[PATCH v4 14/36] [media] v4l2-mc: add a function to inherit controls from a pipeline

[Hans Verkuil]

On 03/17/2017 03:37 PM, Russell King - ARM Linux wrote:
> On Fri, Mar 17, 2017 at 02:51:10PM +0100, Philipp Zabel wrote:
>> On Fri, 2017-03-17 at 10:24 -0300, Mauro Carvalho Chehab wrote:
>> [...]
>>> The big question, waiting for an answer on the last 8 years is
>>> who would do that? Such person would need to have several different
>>> hardware from different vendors, in order to ensure that it has
>>> a generic solution.
>>>
>>> It is a way more feasible that the Kernel developers that already 
>>> have a certain hardware on their hands to add support inside the
>>> driver to forward the controls through the pipeline and to setup
>>> a "default" pipeline that would cover the common use cases at
>>> driver's probe.
>>
>> Actually, would setting pipeline via libv4l2 plugin and letting drivers
>> provide a sane enabled default pipeline configuration be mutually
>> exclusive? Not sure about the control forwarding, but at least a simple
>> link setup and format forwarding would also be possible in the kernel
>> without hindering userspace from doing it themselves later.
> 
> I think this is the exact same problem as controls in ALSA.
> 
> When ALSA started off in life, the requirement was that all controls
> shall default to minimum, and the user is expected to adjust controls
> after the system is running.
> 
> After OSS, this gave quite a marked change in system behaviour, and
> led to a lot of "why doesn't my sound work anymore" problems, because
> people then had to figure out which combination of controls had to be
> set to get sound out of their systems.
> 
> Now it seems to be much better, where install Linux on a platform, and
> you have a working sound system (assuming that the drivers are all there
> which is generally the case for x86.)
> 
> However, it's still possible to adjust all the controls from userspace.
> All that's changed is the defaults.
> 
> Why am I mentioning this - because from what I understand Mauro saying,
> it's no different from this situation.  Userspace will still have the
> power to disable all links and setup its own.  The difference is that
> there will be a default configuration that the kernel sets up at boot
> time that will be functional, rather than the current default
> configuration where the system is completely non-functional until
> manually configured.
> 
> However, at the end of the day, I don't care _where_ the usability
> problems are solved, only that there is some kind of solution.  It's not
> the _where_ that's the real issue here, but the _how_, and discussion of
> the _how_ is completely missing.
> 
> So, let's try kicking off a discussion about _how_ to do things.
> 
> _How_ do we setup a media controller system so that we end up with a
> usable configuration - let's start with the obvious bit... which links
> should be enabled.
> 
> I think the first pre-requisit is that we stop exposing capture devices
> that can never be functional for the hardware that's present on the board,
> so that there isn't this plentora of useless /dev/video* nodes and useless
> subdevices.
> 
> One possible solution to finding a default path may be "find the shortest
> path between the capture device and the sensor and enable intervening
> links".
> 
> Then we need to try configuring that path with format/resolution
> information.
> 
> However, what if something in the shortest path can't handle the format
> that the sensor produces?  I think at that point, we'd need to drop that
> subdev out of the path resolution, re-run the "find the shortest path"
> algorithm, and try again.
> 
> Repeat until success or no path between the capture and sensor exists.
> 
> This works fine if you have just one sensor visible from a capture device,
> but not if there's more than one (which I suspect is the case with the
> Sabrelite board with its two cameras and video receiver.)  That breaks
> the "find the shortest path" algorithm.
> 
> So, maybe it's a lot better to just let the board people provide via DT
> a default setup for the connectivity of the modules somehow - certainly
> one big step forward would be to disable in DT parts of the capture
> system that can never be used (remembering that boards like the RPi /
> Hummingboard may end up using DT overlays to describe this for different
> cameras, so the capture setup may change after initial boot.)

The MC was developed before the device tree came along. But now that the DT
is here, I think this could be a sensible idea to let the DT provide an
initial path.

Sakari, Laurent, Mauro: any opinions?

Regards,

	Hans