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 -- 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
