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. Thanks, Mauro -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
