Re: [PATCH v4 14/36] [media] v4l2-mc: add a function to inherit controls from a pipeline

[Steve Longerbeam <slongerbeam@gmail.com>]

On 03/03/2017 03:06 PM, Russell King - ARM Linux wrote:
> On Thu, Mar 02, 2017 at 06:02:57PM +0200, Sakari Ailus wrote:
>> Hi Steve,
>>
>> On Wed, Feb 15, 2017 at 06:19:16PM -0800, Steve Longerbeam wrote:
>>> v4l2_pipeline_inherit_controls() will add the v4l2 controls from
>>> all subdev entities in a pipeline to a given video device.
>>>
>>> Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
>>> ---
>>>  drivers/media/v4l2-core/v4l2-mc.c | 48 +++++++++++++++++++++++++++++++++++++++
>>>  include/media/v4l2-mc.h           | 25 ++++++++++++++++++++
>>>  2 files changed, 73 insertions(+)
>>>
>>> diff --git a/drivers/media/v4l2-core/v4l2-mc.c b/drivers/media/v4l2-core/v4l2-mc.c
>>> index 303980b..09d4d97 100644
>>> --- a/drivers/media/v4l2-core/v4l2-mc.c
>>> +++ b/drivers/media/v4l2-core/v4l2-mc.c
>>> @@ -22,6 +22,7 @@
>>>  #include <linux/usb.h>
>>>  #include <media/media-device.h>
>>>  #include <media/media-entity.h>
>>> +#include <media/v4l2-ctrls.h>
>>>  #include <media/v4l2-fh.h>
>>>  #include <media/v4l2-mc.h>
>>>  #include <media/v4l2-subdev.h>
>>> @@ -238,6 +239,53 @@ int v4l_vb2q_enable_media_source(struct vb2_queue *q)
>>>  }
>>>  EXPORT_SYMBOL_GPL(v4l_vb2q_enable_media_source);
>>>
>>> +int __v4l2_pipeline_inherit_controls(struct video_device *vfd,
>>> +				     struct media_entity *start_entity)
>>
>> I have a few concerns / questions:
>>
>> - What's the purpose of this patch? Why not to access the sub-device node
>>   directly?
>
> What tools are in existance _today_ to provide access to these controls
> via the sub-device nodes?
>
> v4l-tools doesn't last time I looked - in fact, the only tool in v4l-tools
> which is capable of accessing the subdevices is media-ctl, and that only
> provides functionality for configuring the pipeline.
>
> So, pointing people at vapourware userspace is really quite rediculous.


Hi Russell,

Yes, that's a big reason why I added this capability. The v4l2-ctl
tool won't accept subdev nodes, although Philipp Zabel has a quick hack
to get around this (ignore return code from VIDIOC_QUERYCAP).


>
> The established way to control video capture is through the main video
> capture device, not through the sub-devices.  Yes, the controls are
> exposed through sub-devices too, but that does not mean that is the
> correct way to access them.
>
> The v4l2 documentation (Documentation/media/kapi/v4l2-controls.rst)
> even disagrees with your statements.  That talks about control
> inheritence from sub-devices to the main video device, and the core
> v4l2 code provides _automatic_ support for this - see
> v4l2_device_register_subdev():
>
>         /* This just returns 0 if either of the two args is NULL */
>         err = v4l2_ctrl_add_handler(v4l2_dev->ctrl_handler, sd->ctrl_handler, NULL);
>
> which merges the subdev's controls into the main device's control
> handler.

Actually v4l2_dev->ctrl_handler is not of much use to me. This will
compose a list of controls from all registered subdevs, i.e. _all
possible controls_.

What v4l2_pipeline_inherit_controls() does is compose a list of
controls that are reachable and available in the currently configured
pipeline.

Steve

>
> So, (a) I don't think Steve needs to add this code, and (b) I think
> your statements about not inheriting controls goes against the
> documentation and API compatibility with _existing_ applications,
> and ultimately hurts the user experience, since there's nothing
> existing today to support what you're suggesting in userspace.
>