[PATCH 05/14] media: add a V4L2 OF parser

[Guennadi Liakhovetski]

On Wed, 10 Oct 2012, Sylwester Nawrocki wrote:

> Hi Laurent,
> 
> On 10/10/2012 03:25 PM, Laurent Pinchart wrote:
> > On Tuesday 09 October 2012 13:00:24 Hans Verkuil wrote:
> >> On Tue 9 October 2012 12:34:48 Sylwester Nawrocki wrote:
> >>> On 10/08/2012 11:40 AM, Guennadi Liakhovetski wrote:
> >>>> On Fri, 5 Oct 2012, Sylwester Nawrocki wrote:
> >>>>> I would really like to see more than one user until we add any core
> >>>>> code. No that it couldn't be changed afterwards, but it would be nice
> >>>>> to ensure the concepts are right and proven in real life.
> >>>>
> >>>> Unfortunately I don't have any more systems on which I could easily
> >>>> enough try this. I've got a beagleboard with a camera, but I don't think
> >>>> I'm a particularly good candidate for implementing DT support for OMAP3
> >>>> camera drivers;-) Apart from that I've only got soc-camera based
> >>>> systems, of which none are _really_ DT-ready... At best I could try an
> >>>> i.MX31 based board, but that doesn't have a very well developed .dts and
> >>>> that would be soc-camera too anyway.
> >>>
> >>> I certainly wouldn't expect you would do all the job. I mean it would be
> >>> good to possibly have some other developers adding device tree support
> >>> based on that new bindings and new infrastructure related to them.
> > 
> > As I mentioned in another e-mail, I plan to work on DT support for the OMAP3
> > ISP, but I first need generic clock framework support for OMAP3.
> 
> OK, let's hope it's available soon.
> 
> >>> There have been recently some progress in device tree support for Exynos
> >>> SoCs, including common clock framework support and we hope to add FDT
> >>> support to the Samsung SoC camera devices during this kernel cycle, based
> >>> on the newly designed media bindings. This is going to be a second
> >>> attempt, after our initial RFC from May [1]. It would still be SoC
> >>> specific implementation, but not soc-camera based.
> >>>
> >>> I wasn't a big fan of this asynchronous sub-devices probing, but it now
> >>> seems to be a most complete solution to me. I think it just need to be
> >>> done right at the v4l2-core so individual drivers don't get complicated
> >>> too much.
> >>
> >> After investigating this some more I think I agree with that. There are some
> >> things where we should probably ask for advice from the i2c subsystem devs,
> >> I'm thinking of putting the driver back into the deferred-probe state in
> >> particular.
> > 
> > We might actually not need that, it might be easier to handle the circular
> > dependency problem from the other end. We could add a way (ioctl, sysfs, ...)
> > to force a V4L2 bridge driver to release its subdevs. Once done, the subdev
> > driver could be unloaded and/or the subdev device unregistered, which would
> > release the resources used by the subdev, such as clocks. The bridge driver
> > could then be unregistered.
> 
> That sounds like an option. Perhaps it could be done by v4l2-core, e.g. a sysfs 
> entry could be registered for a media or video device if driver requests it.
> I'm not sure if we should allow subdevs in "released" state, perhaps it's better
> to just unregister subdevs entirely right away ?

What speaks against holding a clock reference only during streaming, or at 
least between open and close? Wouldn't that solve the problem naturally? 
Yes, after giving up your reference to a clock at close() and re-acquiring 
it at open() you will have to make sure the frequency hasn't change, resp. 
adjust it, but I don't see it as a huge problem, I don't think many users 
on embedded systems will compete for your camera master clock. And if they 
do, you have a different problem, IMHO;-)

Thanks
Guennadi

> >> Creating v4l2-core support for this is crucial as it is quite complex and
> >> without core support this is going to be a nightmare for drivers.
> 
> --
> 
> Regards,
> Sylwester

---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/