media: Documentation: v4l: document request API

Document the request API for V4L2 devices, and amend the documentation
of system calls influenced by it.
Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/media/uapi/mediactl/media-controller.rst

@@ -21,6 +21,7 @@ Part IV - Media Controller API
     media-controller-intro
     media-controller-model
     media-types
+    request-api
     media-funcs
     media-header
 

Documentation/media/uapi/mediactl/media-funcs.rst

@@ -16,3 +16,9 @@ Function Reference
     media-ioc-enum-entities
     media-ioc-enum-links
     media-ioc-setup-link
+    media-ioc-request-alloc
+    request-func-close
+    request-func-ioctl
+    request-func-poll
+    media-request-ioc-queue
+    media-request-ioc-reinit

Documentation/media/uapi/mediactl/media-ioc-request-alloc.rst

+.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.1-or-later WITH no-invariant-sections
+
+.. _media_ioc_request_alloc:
+
+*****************************
+ioctl MEDIA_IOC_REQUEST_ALLOC
+*****************************
+
+Name
+====
+
+MEDIA_IOC_REQUEST_ALLOC - Allocate a request
+
+
+Synopsis
+========
+
+.. c:function:: int ioctl( int fd, MEDIA_IOC_REQUEST_ALLOC, int *argp )
+    :name: MEDIA_IOC_REQUEST_ALLOC
+
+
+Arguments
+=========
+
+``fd``
+    File descriptor returned by :ref:`open() <media-func-open>`.
+
+``argp``
+    Pointer to an integer.
+
+
+Description
+===========
+
+If the media device supports :ref:`requests <media-request-api>`, then
+this ioctl can be used to allocate a request. If it is not supported, then
+``errno`` is set to ``ENOTTY``. A request is accessed through a file descriptor
+that is returned in ``*argp``.
+
+If the request was successfully allocated, then the request file descriptor
+can be passed to the :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`,
+:ref:`VIDIOC_G_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>`,
+:ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` and
+:ref:`VIDIOC_TRY_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` ioctls.
+
+In addition, the request can be queued by calling
+:ref:`MEDIA_REQUEST_IOC_QUEUE` and re-initialized by calling
+:ref:`MEDIA_REQUEST_IOC_REINIT`.
+
+Finally, the file descriptor can be :ref:`polled <request-func-poll>` to wait
+for the request to complete.
+
+The request will remain allocated until all the file descriptors associated
+with it are closed by :ref:`close() <request-func-close>` and the driver no
+longer uses the request internally.
+
+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.
+
+ENOTTY
+    The driver has no support for requests.

Documentation/media/uapi/mediactl/media-request-ioc-queue.rst

+.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.1-or-later WITH no-invariant-sections
+
+.. _media_request_ioc_queue:
+
+*****************************
+ioctl MEDIA_REQUEST_IOC_QUEUE
+*****************************
+
+Name
+====
+
+MEDIA_REQUEST_IOC_QUEUE - Queue a request
+
+
+Synopsis
+========
+
+.. c:function:: int ioctl( int request_fd, MEDIA_REQUEST_IOC_QUEUE )
+    :name: MEDIA_REQUEST_IOC_QUEUE
+
+
+Arguments
+=========
+
+``request_fd``
+    File descriptor returned by :ref:`MEDIA_IOC_REQUEST_ALLOC`.
+
+
+Description
+===========
+
+If the media device supports :ref:`requests <media-request-api>`, then
+this request ioctl can be used to queue a previously allocated request.
+
+If the request was successfully queued, then the file descriptor can be
+:ref:`polled <request-func-poll>` to wait for the request to complete.
+
+If the request was already queued before, then ``EBUSY`` is returned.
+Other errors can be returned if the contents of the request contained
+invalid or inconsistent data, see the next section for a list of
+common error codes. On error both the request and driver state are unchanged.
+
+Typically if you get an error here, then that means that the application
+did something wrong and you have to fix the application.
+
+Once a request is queued, then the driver is required to gracefully handle
+errors that occur when the request is applied to the hardware. The
+exception is the ``EIO`` error which signals a fatal error that requires
+the application to stop streaming to reset the hardware state.
+
+It is not allowed to mix queuing requests with queuing buffers directly
+(without a request). ``EPERM`` will be returned if the first buffer was
+queued directly and you next try to queue a request, or vice versa.
+
+A request must contain at least one buffer, otherwise this ioctl will
+return an ``ENOENT`` error.
+
+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
+    The request was already queued.
+EPERM
+    The application queued the first buffer directly, but later attempted
+    to use a request. It is not permitted to mix the two APIs.
+ENOENT
+    The request did not contain any buffers. All requests are required
+    to have at least one buffer. This can also be returned if required
+    controls are missing.
+ENOMEM
+    Out of memory when allocating internal data structures for this
+    request.
+EINVAL
+    The request has invalid data.
+EIO
+    The hardware is in a bad state. To recover, the application needs to
+    stop streaming to reset the hardware state and then try to restart
+    streaming.

Documentation/media/uapi/mediactl/media-request-ioc-reinit.rst

+.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.1-or-later WITH no-invariant-sections
+
+.. _media_request_ioc_reinit:
+
+******************************
+ioctl MEDIA_REQUEST_IOC_REINIT
+******************************
+
+Name
+====
+
+MEDIA_REQUEST_IOC_REINIT - Re-initialize a request
+
+
+Synopsis
+========
+
+.. c:function:: int ioctl( int request_fd, MEDIA_REQUEST_IOC_REINIT )
+    :name: MEDIA_REQUEST_IOC_REINIT
+
+
+Arguments
+=========
+
+``request_fd``
+    File descriptor returned by :ref:`MEDIA_IOC_REQUEST_ALLOC`.
+
+Description
+===========
+
+If the media device supports :ref:`requests <media-request-api>`, then
+this request ioctl can be used to re-initialize a previously allocated
+request.
+
+Re-initializing a request will clear any existing data from the request.
+This avoids having to :ref:`close() <request-func-close>` a completed
+request and allocate a new request. Instead the completed request can just
+be re-initialized and it is ready to be used again.
+
+A request can only be re-initialized if it either has not been queued
+yet, or if it was queued and completed. Otherwise it will set ``errno``
+to ``EBUSY``. No other error codes can be returned.
+
+Return Value
+============
+
+On success 0 is returned, on error -1 and the ``errno`` variable is set
+appropriately.
+
+EBUSY
+    The request is queued but not yet completed.

Documentation/media/uapi/mediactl/request-api.rst

+.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.1-or-later WITH no-invariant-sections
+
+.. _media-request-api:
+
+Request API
+===========
+
+The Request API has been designed to allow V4L2 to deal with requirements of
+modern devices (stateless codecs, complex camera pipelines, ...) and APIs
+(Android Codec v2). One such requirement is the ability for devices belonging to
+the same pipeline to reconfigure and collaborate closely on a per-frame basis.
+Another is support of stateless codecs, which require controls to be applied
+to specific frames (aka 'per-frame controls') in order to be used efficiently.
+
+Supporting these features without the Request API is not always possible and if
+it is, it is terribly inefficient: user-space would have to flush all activity
+on the media pipeline, reconfigure it for the next frame, queue the buffers to
+be processed with that configuration, and wait until they are all available for
+dequeuing before considering the next frame. This defeats the purpose of having
+buffer queues since in practice only one buffer would be queued at a time.
+
+The Request API allows a specific configuration of the pipeline (media
+controller topology + controls for each media entity) to be associated with
+specific buffers. The parameters are applied by each participating device as
+buffers associated to a request flow in. This allows user-space to schedule
+several tasks ("requests") with different parameters in advance, knowing that
+the parameters will be applied when needed to get the expected result. Control
+values at the time of request completion are also available for reading.
+
+Usage
+=====
+
+The Request API is used on top of standard media controller and V4L2 calls,
+which are augmented with an extra ``request_fd`` parameter. Requests themselves
+are allocated from the supporting media controller node.
+
+Request Allocation
+------------------
+
+User-space allocates requests using :ref:`MEDIA_IOC_REQUEST_ALLOC`
+for the media device node. This returns a file descriptor representing the
+request. Typically, several such requests will be allocated.
+
+Request Preparation
+-------------------
+
+Standard V4L2 ioctls can then receive a request file descriptor to express the
+fact that the ioctl is part of said request, and is not to be applied
+immediately. See :ref:`MEDIA_IOC_REQUEST_ALLOC` for a list of ioctls that
+support this. Controls set with a ``request_fd`` parameter are stored instead
+of being immediately applied, and buffers queued to a request do not enter the
+regular buffer queue until the request itself is queued.
+
+Request Submission
+------------------
+
+Once the parameters and buffers of the request are specified, it can be
+queued by calling :ref:`MEDIA_REQUEST_IOC_QUEUE` on the request file descriptor.
+A request must contain at least one buffer, otherwise ``ENOENT`` is returned.
+This will make the buffers associated to the request available to their driver,
+which can then apply the associated controls as buffers are processed. A queued
+request cannot be modified anymore.
+
+.. caution::
+   For :ref:`memory-to-memory devices <codec>` you can use requests only for
+   output buffers, not for capture buffers. Attempting to add a capture buffer
+   to a request will result in an ``EPERM`` error.
+
+If the request contains parameters for multiple entities, individual drivers may
+synchronize so the requested pipeline's topology is applied before the buffers
+are processed. Media controller drivers do a best effort implementation since
+perfect atomicity may not be possible due to hardware limitations.
+
+.. caution::
+
+   It is not allowed to mix queuing requests with directly queuing buffers:
+   whichever method is used first locks this in place until
+   :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` is called or the device is
+   :ref:`closed <func-close>`. Attempts to directly queue a buffer when earlier
+   a buffer was queued via a request or vice versa will result in an ``EPERM``
+   error.
+
+Controls can still be set without a request and are applied immediately,
+regardless of whether a request is in use or not.
+
+.. caution::
+
+   Setting the same control through a request and also directly can lead to
+   undefined behavior!
+
+User-space can :ref:`poll() <request-func-poll>` a request file descriptor in
+order to wait until the request completes. A request is considered complete
+once all its associated buffers are available for dequeuing and all the
+associated controls have been updated with the values at the time of completion.
+Note that user-space does not need to wait for the request to complete to
+dequeue its buffers: buffers that are available halfway through a request can
+be dequeued independently of the request's state.
+
+A completed request contains the state of the request at the time of the
+request completion. User-space can query that state by calling
+:ref:`ioctl VIDIOC_G_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` with the request file
+descriptor. Calling :ref:`ioctl VIDIOC_G_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` for a
+request that has been queued but not yet completed will return ``EBUSY``
+since the control values might be changed at any time by the driver while the
+request is in flight.
+
+Recycling and Destruction
+-------------------------
+
+Finally, a completed request can either be discarded or be reused. Calling
+:ref:`close() <request-func-close>` on a request file descriptor will make
+that file descriptor unusable and the request will be freed once it is no
+longer in use by the kernel. That is, if the request is queued and then the
+file descriptor is closed, then it won't be freed until the driver completed
+the request.
+
+The :ref:`MEDIA_REQUEST_IOC_REINIT` will clear a request's state and make it
+available again. No state is retained by this operation: the request is as
+if it had just been allocated.
+
+Example for a Codec Device
+--------------------------
+
+For use-cases such as :ref:`codecs <codec>`, the request API can be used
+to associate specific controls to
+be applied by the driver for the OUTPUT buffer, allowing user-space
+to queue many such buffers in advance. It can also take advantage of requests'
+ability to capture the state of controls when the request completes to read back
+information that may be subject to change.
+
+Put into code, after obtaining a request, user-space can assign controls and one
+OUTPUT buffer to it:
+
+.. code-block:: c
+
+	struct v4l2_buffer buf;
+	struct v4l2_ext_controls ctrls;
+	int req_fd;
+	...
+	if (ioctl(media_fd, MEDIA_IOC_REQUEST_ALLOC, &req_fd))
+		return errno;
+	...
+	ctrls.which = V4L2_CTRL_WHICH_REQUEST_VAL;
+	ctrls.request_fd = req_fd;
+	if (ioctl(codec_fd, VIDIOC_S_EXT_CTRLS, &ctrls))
+		return errno;
+	...
+	buf.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;
+	buf.flags |= V4L2_BUF_FLAG_REQUEST_FD;
+	buf.request_fd = req_fd;
+	if (ioctl(codec_fd, VIDIOC_QBUF, &buf))
+		return errno;
+
+Note that it is not allowed to use the Request API for CAPTURE buffers
+since there are no per-frame settings to report there.
+
+Once the request is fully prepared, it can be queued to the driver:
+
+.. code-block:: c
+
+	if (ioctl(req_fd, MEDIA_REQUEST_IOC_QUEUE))
+		return errno;
+
+User-space can then either wait for the request to complete by calling poll() on
+its file descriptor, or start dequeuing CAPTURE buffers. Most likely, it will
+want to get CAPTURE buffers as soon as possible and this can be done using a
+regular :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`:
+
+.. code-block:: c
+
+	struct v4l2_buffer buf;
+
+	memset(&buf, 0, sizeof(buf));
+	buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+	if (ioctl(codec_fd, VIDIOC_DQBUF, &buf))
+		return errno;
+
+Note that this example assumes for simplicity that for every OUTPUT buffer
+there will be one CAPTURE buffer, but this does not have to be the case.
+
+We can then, after ensuring that the request is completed via polling the
+request file descriptor, query control values at the time of its completion via
+a call to :ref:`VIDIOC_G_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>`.
+This is particularly useful for volatile controls for which we want to
+query values as soon as the capture buffer is produced.
+
+.. code-block:: c
+
+	struct pollfd pfd = { .events = POLLPRI, .fd = req_fd };
+	poll(&pfd, 1, -1);
+	...
+	ctrls.which = V4L2_CTRL_WHICH_REQUEST_VAL;
+	ctrls.request_fd = req_fd;
+	if (ioctl(codec_fd, VIDIOC_G_EXT_CTRLS, &ctrls))
+		return errno;
+
+Once we don't need the request anymore, we can either recycle it for reuse with
+:ref:`MEDIA_REQUEST_IOC_REINIT`...
+
+.. code-block:: c
+
+	if (ioctl(req_fd, MEDIA_REQUEST_IOC_REINIT))
+		return errno;
+
+... or close its file descriptor to completely dispose of it.
+
+.. code-block:: c
+
+	close(req_fd);
+
+Example for a Simple Capture Device
+-----------------------------------
+
+With a simple capture device, requests can be used to specify controls to apply
+for a given CAPTURE buffer.
+
+.. code-block:: c
+
+	struct v4l2_buffer buf;
+	struct v4l2_ext_controls ctrls;
+	int req_fd;
+	...
+	if (ioctl(media_fd, MEDIA_IOC_REQUEST_ALLOC, &req_fd))
+		return errno;
+	...
+	ctrls.which = V4L2_CTRL_WHICH_REQUEST_VAL;
+	ctrls.request_fd = req_fd;
+	if (ioctl(camera_fd, VIDIOC_S_EXT_CTRLS, &ctrls))
+		return errno;
+	...
+	buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+	buf.flags |= V4L2_BUF_FLAG_REQUEST_FD;
+	buf.request_fd = req_fd;
+	if (ioctl(camera_fd, VIDIOC_QBUF, &buf))
+		return errno;
+
+Once the request is fully prepared, it can be queued to the driver:
+
+.. code-block:: c
+
+	if (ioctl(req_fd, MEDIA_REQUEST_IOC_QUEUE))
+		return errno;
+
+User-space can then dequeue buffers, wait for the request completion, query
+controls and recycle the request as in the M2M example above.

Documentation/media/uapi/mediactl/request-func-close.rst

+.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.1-or-later WITH no-invariant-sections
+
+.. _request-func-close:
+
+***************
+request close()
+***************
+
+Name
+====
+
+request-close - Close a request file descriptor
+
+
+Synopsis
+========
+
+.. code-block:: c
+
+    #include <unistd.h>
+
+
+.. c:function:: int close( int fd )
+    :name: req-close
+
+Arguments
+=========
+
+``fd``
+    File descriptor returned by :ref:`MEDIA_IOC_REQUEST_ALLOC`.
+
+
+Description
+===========
+
+Closes the request file descriptor. Resources associated with the request
+are freed once all file descriptors associated with the request are closed
+and the driver has completed the request.
+
+
+Return Value
+============
+
+:ref:`close() <request-func-close>` returns 0 on success. On error, -1 is
+returned, and ``errno`` is set appropriately. Possible error codes are:
+
+EBADF
+    ``fd`` is not a valid open file descriptor.

Documentation/media/uapi/mediactl/request-func-ioctl.rst

+.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.1-or-later WITH no-invariant-sections
+
+.. _request-func-ioctl:
+
+***************
+request ioctl()
+***************
+
+Name
+====
+
+request-ioctl - Control a request file descriptor
+
+
+Synopsis
+========
+
+.. code-block:: c
+
+    #include <sys/ioctl.h>
+
+
+.. c:function:: int ioctl( int fd, int cmd, void *argp )
+    :name: req-ioctl
+
+Arguments
+=========
+
+``fd``
+    File descriptor returned by :ref:`MEDIA_IOC_REQUEST_ALLOC`.
+
+``cmd``
+    The request ioctl command code as defined in the media.h header file, for
+    example :ref:`MEDIA_REQUEST_IOC_QUEUE`.
+
+``argp``
+    Pointer to a request-specific structure.
+
+
+Description
+===========
+
+The :ref:`ioctl() <request-func-ioctl>` function manipulates request
+parameters. The argument ``fd`` must be an open file descriptor.
+
+The ioctl ``cmd`` code specifies the request function to be called. It
+has encoded in it whether the argument is an input, output or read/write
+parameter, and the size of the argument ``argp`` in bytes.
+
+Macros and structures definitions specifying request ioctl commands and
+their parameters are located in the media.h header file. All request ioctl
+commands, their respective function and parameters are specified in
+:ref:`media-user-func`.
+
+
+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.
+
+Command-specific error codes are listed in the individual command
+descriptions.
+
+When an ioctl that takes an output or read/write parameter fails, the
+parameter remains unmodified.

Documentation/media/uapi/mediactl/request-func-poll.rst

+.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.1-or-later WITH no-invariant-sections
+
+.. _request-func-poll:
+
+**************
+request poll()
+**************
+
+Name
+====
+
+request-poll - Wait for some event on a file descriptor
+
+
+Synopsis
+========
+
+.. code-block:: c
+
+    #include <sys/poll.h>
+
+
+.. c:function:: int poll( struct pollfd *ufds, unsigned int nfds, int timeout )
+   :name: request-poll
+
+Arguments
+=========
+
+``ufds``
+   List of file descriptor events to be watched
+
+``nfds``
+   Number of file descriptor events at the \*ufds array
+
+``timeout``
+   Timeout to wait for events
+
+
+Description
+===========
+
+With the :c:func:`poll() <request-func-poll>` function applications can wait
+for a request to complete.
+
+On success :c:func:`poll() <request-func-poll>` returns the number of file
+descriptors that have been selected (that is, file descriptors for which the
+``revents`` field of the respective struct :c:type:`pollfd`
+is non-zero). Request file descriptor set the ``POLLPRI`` flag in ``revents``
+when the request was completed.  When the function times out it returns
+a value of zero, on failure it returns -1 and the ``errno`` variable is
+set appropriately.
+
+Attempting to poll for a request that is completed or not yet queued will
+set the ``POLLERR`` flag in ``revents``.
+
+
+Return Value
+============
+
+On success, :c:func:`poll() <request-func-poll>` returns the number of
+structures which have non-zero ``revents`` fields, or zero if the call
+timed out. On error -1 is returned, and the ``errno`` variable is set
+appropriately:
+
+``EBADF``
+    One or more of the ``ufds`` members specify an invalid file
+    descriptor.
+
+``EFAULT``
+    ``ufds`` references an inaccessible memory area.
+
+``EINTR``
+    The call was interrupted by a signal.
+
+``EINVAL``
+    The ``nfds`` value exceeds the ``RLIMIT_NOFILE`` value. Use
+    ``getrlimit()`` to obtain this value.

Documentation/media/uapi/v4l/buffer.rst

@@ -306,10 +306,15 @@ struct v4l2_buffer
       - A place holder for future extensions. Drivers and applications
 	must set this to 0.
     * - __u32
-      - ``reserved``
+      - ``request_fd``
       -
-      - A place holder for future extensions. Drivers and applications
-	must set this to 0.
+      - The file descriptor of the request to queue the buffer to. If specified
+        and flag ``V4L2_BUF_FLAG_REQUEST_FD`` is set, then the buffer will be
+	queued to that request. This is set by the user when calling
+	:ref:`ioctl VIDIOC_QBUF <VIDIOC_QBUF>` and ignored by other ioctls.
+	If the device does not support requests, then ``EPERM`` will be returned.
+	If requests are supported but an invalid request FD is given, then
+	``ENOENT`` will be returned.
 
 
 
@@ -514,6 +519,11 @@ Buffer Flags
 	streaming may continue as normal and the buffer may be reused
 	normally. Drivers set this flag when the ``VIDIOC_DQBUF`` ioctl is
 	called.
+    * .. _`V4L2-BUF-FLAG-IN-REQUEST`:
+
+      - ``V4L2_BUF_FLAG_IN_REQUEST``
+      - 0x00000080
+      - This buffer is part of a request that hasn't been queued yet.
     * .. _`V4L2-BUF-FLAG-KEYFRAME`:
 
       - ``V4L2_BUF_FLAG_KEYFRAME``
@@ -589,6 +599,11 @@ Buffer Flags
 	the format. Any Any subsequent call to the
 	:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl will not block anymore,
 	but return an ``EPIPE`` error code.
+    * .. _`V4L2-BUF-FLAG-REQUEST-FD`:
+
+      - ``V4L2_BUF_FLAG_REQUEST_FD``
+      - 0x00800000
+      - The ``request_fd`` field contains a valid file descriptor.
     * .. _`V4L2-BUF-FLAG-TIMESTAMP-MASK`:
 
       - ``V4L2_BUF_FLAG_TIMESTAMP_MASK``

Documentation/media/uapi/v4l/vidioc-g-ext-ctrls.rst

@@ -95,6 +95,26 @@ appropriate. In the first case the new value is set in struct
 is inappropriate (e.g. the given menu index is not supported by the menu
 control), then this will also result in an ``EINVAL`` error code error.
 
+If ``request_fd`` is set to a not-yet-queued :ref:`request <media-request-api>`
+file descriptor and ``which`` is set to ``V4L2_CTRL_WHICH_REQUEST_VAL``,
+then the controls are not applied immediately when calling
+:ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>`, but instead are applied by
+the driver for the buffer associated with the same request.
+If the device does not support requests, then ``EPERM`` will be returned.
+If requests are supported but an invalid request FD is given, then
+``ENOENT`` will be returned.
+
+An attempt to call :ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` for a
+request that has already been queued will result in an ``EBUSY`` error.
+
+If ``request_fd`` is specified and ``which`` is set to ``V4L2_CTRL_WHICH_REQUEST_VAL``
+during a call to :ref:`VIDIOC_G_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>`, then the
+returned values will be the values currently set for the request (or the
+hardware value if none is set) if the request has not yet been queued, or the
+values of the controls at the time of request completion if it has already
+completed. Attempting to get controls while the request has been queued but
+not yet completed will result in an ``EBUSY`` error.
+
 The driver will only set/get these controls if all control values are
 correct. This prevents the situation where only some of the controls
 were set/get. Only low-level errors (e. g. a failed i2c command) can
@@ -209,13 +229,17 @@ still cause this situation.
       - ``which``
       - Which value of the control to get/set/try.
 	``V4L2_CTRL_WHICH_CUR_VAL`` will return the current value of the
-	control and ``V4L2_CTRL_WHICH_DEF_VAL`` will return the default
-	value of the control.
+	control, ``V4L2_CTRL_WHICH_DEF_VAL`` will return the default
+	value of the control and ``V4L2_CTRL_WHICH_REQUEST_VAL`` indicates that
+	these controls have to be retrieved from a request or tried/set for
+	a request. In the latter case the ``request_fd`` field contains the
+	file descriptor of the request that should be used. If the device
+	does not support requests, then ``EPERM`` will be returned.
 
 	.. note::
 
-	   You can only get the default value of the control,
-	   you cannot set or try it.
+	   When using ``V4L2_CTRL_WHICH_DEF_VAL`` note that You can only
+	   get the default value of the control, you cannot set or try it.
 
 	For backwards compatibility you can also use a control class here
 	(see :ref:`ctrl-class`). In that case all controls have to
@@ -272,8 +296,15 @@ still cause this situation.
 	then you can call :ref:`VIDIOC_TRY_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` to try to discover the
 	actual control that failed the validation step. Unfortunately,
 	there is no ``TRY`` equivalent for :ref:`VIDIOC_G_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>`.
+    * - __s32
+      - ``request_fd``
+      - File descriptor of the request to be used by this operation. Only
+	valid if ``which`` is set to ``V4L2_CTRL_WHICH_REQUEST_VAL``.
+	If the device does not support requests, then ``EPERM`` will be returned.
+	If requests are supported but an invalid request FD is given, then
+	``ENOENT`` will be returned.
     * - __u32
-      - ``reserved``\ [2]
+      - ``reserved``\ [1]
       - Reserved for future extensions.
 
 	Drivers and applications must set the array to zero.
@@ -362,7 +393,9 @@ ERANGE
 EBUSY
     The control is temporarily not changeable, possibly because another
     applications took over control of the device function this control
-    belongs to.
+    belongs to, or (if the ``which`` field was set to
+    ``V4L2_CTRL_WHICH_REQUEST_VAL``) the request was queued but not yet
+    completed.
 
 ENOSPC
     The space reserved for the control's payload is insufficient. The
@@ -372,3 +405,11 @@ ENOSPC
 EACCES
     Attempt to try or set a read-only control or to get a write-only
     control.
+
+EPERM
+    The ``which`` field was set to ``V4L2_CTRL_WHICH_REQUEST_VAL`` but the
+    device does not support requests.
+
+ENOENT
+    The ``which`` field was set to ``V4L2_CTRL_WHICH_REQUEST_VAL`` but the
+    the given ``request_fd`` was invalid.

Documentation/media/uapi/v4l/vidioc-qbuf.rst

@@ -65,7 +65,7 @@ To enqueue a :ref:`memory mapped <mmap>` buffer applications set the
 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.
+``EINVAL`` error code.
 
 To enqueue a :ref:`user pointer <userp>` buffer applications set the
 ``memory`` field to ``V4L2_MEMORY_USERPTR``, the ``m.userptr`` field to
@@ -98,6 +98,25 @@ dequeued, until the :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or
 :ref:`VIDIOC_REQBUFS` ioctl is called, or until the
 device is closed.
 
+The ``request_fd`` field can be used with the ``VIDIOC_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 ``EPERM`` will be returned.
+If requests are supported but an invalid request FD is given, then
+``ENOENT`` will be returned.
+
+.. caution::
+   It is not allowed to mix queuing requests with queuing buffers directly.
+   ``EPERM`` will be returned if the first buffer was queued directly and
+   then the application tries to queue a request, or vice versa.
+
+   For :ref:`memory-to-memory devices <codec>` 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 ``EPERM`` error.
+
 Applications call the ``VIDIOC_DQBUF`` ioctl to dequeue a filled
 (capturing) or displayed (output) buffer from the driver's outgoing
 queue. They just set the ``type``, ``memory`` and ``reserved`` fields of
@@ -153,3 +172,14 @@ EPIPE
     ``VIDIOC_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.
+
+EPERM
+    The ``V4L2_BUF_FLAG_REQUEST_FD`` flag was set but the device does not
+    support requests. Or 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). Or an attempt is made to queue a
+    CAPTURE buffer to a request for a :ref:`memory-to-memory device <codec>`.
+
+ENOENT
+    The ``V4L2_BUF_FLAG_REQUEST_FD`` flag was set but the the given
+    ``request_fd`` was invalid.

Documentation/media/videodev2.h.rst.exceptions

@@ -514,6 +514,7 @@ ignore define V4L2_CTRL_DRIVER_PRIV
 ignore define V4L2_CTRL_MAX_DIMS
 ignore define V4L2_CTRL_WHICH_CUR_VAL
 ignore define V4L2_CTRL_WHICH_DEF_VAL
+ignore define V4L2_CTRL_WHICH_REQUEST_VAL
 ignore define V4L2_OUT_CAP_CUSTOM_TIMINGS
 ignore define V4L2_CID_MAX_CTRLS