Re: [PATCH 07/14] media: soc-camera: support deferred probing of clients

[Guennadi Liakhovetski <g.liakhovetski@gmx.de>]

On Wed, 10 Apr 2013, Barry Song wrote:

> Hi Guennadia,

There's a typo above.

> Thanks!
> 
> 2013/4/10 Guennadi Liakhovetski <g.liakhovetski@gmx.de>:
> > Hi Barry
> >
> > On Wed, 10 Apr 2013, Barry Song wrote:
> >
> >> Hi Guennadia,
> >>
> >> 2012/9/27 Guennadi Liakhovetski <g.liakhovetski@gmx.de>:
> >> > Currently soc-camera doesn't work with independently registered I2C client
> >> > devices, it has to register them itself. This patch adds support for such
> >> > configurations, in which case client drivers have to request deferred
> >> > probing until their host becomes available and enables the data interface.
> >> >
> >> > Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
> >> > ---
> >>
> >> it seems deferred probing for i2c camera sensors is a more workaround
> >> than a solution.
> >> currently,  soc-camera-pdrv is the manager of the whole initilization
> >> flow. it all requires the host/client registerred and initilized
> >> synchronously. so that results in strange things like that we fill a
> >> i2c_board_info structure in arch/arm/mach-xxx but we never call
> >> anything like i2c_new_device() to add the i2c client in mach. because
> >> we need to do that in the soc-camera-pdrv driver to make all things
> >> happen orderly.
> >>
> >> but now after we move to DT, all i2c device will be registerred
> >> automatically by of_i2c_register_devices() in i2c_host 's probe, that
> >> makes the problem much worse and very urgent to get fixed.
> >>
> >> returning DEFERRED_PROBE error until getting the private data filled
> >> by the manager,
> >
> > This hasn't been the case since several versions of these patches. We no
> > longer use private data to decide whether subdevices can probe
> > successfully or have to defer probing.
> 
> sorry for missing.  i will refer to your newer versions.
> 
> >
> >> indirectly, makes the things seem to be asynchronous,
> >> but essentially it is still synchronous because the overall timing
> >> line is still controller by soc-camera-pdrv.
> >
> > It isn't. If your subdevice driver doesn't have any dependencies, like
> > e.g. sh_mobile_csi2.c, it will probe asynchronously whenever it's loaded.
> > It is the task of a bridge driver, in our case of the soc-camera core, to
> > register notifiers and a list of expected subdevices with the v4l2-async
> > subsystem. As subdevices complete their probing they signal that to the
> > v4l2-async too, which then calls bridge's notifiers, which then can build
> > the pipeline.
> 
> it seems we didn't describle my idea clearly in the last mail. i
> actually mean we don't need that if we put the pipeline building to
> the last stage after all things have been placed there.
> 
> >
> >> what about another possible way:
> >> we let all host and i2c client driver probed in any order,
> >
> > This cannot work, because some I2C devices, e.g. sensors, need a clock
> > signal from the camera interface to probe. Before the bridge driver has
> > completed its probing and registered a suitable clock source with the
> > v4l2-clk framework, sensors cannot be probed. And no, we don't want to
> > fake successful probing without actually being able to talk to the
> > hardware.
> 
> i'd say same dependency also exists on ASoC.  a "fake" successful
> probing doesn't mean it should really begin to work if there is no
> external trigger source.  ASoC has successfully done that by a machine
> driver to connect all DAI.
> a way is we put all things ready in their places, finally we connect
> them together and launch the whole hardware flow.
> 
> anyway, if you have maken the things work by some simple hacking and
> that means minimial changes to current soc-camera, i think we can
> follow.

If you want to volunteer to step up as a new soc-camera maintainer to 
replace my simple hacking with your comprehencive and advanced designs - 
feel free, I'll ack straight away.

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