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2018-06-04  

Report of the Linux Media Summit 2017 – Prague, October, 2017

This is the report of the Linux Media Summit 2017, with happened in Prague, October, 2017.

group photo

First day

The first day was mostly concentrated on discussing the media development process. No notes were done at the time of the meeting, so any report here is based on my own recollection may be unrepresentative of the entire meeting.

Attendees:

Sakari Ailus
Mauro Carvalho Chehab
Shuah Khan
Mike Krufky
Gustavo Padovan
Laurent Pinchart
Niklas Söderlund
Hans Verkuil

Media development statistics

Mauro gave a presentation on media tree development statistics he's been collecting. The slides can be found here:

https://linuxtv.org/downloads/presentations/media_summit_2017/media_summit_process.pdf

Media device sharing

Media device sharing was discussed during the first day as well. The use case is related to a USB device that in Linux has several interfaces (?) which in Linux are devices. The purpose of this would be to show entities backed by different devices from the same USB device, without either of the drivers being necessarily aware of each other. The USB device is also hot unpluggable, meaning that the devices backing the media entities in the media graph may disappear, and do so in any order as they could appear.

Media device sharing requires improvements to the Media controller framework. Especially:

  • The Media controller framework assumes the media device is backed by a single struct device. There's also an ops struct, also related to a single driver. There may no longer be 1:1 relation between these two and the media device.
  • Object lifetime management. Object lifetime management has been found to be not designed well enough in Media controller in general. The original use case was non-hotpluggable devices and the early efforts address the problem were focussed in preventing unbinding of drivers from devices in various ways. This need to be properly addressed, with refcounts, so that the necessary memory objects for various operations will stay where needed as long as needed. (I'm not going to details here. Work has been done to this end but it is not complete yet.)

Second day

The report on the second day is based on the notes written during the meeting. The raw notes can be found here:

http://www.retiisi.org.uk/v4l2/notes/media-summit-2017.html

Attendees

Jacopo Mondi
Mauro Chehab
Niklas Söderlund
Benoit Parrot
Pavel Machek
Ricardo Ribalda
Dimitrios Katasaros
Laurent Pinchart
Hans Verkuil
Gustavo Padovan
Alexandre Courbot
Sakari Ailus
Mike Krufky
Brad Love
Tomas Dratva

CEC status

Hans gave a presentation on CEC status. The slides are available here:

https://linuxtv.org/downloads/presentations/media_summit_2017/cec-status.pdf

Latest CEC status is always available here:

https://hverkuil.home.xs4all.nl/cec-status.txt

Side note: should we use the new serial bus (serdev) for the RC serial driver?

Explicit synchronization in V4L2

Gustavo gave a presentation on explicit synchronisation in V4L2. It's available here:

https://linuxtv.org/downloads/presentations/media_summit_2017/2017-MediaSummit-Fences.pdf

In-fences are easy, there is no large issue in the RFC.

Out-fences, however, are subject to buffer reordering issues. A new event has been created to report buffer order to userspace. Fences are created at QBUF time, but can't be used by userspace until an event reports which buffer will be available next.

Not all drivers perform reordering. As an optimization a capability flag could tell userspace that buffers are always ordered and that events are not needed. The capability flag should likely happen at/after setting format, as out-of-order depends on the video format

Reordering is mostly useful for codecs, and can also occur from buffer recycling internally in drivers when capture errors happen. While recycling is allowed by V4L2, we could forbid it when fences are used. In that case all buffers would be returned to userspace in order, and errors be reported with the error flag set.

We may need to make capture queues not reorder buffers as well, so basically if we are using in-fences we don't let the bufffers reorder.

So when there is no reorder we can return the buffer's fence at the QBUF time and not need the OUT_FENCE event for these cases.

If the user doesn't pickup the outstanding OUT_FENCE event before DQBUF just set the fence to -1 at DQBUF time.

Discussion if timestamps are enough for audio/video synchronization... and what kind of timestamps to use. Wall clock timestamps are not usable for synchronisation, monotonic is the way to go. Converting device timestmaps to monotonic in the kernel can be troublesome. Time stamping with fences --- time stamps related to buffers are only available at dqbuf time (unless SoF events are being used, and they're supported by only some hardware). This is too late in some use cases, if the timestamp is used to determine when the frame should be displayed.

Capture to networking (a.k.a. partial fences) - We need to move from a frame-based API to a slice-based API. The problem isn't limited to fences, the whole V4L2 API is based on frames. Latency could be lowered in this way, but it isn't clear whether frame-based handling is the main source of latency in any use case. To support very low latency (mostly for professional video use cases) we would need to redesign all buffer handling in V4L2. We'll postpone this for now until someone comes with a convincing use case and enough resources to implement it.

Media Subsystem Maintainance

Some years ago V4L2 sub-maintainers were introduced to address recognised bottlenecks (there's only one Mauro, for instance). Today we have more contributors but the number of V4L2 and Media core developers who actively review patches submitted to the list remains small. The current model appears to have its limits as well, which is also why we're looking how others are doing their work.

Gustavo explained how the DRM subsystem is maintained. The DRM subsystem consists of the core of the subsystem and individual drivers, a large part of which are maintained in the drm-misc tree. The drm-misc tree is also the target for small core changes and DRM-wide changes.

The drm-misc tree has a number of committers (37 at the time of the presentation was given) who also review each others patches. The model switch was done as there were many contributors and bottlenecks due to that.

Each drm-misc committer has the ability to push patches to the same, common branch, naturally requiring the patches to be reviewed on the mailing list. Once a consensus is eached among the drm-misc developers reviewing the patch, a patch may be merged.

A pull request is sent, by one or the drm-misc maintainers (of which there are three), to David Arlie from the common branch weekly. Dave merges this pull request to his tree.

Committers of the drm-misc tree take great care in ensuring the quality of their code base. This has lead to improved developer skills among the existing developers due to more challenging and responsible work. This, in turn, makes it harder for patches to fall through the cracks as there are many reviewers and committers capable of doing the required work to get the code merged.

Such model has attracted more contributors and solved process bottlenecks. There is a policy that would lead on removing committer rights if abused. However, there wasn't any incident so far that justified the removal of committer right.

Bad patches getting merged will be reverted, or hopefully, not getting merged in the first place.

Could media subsystem switch to this model?

The DRM guys have a set of tools that support their work flow. These tools are nor DRM specific as such but they currently do contain DRM specific configuration that would need to be moved to a configuration file before the tools were usable for media tree development.

Right now, the media subsystem is rather complex, containing multiple different kernel APIs and frameworks (e.g. V4L2, DVB, Media controller) and a large number of drivers using them. Mauro, the media tree maintainer, does have checks in place to catch patches that could potentially cause havoc if merged to Linus's tree in form of breaking the user space API. If the media tree would be maintained by a group of committers instead, similar checks would have to be in place in order to avoid breakages that are visible outside the subsystem.

Developers often test the patches and may perform compile tests (e.g. through 01.org's kbuild test robot) but they may be missed just after the final, trivial changes that sometimes end up breaking things. For instance, before pushing new patches to the master branch, the following checks should be automatically performed so that developers don't end up pushing stuff that's not good to go there:

  • Perform checkpatch.pl checks, ensures the branch has the correct baseline and does check for many errors
  • Compile the new patches.
  • Destructive push to the master branch need to be prevented.
  • Question to Mauro: what kind of checks are you actually performing on the patches you apply or pull requests you pick the patches from?

Could Jenkins or 0day be helpful here? Jenkins would require more infra on our side whereas 0day already exists. 0day requires explicit instructions to check a kernel git tree periodically, this is managed by Fengguang Wu (fengguang.wu at intel.com).

Security

Having more committers places more strict requirements on developer machine security. This is because many more people have push access to the master branch, essentially there are many more points where development tree integrity may be compromised. (Note: nothing will tell you this has happened; push done by an attacker on a developer's machine looks exactly like a developer would have done that.)

Konstantin Ryabitsev gave an excellent presentation on securing developer workstations (outside Media summit); I couldn't find a video (I guess there was no camera) but the slides are online:

https://mricon.com/talks/osseu17.pdf

Patchwork

  • No information in patchwork e-mails on who acted on a patch. This would be important to add.
  • patchwork.linuxtv.org is using 1.x version of Patchwork. Switch to 2.x should be done, but there are database incompatibilities between the two. Is there a migration script?
  • The alternative is to use kernel.org patchwork. This has been troublesome in the past and it is not known it'd be less so now. (Why so escaped me.)

Media subsystem documentation

Mauro made a presentation on the state of the documentation of the media subsystem. It is available here:

https://linuxtv.org/downloads/presentations/media_summit_2017/media_summit_documentation.pdf

In the following discussion it was pointed out that videobuf2 has been mostly undocumented but this hasn't been a problem in practice: the API is sensible, and the drivers using videobuf2 are mostly doing the right thing. This does not excuse the lack of documentation however.

There's a push to split kAPI and uAPI documentation in the kernel. V4L2 now has them combined in a single document. The reason for the push is that the audiences to the two APIs are largely different.

Related to the above Mauro gave this presentation in the KS documentation session:

https://linuxtv.org/downloads/presentations/media_summit_2017/ks_documentation_session.pdf

While the V4L2 reference documentation is in a very good shape, how-to-type documentation is not available for application developers who have little idea of how to use the API. The reference documentation does not help here.

Where should KernelDoc function documentation go to?

It was discussed whether KernelDoc documentation should be placed in headers vs. in .c files. The points raised were (the list may be inconclusive):

  • Keeping the documentation in headers may result in changing the function but missing changing the documentation. This could happen as the developer misses the documentation in header file, which is entirely plausible if the function prototype is not changed.
  • Some prefer to have the documentation with the interface definition, on the grounds it is easier to browse it that way. Tools such as C-scope do find definitions of functions based on the function definition itself, not the prototype in the header.
  • Some saw extensive documentation fitting better to the header. The .c files could grow significantly due to documentation, making it more difficult to work with the code in them.

It was decided to keep KernelDoc function documentation in the header files.

SoC camera removal

There are not many drivers left using the SoC camera framework.

sh-ceu is being converted by Jacopo to a proper V4L2 driver. This work is ongoing.

V4L2 clock used by SoC camera, pxa_camera and two i2c sensor drivers only. One option for the two sensor drivers could be to just remove them; they can be brought back if needed.

Videobuf1 removal

atomisp staging driver and more than 10 non-staging drivers still use videobuf1. One of the most notable ones are bttv. Work would be needed to convert these drivers to videobuf2.

Request API

Alexandre Courbot gave a presentation on the V4L2 jobs API, which is effectively intended to address the same problem area than the Media request API.

There is interest in getting request API to upstream. Several classes of hardware need it, including cameras supporting Android camera HAL v3 and stateless video codecs, the two main use cases at hand at the moment. The request API involves, in principle, all API elements and aspects of the V4L2 and Media controller APIs. This requires forward-looking approach in the implementation which will unlikely to be a complete one in the beginning.

The Request API approach uses the media controller to create request objects. Note that this does not work if there are multiple media controllers involved that need to coordinate. As a consequence, devices managed by different drivers cannot be part of the same request.

The MC approach requires that stateless codec drivers implement a media graph which is just a video node with two pads and a codec subdev with two pads.

An old version of the Request API is in use by ChromeOS for several HW codecs for several years now. Support for this should be done first to prevent the risk that vendors will drop V4L2 support and find other solutions. The API should be extensible to the complex video pipeline use-case, though.

In order to support stateless codecs we add support for the request API to the V4L2 ioctls that need this (ioctls operating on struct v4l2_buffer and v4l2_ext_controls). Creating, cloning, applying and deleting request objects is done through the media controller.

In the long run we want to be able to set the request data for all bridge and subdev drivers with a single MC ioctl. As part of this we could make the control framework more generic: i.e. no longer V4L2 specific but a generic media framework. This would be similar to the DRM atomic framework.

Note that Media device complexes may have devices that are out of scope of requests such as slow i2c devices where you still have to hit the frame related to the request. So what do you do if you miss? This would require additional complex cross-driver mechanisms in kernel. At least for now make user space responsible for applying configurations to such devices.

A request object is essentially created empty from the point of view of the user. Only values that are changed in the request are part of the request, other values remain unchanged (unless those values change due to a side-effect of setting a request value). When a request completes, make a copy of the volatile controls (since that's the value at that point in time). This is needed for auto-<whatever> functions that need to know such volatile values.

It should be possible to poll on the request's file handle and be woken up when the request is done.

When dequeuing a buffer the kernel will set the request field to a request filehandle if the buffer is associated with a request. The kernel knows this information, so userspace doesn't have to set this field.

Todo: Implement polling for request file handles. This allows polling for specific requests, e.g. as get woken up when the last request in a set of requests is done.

Actions

  • Laurent: check the suitability of DRM tooling for V4L2
  • Mauro: check if kernel.org improved patchwork support.
  • Hans: tentative (not this year) convert old videobuf documentation to videobuf2.
  • Alexandre: MC Request API (REQUEST_CMD), with request components through V4L2 devices, request creation and queueing through MC
  • Hans: control framework API: sync with Alexandre by 2nd week of November at the latest.

mchehab

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