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

From: Mauro Carvalho Chehab
Date: Sat Mar 11 2017 - 08:17:15 EST

Em Sat, 11 Mar 2017 12:32:43 +0100
Hans Verkuil <hverkuil@xxxxxxxxx> escreveu:

> On 10/03/17 16:09, Mauro Carvalho Chehab wrote:
> > Em Fri, 10 Mar 2017 13:54:28 +0100
> > Hans Verkuil <hverkuil@xxxxxxxxx> escreveu:
> >
> >>> Devices that have complex pipeline that do essentially require using the
> >>> Media controller interface to configure them are out of that scope.
> >>>
> >>
> >> Way too much of how the MC devices should be used is in the minds of developers.
> >> There is a major lack for good detailed documentation, utilities, compliance
> >> test (really needed!) and libv4l plugins.
> >
> > Unfortunately, we merged an incomplete MC support at the Kernel. We knew
> > all the problems with MC-based drivers and V4L2 applications by the time
> > it was developed, and we requested Nokia developers (with was sponsoring MC
> > develoment, on that time) to work on a solution to allow standard V4L2
> > applications to work with MC based boards.
> >
> > Unfortunately, we took the decision to merge MC without that, because
> > Nokia was giving up on Linux development, and we didn't want to lose the
> > 2 years of discussions and work around it, as Nokia employers were leaving
> > the company. Also, on that time, there was already some patches floating
> > around adding backward support via libv4l. Unfortunately, those patches
> > were never finished.
> >
> > The net result is that MC was merged with some huge gaps, including
> > the lack of a proper solution for a generic V4L2 program to work
> > with V4L2 devices that use the subdev API.
> >
> > That was not that bad by then, as MC was used only on cell phones
> > that run custom-made applications.
> >
> > The reallity changed, as now, we have lots of low cost SoC based
> > boards, used for all sort of purposes. So, we need a quick solution
> > for it.
> >
> > In other words, while that would be acceptable support special apps
> > on really embedded systems, it is *not OK* for general purpose SoC
> > harware[1].
> >
> > [1] I'm calling "general purpose SoC harware" those ARM boards
> > like Raspberry Pi that are shipped to the mass and used by a wide
> > range of hobbyists and other people that just wants to run Linux on
> > ARM. It is possible to buy such boards for a very cheap price,
> > making them to be used not only on special projects, where a custom
> > made application could be interesting, but also for a lot of
> > users that just want to run Linux on a low cost ARM board, while
> > keeping using standard V4L2 apps, like "camorama".
> >
> > That's perhaps one of the reasons why it took a long time for us to
> > start receiving drivers upstream for such hardware: it is quite
> > intimidating and not logical to require developers to implement
> > on their drivers 2 complex APIs (MC, subdev) for those
> > hardware that most users won't care. From user's perspective,
> > being able to support generic applications like "camorama" and
> > "zbar" is all they want.
> >
> > In summary, I'm pretty sure we need to support standard V4L2
> > applications on boards like Raspberry Pi and those low-cost
> > SoC-based boards that are shipped to end users.
> >
> >> Anyway, regarding this specific patch and for this MC-aware driver: no, you
> >> shouldn't inherit controls from subdevs. It defeats the purpose.
> >
> > Sorry, but I don't agree with that. The subdev API is an optional API
> > (and even the MC API can be optional).
> >
> > I see the rationale for using MC and subdev APIs on cell phones,
> > ISV and other embedded hardware, as it will allow fine-tuning
> > the driver's support to allow providing the required quality for
> > certain custom-made applications. but on general SoC hardware,
> > supporting standard V4L2 applications is a need.
> >
> > Ok, perhaps supporting both subdev API and V4L2 API at the same
> > time doesn't make much sense. We could disable one in favor of the
> > other, either at compilation time or at runtime.
> Right. If the subdev API is disabled, then you have to inherit the subdev
> controls in the bridge driver (how else would you be able to access them?).
> And that's the usual case.
> If you do have the subdev API enabled, AND you use the MC, then the
> intention clearly is to give userspace full control and inheriting controls
> no longer makes any sense (and is highly confusing IMHO).

I tend to agree with that.

> >
> > This way, if the subdev API is disabled, the driver will be
> > functional for V4L2-based applications that don't support neither
> > MC or subdev APIs.
> I'm not sure if it makes sense for the i.MX driver to behave differently
> depending on whether the subdev API is enabled or disabled. I don't know
> enough of the hardware to tell if it would ever make sense to disable the
> subdev API.

Yeah, I don't know enough about it either. The point is: this is
something that the driver maintainer and driver users should
decide if it either makes sense or not, based on the expected use cases.