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

[Steve Longerbeam]

On 03/03/2017 11:17 AM, Sakari Ailus wrote:
> Hi Steve,
>
> On Thu, Mar 02, 2017 at 06:12:43PM -0800, Steve Longerbeam wrote:
>>
>>
>> On 03/02/2017 03:48 PM, Steve Longerbeam wrote:
>>>
>>>
>>> On 03/02/2017 08:02 AM, 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 at 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?
>>>
>>>
>>> I don't really understand what you are trying to say.<snip>
>>>
>>
>> Actually I think I understand what you mean now. Yes, the user can
>> always access a subdev's control directly from its /dev/v4l-subdevXX.
>> I'm only providing this feature as a convenience to the user, so that
>> all controls in a pipeline can be accessed from one place, i.e. the
>> main capture device node.
>
> No other MC based V4L2 driver does this. You'd be creating device specific
> behaviour that differs from what the rest of the drivers do. The purpose of
> MC is to provide the user with knowledge of what devices are there, and the
> V4L2 sub-devices interface is used to access them in this case.

Well, again, I don't mind removing this. As I said it is only a
convenience (although quite a nice one in my opinion). I'd like
to hear from others whether this is worth keeping though.


>
> It does matter where a control is implemented, too. If the pipeline contains
> multiple sub-devices that implement the same control, only one of them may
> be accessed. The driver calling the function (or even less the function)
> would not know which one of them should be ignored.

Yes the pipeline should not have any duplicate controls. On imx-media no
pipelines that can be configured have duplicate controls.

Steve

>
> If you need such functionality, it should be implemented in the user space
> instead.
>