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

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Date:   Fri, 3 Mar 2017 14:47:24 -0800
From:   Steve Longerbeam <slongerbeam@...il.com>
To:     Sakari Ailus <sakari.ailus@....fi>
Cc:     robh+dt@...nel.org, mark.rutland@....com, shawnguo@...nel.org,
        kernel@...gutronix.de, fabio.estevam@....com,
        linux@...linux.org.uk, mchehab@...nel.org, hverkuil@...all.nl,
        nick@...anahar.org, markus.heiser@...marIT.de,
        p.zabel@...gutronix.de, laurent.pinchart+renesas@...asonboard.com,
        bparrot@...com, geert@...ux-m68k.org, arnd@...db.de,
        sudipm.mukherjee@...il.com, minghsiu.tsai@...iatek.com,
        tiffany.lin@...iatek.com, jean-christophe.trotin@...com,
        horms+renesas@...ge.net.au, niklas.soderlund+renesas@...natech.se,
        robert.jarzmik@...e.fr, songjun.wu@...rochip.com,
        andrew-ct.chen@...iatek.com, gregkh@...uxfoundation.org,
        shuah@...nel.org, sakari.ailus@...ux.intel.com, pavel@....cz,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org,
        devel@...verdev.osuosl.org,
        Steve Longerbeam <steve_longerbeam@...tor.com>
Subject: Re: [PATCH v4 14/36] [media] v4l2-mc: add a function to inherit
 controls from a pipeline



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@...tor.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.
>