[Ksummit-discuss] [TECH TOPIC] Media Controller

[Mauro Carvalho Chehab]

Along the years, we've been developing a mechanism at the Kernel to store
and present graphs to userspace, called Media Controller. The original
scope of the Media Controller were to store and represent pipeline connections
between hardware components on complex streaming devices, like the ones
that are used on cell phones, where there are typically two camera sensors,
and lots of processing units to enhance the image and to convert it into
different formats.

Since the beginning, we found that it would be useful to use it some day
on other subsystems. Well, now such day has come ;)

Driven by the need of using the media controller on TV sets, where the
pipelines can be really complex and may cover more than one subsystem,
we're reworking at both the core implementation and at the userspace API
to allow it to be used where needed.

On a TV set, the streaming data pipeline covers camera sensors, analog TV,
digital TV, crypto/decrypto modules, audio and GPU. Also, TVs have network
interfaces that may be provided via DVB (like CATV). If we want/need to 
represent and control the full pipeline, the media controller has to have
support on different subsystems: V4L2, DVB, ALSA, DRM and Network. It also
needs a way to share a common struct between those subsystems without making
them depend on each other. So, it should use devres.

Also, on the MC summit, it was pointed that other subsystems like IIO
(Linux Industrial I/O) have similar needs and also need to track complex
pipelines.

When we have such complex pipelines that interact with different subsystems,
we need to better describe the interfaces between userspace and the
pipeline. In the past, we only needed to represent device nodes, but now
we'll need to be capable of representing network interfaces and sysfs.

We also need to allow dynamic changes in terms of removing and adding
objects at the graph.

Due to that, we're redesigning the MC core, making it more generic and
more capable of representing complex devices. So, we'd like to discuss
it with the KS audience, in order to check if there are other use cases
for it, and if the solution we're designing would require more changes
to fit such other use cases.

For this discussion, I nominate myself and Laurent Pinchart, Hans Verkuil,
Sakari Ailus, Shuah Khan and Lars-Peter Clausen.

PS.: I know we're a week late, but we've been busy this month with the
discussions, including a Media Controller 3-days summit in Helsinki last
week.