDIOC_EXT_CREATE_BUFS` ioctl is called, or until the
> >> +device is closed.
> >
> > Ditto.
> >
> >> +
> >> +The ``request_fd`` field can be used with the ``VIDIOC_EXT_QBUF`` ioctl to specify
> >> +the file descriptor of a :ref:`request <media-request-api>`, if requests are
> >> +in use. Setting it means that the buffer will not be passed to the driver
> >> +until the request itself is queued. Also, the driver will apply any
> >> +settings associated with the request for this buffer. This field will
> >> +be ignored unless the ``V4L2_BUF_FLAG_REQUEST_FD`` flag is set.
> >> +If the device does not support requests, then ``EBADR`` will be returned.
> >> +If requests are supported but an invalid request file descriptor is given,
> >> +then ``EINVAL`` will be returned.
> >> +
> >> +.. caution::
> >> +   It is not allowed to mix queuing requests with queuing buffers directly.
> >> +   ``EBUSY`` will be returned if the first buffer was queued directly and
> >> +   then the application tries to queue a request, or vice versa. After
> >> +   closing the file descriptor, calling
> >> +   :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or calling :ref:`VIDIOC_EXT_CREATE_BUFS`
> >> +   the check for this will be reset.
> >> +
> >> +   For :ref:`memory-to-memory devices <mem2mem>` you can specify the
> >> +   ``request_fd`` only for output buffers, not for capture buffers. Attempting
> >> +   to specify this for a capture buffer will result in an ``EBADR`` error.
> >> +
> >> +Applications call the ``VIDIOC_EXT_DQBUF`` ioctl to dequeue a filled
> >> +(capturing) or displayed (output) buffer from the driver's outgoing
> >
> > "displayed" is quite inaccurate given that the majority of use cases for
> > OUTPUT in V4L2 now are m2m devices. Maybe "consumed"?
> >
> >> +queue. They just set the ``type``, ``planes[i].memory`` and ``reserved`` fields of
> >
> > Why does the application have to set the memory field? It should be already
> > known by the kernel which owns the buffer at the given time.
> >
> >> +a struct :c:type:`v4l2_ext_buffer` as above, when
> >> +``VIDIOC_EXT_DQBUF`` is called with a pointer to this structure the driver
> >> +fills the remaining fields or returns an error code. The driver may also
> >> +set ``V4L2_BUF_FLAG_ERROR`` in the ``flags`` field. It indicates a
> >> +non-critical (recoverable) streaming error. In such case the application
> >> +may continue as normal, but should be aware that data in the dequeued
> >> +buffer might be corrupted. When using the multi-planar API, the planes
> >> +array must be passed in as well.
> >> +
> >> +If the application sets the ``memory`` field to ``V4L2_MEMORY_DMABUF`` to
> >> +dequeue a :ref:`DMABUF <dmabuf>` buffer, the driver fills the ``m.fd`` field
> >> +with a file descriptor numerically the same as the one given to ``VIDIOC_EXT_QBUF``
> >> +when the buffer was enqueued. No new file descriptor is created at dequeue time
> >> +and the value is only for the application convenience.
> >
> > Honestly, I don't like this aspect of the current API. It kind of implies
> > that the value returned is valid, but the file descriptor might have been
> > already closed. Could we just remove this and explicitly state that for
> > DMA-buf, it is the user space responsibility to track the mapping between
> > V4L2 indexes and its own buffer objects?
> >
> >> +
> >> +By default ``VIDIOC_EXT_DQBUF`` blocks when no buffer is in the outgoing
> >> +queue. When the ``O_NONBLOCK`` flag was given to the
> >> +:ref:`open() <func-open>` function, ``VIDIOC_EXT_DQBUF`` returns
> >> +immediately with an ``EAGAIN`` error code when no buffer is available.
> >> +
> >> +
> >> +.. kernel-doc:: include/uapi/linux/videodev2.h
> >> +   :functions: v4l2_ext_buffers
> >> +
> >> +
> >> +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.
> >> +
> >> +EAGAIN
> >> +    Non-blocking I/O has been selected using ``O_NONBLOCK`` and no
> >> +    buffer was in the outgoing queue.
> >> +
> >> +EINVAL
> >> +    The buffer ``type`` is not supported, or the ``index`` is out of
> >> +    bounds, or no buffers have been allocated yet, or the ``userptr`` or
> >> +    ``length`` are invalid, or the ``V4L2_BUF_FLAG_REQUEST_FD`` flag was
> >
> > There is no 'length' now.
> >
> >> +    set but the the given ``request_fd`` was invalid, or ``m.fd`` was
> >> +    an invalid DMABUF file descriptor.
> >> +
> >> +EIO
> >> +    ``VIDIOC_EXT_DQBUF`` failed due to an internal error. Can also indicate
> >> +    temporary problems like signal loss.
>
> Drop the last sentence, -EIO should never be used for temporary problems like
> that. EIO really means that the DMA engine crashed and userspace will have to call
> STREAMOFF and try to restart the DMA with STREAMON.
>

I suspect the users of the new API wouldn't actually have to deal with
a signal loss, but if not, what should be the behavior then?

> >> +
> >> +    .. note::
> >> +
> >> +       The driver might dequeue an (empty) buffer despite returning
> >> +       an error, or even stop capturing. Reusing such buffer may be unsafe
> >> +       though and its details (e.g. ``index``) may not be returned either.
>
> The first part of this note is rather vague. I'm not sure what it meant with this.
>
> >> +       It is recommended that drivers indicate recoverable errors by setting
> >> +       the ``V4L2_BUF_FLAG_ERROR`` and returning 0 instead. In that case the
> >> +       application should be able to safely reuse the buffer and continue
> >> +       streaming.
> >
> > What is this for? It sounds like some legacy behavior, which maybe we could
> > drop with this new API?
>
> No, V4L2_BUF_FLAG_ERROR is really needed. Some hardware can really have
> intermittent failures causing video frames to be corrupt. And yes, that's
> typically due to hardware bugs. It usually either happens when you start
> streaming or just intermittent, but it is not severe enough to stop streaming
> completely (that's the -EIO error due to a call to vb2_queue_error).
>
> It's also used when you stop streaming and pending buffers are returned to
> userspace. They are marked as ERROR since they don't contain any video, or
> only partial video (if the DMA was stopped midway a frame).
>

Sorry, I meant to insert that comment below the first half. The second
half is fine.

> >
> >> +
> >> +EPIPE
> >> +    ``VIDIOC_EXT_DQBUF`` returns this on an empty capture queue for mem2mem
> >> +    codecs if a buffer with the ``V4L2_BUF_FLAG_LAST`` was already
> >> +    dequeued and no new buffers are expected to become available.
> >> +
> >> +EBADR
> >> +    The ``V4L2_BUF_FLAG_REQUEST_FD`` flag was set but the device does not
> >> +    support requests for the given buffer type, or
> >> +    the ``V4L2_BUF_FLAG_REQUEST_FD`` flag was not set but the device requires
> >> +    that the buffer is part of a request.
> >> +
> >> +EBUSY
> >> +    The first buffer was queued via a request, but the application now tries
> >> +    to queue it directly, or vice versa (it is not permitted to mix the two
> >> +    APIs).
> >> diff --git a/Documentation/userspace-api/media/v4l/vidioc-ext-querybuf.rst b/Documentation/userspace-api/media/v4l/vidioc-ext-querybuf.rst
> >> new file mode 100644
> >> index 0000000000000..f2a12017253f6
> >> --- /dev/null
> >> +++ b/Documentation/userspace-api/media/v4l/vidioc-ext-querybuf.rst
> >> @@ -0,0 +1,79 @@
> >> +.. 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
> >> +
> >> +.. _VIDIOC_EXT_QUERYBUF:
> >> +
> >> +*************************
> >> +ioctl VIDIOC_EXT_QUERYBUF
> >> +*************************
> >> +
> >> +Name
> >> +====
> >> +
> >> +VIDIOC_EXT_QUERYBUF - Query the status of a buffer
> >> +
> >> +
> >> +Synopsis
> >> +========
> >> +
> >> +.. c:function:: int ioctl( int fd, VIDIOC_EXT_QUERYBUF, struct v4l2_ext_buffer *argp )
> >> +    :name: VIDIOC_EXT_QUERYBUF
> >> +
> >> +
> >> +Arguments
> >> +=========
> >> +
> >> +``fd``
> >> +    File descriptor returned by :ref:`open() <func-open>`.
> >> +
> >> +``argp``
> >> +    Pointer to struct :c:type:`v4l2_ext_buffer`.
> >> +
> >> +
> >> +Description
> >> +===========
> >> +
> >> +This ioctl is part of the :ref:`streaming <mmap>` I/O method. It can
> >> +be used to query the status of a buffer at any time after buffers have
> >> +been allocated with the :ref:`VIDIOC_EXT_CREATE_BUFS` ioctl.
> >> +
> >> +Applications set the ``type`` field of a struct
> >> +:c:type:`v4l2_ext_buffer` to the same buffer type as was
> >> +previously used with struct :c:type:`v4l2_ext_pix_format` ``type``,
> >> +and the ``index`` field. Valid index numbers range from zero to the
> >> +number of buffers allocated with
> >> +:ref:`VIDIOC_EXT_CREATE_BUFS` (struct
> >> +:c:type:`v4l2_ext_create_buffers` ``count``) minus
> >> +one. The ``reserved`` field must be set to 0.
> >> +
> >> +In the ``flags`` field the ``V4L2_BUF_FLAG_MAPPED``,
> >> +``V4L2_BUF_FLAG_PREPARED``, ``V4L2_BUF_FLAG_QUEUED`` and
> >> +``V4L2_BUF_FLAG_DONE`` flags will be valid. The ``planes.memory`` fields will be
> >> +set to the current I/O method for each plane.
> >> +
> >> +For every valid plane, an entry in ``planes`` will be filled, and zeroed for
> >> +invalid ones. ``planes[i].buffer_length`` is the size of the memory buffer
> >> +which contains the plane, ``planes[i].plane_length`` is the length of the plane,
> >
> > What is the length of the plane here? It seems like depending on the
> > context, the field can mean size of the payload (for filled buffer) or
> > total space for the payload (for empty buffer), which is a bit confusing.
> >
> >> +and ``planes[i].offset` is where the plane is placed in the memory buffer.
> >> +
> >> +The size of the ``planes`` array can be calculated by the number of sequential
> >> +planes with ``planes[i].buffer_length`` that differs from zero up to the max
> >> +size of the array.
> >
> > Not sure if it's just me, but "size of array" sounds to me like the
> > total memory size of the array, i.e. the max number of elements, which
> > is fixed for this struct. How about
> >
> >   "The entries of the planes array are valid up to the last entry with
> >   non-zero buffer_length or the last entry of the array, whichever is
> >   first."
> >
> > ?
> >
> >> +
> >> +
> >> +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.
> >> +
> >> +EINVAL
> >> +    The buffer ``type`` is not supported, or the ``index`` is out of
> >> +    bounds.
> >> diff --git a/Documentation/userspace-api/media/v4l/vidioc-g-ext-pix-fmt.rst b/Documentation/userspace-api/media/v4l/vidioc-g-ext-pix-fmt.rst
> >> new file mode 100644
> >> index 0000000000000..008e6c98a88a5
> >> --- /dev/null
> >> +++ b/Documentation/userspace-api/media/v4l/vidioc-g-ext-pix-fmt.rst
> >> @@ -0,0 +1,117 @@
> >> +.. 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
> >> +
> >> +.. _VIDIOC_G_EXT_PIX_FMT:
> >> +
> >> +************************************************************************
> >> +ioctl VIDIOC_G_EXT_PIX_FMT, VIDIOC_S_EXT_PIX_FMT, VIDIOC_TRY_EXT_PIX_FMT
> >> +************************************************************************
> >> +
> >> +Name
> >> +====
> >> +
> >> +VIDIOC_G_EXT_PIX_FMT - VIDIOC_S_EXT_PIX_FMT - VIDIOC_TRY_EXT_PIX_FMT - Get or set the data format, try a format
> >> +
> >> +
> >> +Synopsis
> >> +========
> >> +
> >> +.. c:function:: int ioctl( int fd, VIDIOC_G_EXT_PIX_FMT, struct v4l2_ext_pix_format *argp )
> >> +    :name: VIDIOC_G_EXT_PIX_FMT
> >> +
> >> +.. c:function:: int ioctl( int fd, VIDIOC_S_EXT_PIX_FMT, struct v4l2_ext_pix_format *argp )
> >> +    :name: VIDIOC_S_EXT_PIX_FMT
> >> +
> >> +.. c:function:: int ioctl( int fd, VIDIOC_TRY_EXT_PIX_FMT, struct v4l2_ext_pix_format *argp )
> >> +    :name: VIDIOC_TRY_EXT_PIX_FMT
> >> +
> >> +Arguments
> >> +=========
> >> +
> >> +``fd``
> >> +    File descriptor returned by :ref:`open() <func-open>`.
> >> +
> >> +``argp``
> >> +    Pointer to struct :c:type:`v4l2_ext_pix_format`.
> >> +
> >> +
> >> +Description
> >> +===========
> >> +
> >> +These ioctls are used to negotiate the format of data (typically image
> >> +format) exchanged between driver and application.
> >> +
> >> +To query the current parameters applications set the ``type`` field of a
> >> +struct :c:type:`v4l2_ext_pix_format` to  ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or
> >> +``V4L2_BUF_TYPE_VIDEO_OUTPUT``, all the other types are invalid in this API,
> >> +and multiplanar is supported through modifiers.
> >> +
> >> +When the application calls the
> >> +:ref:`VIDIOC_G_EXT_PIX_FMT <VIDIOC_G_EXT_PIX_FMT>` ioctl with a pointer to this
> >> +structure the driver fills the other members.
> >> +When the requested buffer type is not supported drivers return
> >> +an ``EINVAL`` error code.
> >> +
> >> +To change the current format parameters applications initialize all
> >> +the fields in the struct.
> >> +For details see the documentation of the various devices types in
> >> +:ref:`devices`. Good practice is to query the current parameters
> >> +first, and to modify only those parameters not suitable for the
> >> +application. When the application calls the :ref:`VIDIOC_S_EXT_PIX_FMT <VIDIOC_G_EXT_PIX_FMT>` ioctl with
> >> +a pointer to a struct :c:type:`v4l2_ext_pix_format` structure the driver
> >> +checks and adjusts the parameters against hardware abilities. Drivers
> >> +should not return an error code unless the ``type`` field is invalid,
> >> +this is a mechanism to fathom device capabilities and to approach
> >> +parameters acceptable for both the application and driver. On success
> >> +the driver may program the hardware, allocate resources and generally
> >> +prepare for data exchange. Finally the :ref:`VIDIOC_S_EXT_PIX_FMT <VIDIOC_G_EXT_PIX_FMT>` ioctl returns
> >> +the current format parameters as :ref:`VIDIOC_G_EXT_PIX_FMT <VIDIOC_G_EXT_PIX_FMT>` does. Very simple,
> >> +inflexible devices may even ignore all input and always return the
> >> +default parameters. However all V4L2 devices exchanging data with the
> >> +application must implement the :ref:`VIDIOC_G_EXT_PIX_FMT <VIDIOC_G_EXT_PIX_FMT>` and :ref:`VIDIOC_S_EXT_PIX_FMT <VIDIOC_G_EXT_PIX_FMT>`
> >> +ioctl. When the requested buffer type is not supported drivers return an
> >> +EINVAL error code on a :ref:`VIDIOC_S_EXT_PIX_FMT <VIDIOC_G_EXT_PIX_FMT>` attempt. When I/O is already in
> >> +progress or the resource is not available for other reasons drivers
> >> +return the ``EBUSY`` error code.
> >> +
> >> +The :ref:`VIDIOC_TRY_EXT_PIX_FMT <VIDIOC_G_EXT_PIX_FMT>` ioctl is equivalent to :ref:`VIDIOC_S_EXT_PIX_FMT <VIDIOC_G_EXT_PIX_FMT>` with one
> >> +exception: it does not change driver state. It can also be called at any
> >> +time, never returning ``EBUSY``. This function is provided to negotiate
> >> +parameters, to learn about hardware limitations, without disabling I/O
> >> +or possibly time consuming hardware preparations. Although strongly
> >> +recommended drivers are not required to implement this ioctl.
> >> +
> >
> > Could we make this mandatory to avoid ambiguities in the API? In
>
> Yes, please!
>
> > general, most drivers implemented it anyway, without much added
> > complexity, because one can then simplify S_FMT to TRY_FMT + state
> > update.
>
> Ideally we would also fix the ambiguous situation regarding TRY/S_FMT
> and unsupported pixelformats: some drivers will replace unsupported
> pixelformats with a valid pixelformat, others will return an error.
>
> v4l2-compliance warns about drivers that return an error and point to
> this very old thread:
>
> http://www.mail-archive.com/linux-media@vger.kernel.org/msg56550.html
>
> However, I'm not really sure if we can resolve this ambiguity. I believe
> that it is only older webcam drivers that return an error.

I guess we can at least enforce a specific behavior in the new API,
before we get compatibility obligations. :)

Best regards,
Tomasz

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