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Subject[RFC PATCH 9/9] [media] document jobs API
Date
Still a work-in-progress, but hopefully conveys the general idea.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
---
Documentation/media/intro.rst | 2 +
Documentation/media/media_uapi.rst | 1 +
Documentation/media/uapi/jobs/jobs-api.rst | 23 +++
Documentation/media/uapi/jobs/jobs-example.rst | 69 ++++++++
Documentation/media/uapi/jobs/jobs-intro.rst | 61 +++++++
Documentation/media/uapi/jobs/jobs-queue.rst | 73 ++++++++
Documentation/media/uapi/jobs/jobs-queue.svg | 192 +++++++++++++++++++++
.../media/uapi/v4l/vidioc-g-ext-ctrls.rst | 6 +
8 files changed, 427 insertions(+)
create mode 100644 Documentation/media/uapi/jobs/jobs-api.rst
create mode 100644 Documentation/media/uapi/jobs/jobs-example.rst
create mode 100644 Documentation/media/uapi/jobs/jobs-intro.rst
create mode 100644 Documentation/media/uapi/jobs/jobs-queue.rst
create mode 100644 Documentation/media/uapi/jobs/jobs-queue.svg

diff --git a/Documentation/media/intro.rst b/Documentation/media/intro.rst
index 9ce2e23a0236..e39a9dd3444a 100644
--- a/Documentation/media/intro.rst
+++ b/Documentation/media/intro.rst
@@ -38,6 +38,8 @@ divided into five parts.

5. The :ref:`fifth part <cec>` covers the CEC (Consumer Electronics Control) API.

+6. The :ref:`sixth part <jobsapi>` covers the jobs API.
+
It should also be noted that a media device may also have audio components, like
mixers, PCM capture, PCM playback, etc, which are controlled via ALSA API. For
additional information and for the latest development code, see:
diff --git a/Documentation/media/media_uapi.rst b/Documentation/media/media_uapi.rst
index fd8ebe002cd2..254e3d085abc 100644
--- a/Documentation/media/media_uapi.rst
+++ b/Documentation/media/media_uapi.rst
@@ -27,5 +27,6 @@ License".
uapi/rc/remote_controllers
uapi/mediactl/media-controller
uapi/cec/cec-api
+ uapi/jobs/jobs-api
uapi/gen-errors
uapi/fdl-appendix
diff --git a/Documentation/media/uapi/jobs/jobs-api.rst b/Documentation/media/uapi/jobs/jobs-api.rst
new file mode 100644
index 000000000000..3a7aa8568e93
--- /dev/null
+++ b/Documentation/media/uapi/jobs/jobs-api.rst
@@ -0,0 +1,23 @@
+.. -*- coding: utf-8; mode: rst -*-
+
+.. include:: <isonum.txt>
+
+.. _jobsapi:
+
+##################
+Part VI - Jobs API
+##################
+
+This part describes the V4L2 Jobs API.
+
+.. class:: toc-title
+
+ Table of Contents
+
+.. toctree::
+ :maxdepth: 5
+ :numbered:
+
+ jobs-intro
+ jobs-queue
+ jobs-example
diff --git a/Documentation/media/uapi/jobs/jobs-example.rst b/Documentation/media/uapi/jobs/jobs-example.rst
new file mode 100644
index 000000000000..0b725dfb58bd
--- /dev/null
+++ b/Documentation/media/uapi/jobs/jobs-example.rst
@@ -0,0 +1,69 @@
+.. -*- coding: utf-8; mode: rst -*-
+
+.. _jobs-example:
+
+===========================================
+Example: Using the Jobs API for HDR capture
+===========================================
+
+HDR capturing involves taking two shots of the same image with different
+exposure settings. When using the V4L2 API, one must wait for the first shot to
+be completed before updating the exposure and taking the second one, introducing
+synchronization with user-space and potential delays for the second image.
+
+The Jobs API allows us to simply submit two jobs with different exposure
+parameters, and to dequeue their buffers to get the result.
+
+The following example shows how to do this on a capture device with the exposure
+control. It assumes that the capture device has already been opened and its
+format set. To keep the code simple this code does not handle errors.
+
+.. code-block:: c
+
+ struct v4l2_ext_control ctrl[1];
+ struct v4l2_ext_controls ctrls;
+ struct v4l2_jobqueue_init jq_init;
+ struct v4l2_buffer buf[2];
+ int jq_fd;
+
+ /* FD for the jobs queue */
+ jq_fd = open("/dev/v4l2_jobqueue");
+
+ /* Initialize the jobs queue */
+ jq_init.nb_devs = 1;
+ jq_init.fd[0] = capture_fd;
+ ioctl(jq_fd, VIDIOC_JOBQUEUE_INIT, &jq_init);
+
+ /* Prepare and submit the first capture job, low exposure */
+ ctrl[0].id = V4L2_CID_EXPOSURE;
+ ctrl[0].value = 32;
+ ctrls.which = V4L2_CTRL_WHICH_CURJOB_VAL;
+ ctrls.count = 1;
+ ctrsl.controls = ctrl;
+ ioctl(capture_fd, VICIOC_S_EXT_CTRLS, &ctrls);
+ ioctl(capture_fd, VIDIOC_QBUF, &buf[0]);
+ ioctl(jq_fd, VIDIOC_JOBQUEUE_QJOB, NULL);
+
+ /* Prepare and submit the second capture job, high exposure */
+ ctrl[0].id = V4L2_CID_EXPOSURE;
+ ctrl[0].value = 192;
+ ctrls.which = V4L2_CTRL_WHICH_CURJOB_VAL;
+ ctrls.count = 1;
+ ctrsl.controls = ctrl;
+ ioctl(capture_fd, VICIOC_S_EXT_CTRLS, &ctrls);
+ ioctl(capture_fd, VIDIOC_QBUF, &buf[1]);
+ ioctl(jq_fd, VIDIOC_JOBQUEUE_QJOB, NULL);
+
+ /* Dequeue buffers with the captured data */
+ ioctl(capture_fd, VIDIOC_DQBUF, &buf[0]);
+ ioctl(capture_fd, VIDIOC_DQBUF, &buf[1]);
+
+ /* Dequeue jobs and confirm exposure parameters */
+ ioctl(jq_fd, VIDIOC_JOBQUEUE_DQJOB, NULL);
+ ctrls.which = V4L2_CTRL_WHICH_DEQJOB_VAL
+ ioctl(fd, VIDIOC_G_EXT_CTRLS, &ctrls);
+ printf("Exposure for first capture: %d\n", ctrl[0].value);
+
+ ioctl(jq_fd, VIDIOC_JOBQUEUE_DQJOB, NULL);
+ ioctl(fd, VIDIOC_G_EXT_CTRLS, &ctrls);
+ printf("Exposure for second capture: %d\n", ctrl[0].value);
diff --git a/Documentation/media/uapi/jobs/jobs-intro.rst b/Documentation/media/uapi/jobs/jobs-intro.rst
new file mode 100644
index 000000000000..34d7efe5bcfb
--- /dev/null
+++ b/Documentation/media/uapi/jobs/jobs-intro.rst
@@ -0,0 +1,61 @@
+.. -*- coding: utf-8; mode: rst -*-
+
+.. _jobs-intro:
+
+============
+Introduction
+============
+
+The Jobs API allows to submit units of work ("jobs") to be processed
+cooperatively by a set of opened devices. The state of jobs can be set and
+queried independently from the actual hardware state, which means that
+user-space does not need to explicitly wait for a job to finish before
+submitting a new one with different parameters.
+
+The Jobs API is suitable for several workflows:
+
+- A set of devices need to cooperate on a given work, e.g. MIPI capture devices.
+In this case the Jobs API ensures that all devices are properly set up for
+processing the work.
+
+- Asynchronous submission of works with different parameters. Especially useful
+for stateless codecs, but can also facilitate capture use-cases like HDR where
+two frames with different exposures need to be captured as quickly as possible.
+
+- Quick switch between several states without having to explicitly reprogram
+them. A job can be kept in memory and be reapplied at a future time.
+
+A job's will usually be used as follows:
+
+1. The state of the job is defined using the standard V4L2 API: controls,
+formats, and other parameters are set, and buffers are queued on all devices
+that take part in the job.
+
+2. The job is queued. All set parameters are applied, and the queued buffers are
+processed.
+
+3. The job is dequeued. Output controls at the time of job completion can be
+read back. If another job is queued, it is processed.
+
+Jobs are submitted to a jobs queue which processes them in sequential order.
+Similarly to buffers, jobs must be dequeued and the state of the devices at the
+time of job completion can then be read back.
+
+The Jobs API extends the existing V4L2 API and slightly changes the behavior of
+some existing commands when in use. This part will describe the new APIs and how
+to use them.
+
+WARNING: This is a work-in-progress. Comments are welcome. Many pieces are
+missing, while some are subject to change. The list of missing pieces and open
+questions include notably:
+
+- No format or crop setting
+- No support for non-integer controls (this requires some rework of the control
+framework)
+- No support for media controller devices yet
+- No error reporting when something unexpected happens while a job is being
+processed
+- It is probably not desirable to have a /dev/v4l2_jobsqueue device node for
+managing jobs, but this makes testing easier at the moment.
+- Jobs should have identifiers to make them easier to identify by user-space
+- Dequeued buffers should carry the id of the job they were associated to
diff --git a/Documentation/media/uapi/jobs/jobs-queue.rst b/Documentation/media/uapi/jobs/jobs-queue.rst
new file mode 100644
index 000000000000..ecfc8dd46302
--- /dev/null
+++ b/Documentation/media/uapi/jobs/jobs-queue.rst
@@ -0,0 +1,73 @@
+.. -*- coding: utf-8; mode: rst -*-
+
+.. _jobs-queue:
+
+==============
+The Jobs Queue
+==============
+
+A jobs queue is instanciated from a set of already opened V4L2 devices, and
+controls their behavior with atomic (from the user perspective) units of work
+called jobs. User-space prepares a job by setting its desired state through
+controls and formats, queuing the buffers to be processed for the job, and
+finally queuing the job itself once it is ready. The hardware state will not be
+affected until the job is effectively processed.
+
+Figure :ref:`jobs-queue` shows the life cycle of jobs within the queue:
+
+.. kernel-figure:: jobs-queue.svg
+ :alt: Life of a job inside the jobs queue
+ :align: center
+
+User-space creates a jobs queue by opening the jobs queue device and invoking
+the :c:func:`VIDIOC_JOBQUEUE_INIT` ioctl with the list of opened file
+descriptors of the devices being part of the new queue. After this, all the
+passed devices are ready to perform according to the jobs API semantics, and the
+opened jobs queue can accept queue and dequeue ioctls.
+
+Upon job queue creation, the first job is instanciated and ready to be prepared
+and queued (we refer to this as the "current job"). User-space prepares the job
+using variants of the regular V4L2 ioctls: for instance, in order to set
+controls on a job, one will invoke :c:func:`VIDIOC_S_EXT_CTRLS` with
+V4L2_CTRL_WHICH_CURJOB_VAL.
+
+The current job can be submitted using the :c:func:`VIDIOC_JOBQUEUE_QJOB` ioctl
+when it is ready. A new job is immediately created and becomes the new active
+job.
+
+Queued jobs are processed sequentially. As a job is processed, its parameters
+for every device that is part of the queue are applied, and all its buffers are
+processed. Once completed, the job is ready to be dequeued.
+
+Buffers can be dequeued independently of jobs. This allows a buffer that is
+produced in the middle of the pipeline to be dequeued and used before the entire
+job completes. However jobs **must** be dequeued at some point to avoid
+accumulation of completed jobs. This is done using the
+:c:func:`VIDIOC_JOBQUEUE_DQJOB` ioctl. User-space can then read the state of the
+dequeued job at the time of its completion. For instance, in order to read the
+values of controls at the time the dequeued job completed, one will invoke
+:c:func:`VIDIOC_QUERY_EXT_CTRL` with V4L2_CTRL_WHICH_DEQJOB_VAL.
+
+Note that regular, unmodified ioctls will continue working as usual: for
+instance, invoking :c:func:`VIDIOC_S_EXT_CTRLS` with V4L2_CTRL_WHICH_CUR_VAL
+will immediately change the value of submitted controls. The only difference
+when the jobs API is used is that buffers queued on devices that are part of a
+jobs queue are not processed until the current job of that queue is submitted.
+
+By default, jobs are purely sequential: a new job is created as soon as the
+current job is queued, and becomes the new current job. This new current job
+initially has an empty state, meaning that its expected state is the state of
+the previously queued job after it completes. Once a dequeued job is replaced by
+the next one, it is deleted.
+
+For some use-cases however, it may make sense to reapply the exact state of a
+previous job. The jobs API allows to do so using the
+:c:func:`VIDIOC_JOBQUEUE_EXPORT_JOB` and :c:func:`VIDIOC_JOBQUEUE_IMPORT_JOB`
+ioctls.
+
+:c:func:`VIDIOC_JOBQUEUE_EXPORT_JOB` returns a file descriptor that represents
+the current job, which will be preserved in memory until that file descriptor is
+closed. By passing that file descriptor to the
+:c:func:`VIDIOC_JOBQUEUE_IMPORT_JOB`, the current job is replaced by the job
+represented by the file descriptor. User-space can then tune the state of the
+job, queue new buffers, and submit the job to be processed again.
diff --git a/Documentation/media/uapi/jobs/jobs-queue.svg b/Documentation/media/uapi/jobs/jobs-queue.svg
new file mode 100644
index 000000000000..e6d9408fd1df
--- /dev/null
+++ b/Documentation/media/uapi/jobs/jobs-queue.svg
@@ -0,0 +1,192 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN" "http://www.w3.org/TR/2001/PR-SVG-20010719/DTD/svg10.dtd">
+<svg width="33cm" height="23cm" viewBox="96 61 644 457" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+ <text font-size="12.7998" style="fill: #000000;text-anchor:start;font-family:sans-serif;font-style:normal;font-weight:normal" x="198.6" y="319.8">
+ <tspan x="198.6" y="319.8"></tspan>
+ </text>
+ <g>
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+ <tspan x="198.6" y="435.322">Current</tspan>
+ </text>
+ </g>
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+ </text>
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+ <tspan x="613.6" y="325.322">Completed</tspan>
+ </text>
+ </g>
+ <text font-size="12.7998" style="fill: #000000;text-anchor:start;font-family:sans-serif;font-style:normal;font-weight:normal" x="648.1" y="316.3">
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+ </text>
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diff --git a/Documentation/media/uapi/v4l/vidioc-g-ext-ctrls.rst b/Documentation/media/uapi/v4l/vidioc-g-ext-ctrls.rst
index 5ab8d2ac27b9..035d5e29bd2e 100644
--- a/Documentation/media/uapi/v4l/vidioc-g-ext-ctrls.rst
+++ b/Documentation/media/uapi/v4l/vidioc-g-ext-ctrls.rst
@@ -216,6 +216,12 @@ still cause this situation.
You can only get the default value of the control,
you cannot set or try it.

+ When the Jobs API is active ``V4L2_CTRL_WHICH_CURJOB_VAL`` will return or set the value of the current job, while ``V4L2_CTRL_WHICH_DEQJOB_VAL`` returns the value of the currently dequeued job.
+
+ .. note::
+
+ It is invalid to try and set the value of the dequeued job.
+
For backwards compatibility you can also use a control class here
(see :ref:`ctrl-class`). In that case all controls have to
belong to that control class. This usage is deprecated, instead
--
2.14.2.822.g60be5d43e6-goog
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