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

[Guennadi Liakhovetski]

Hi Barry

On Wed, 10 Apr 2013, Barry Song wrote:

> Hi Guennadia,
> 
> 2012/9/27 Guennadi Liakhovetski <g.liakhovetski at 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 at 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.

> 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.

> 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.

Thanks
Guennadi

> but let the
> final soc-camera-pdrv is the connection of all things. the situation
> of soc_camera is very similar with ALSA SoC. it turns out ASoC has
> done that very well.
> 
> -barry
> 

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