Re: [RFC PATCH 1/4] v4l: Add V4L2_CID_PRESET_WHITE_BALANCE menu control

[Sylwester Nawrocki <snjw23@gmail.com>]

Hi HeungJun,

On 01/02/2012 05:38 AM, Kim, Heungjun wrote:
> On 2011년 12월 30일 19:30, Sylwester Nawrocki wrote:
>> On 12/29/2011 06:08 AM, HeungJun, Kim wrote:
>>> I guess the WBP menu controls of pwc driver is probably defined in the other
>>> headers, for users being well known the PWC hardware. So it should be managed
>>> separately to videodev2.h. Is it right? Even if the way might be slightly
>>> different, it can't avoid to be "managed separately".
>>>
>>> It means the users being not well known the specific hardware like PWC,
>>> have difficulty to use that driver well.
>>> And, at least, It doesn't looks generic API for me.
>>> In this case, the unfamiliar user with such unique hardware, can use
>>> whatever he wants to use finally, after finding&  looking around the headers.
>> Applications can query drivers for supported controls and populate user control
>> panels dynamically, based on information from VIDIOC_QUERYCTRL and
>> VIDIOC_QUERYMENU
>> ioctls. Not needing to rely on menu items definition in videodev2.h.
>> I had a feeling you weren't considering such case. :)
> You're right in that meaning. And it might be a good point.
> But, I think these 2 ioctl can not handle about this issue.
> 
> Before using VIDIOC_QUERYCTRL and VIDIOC_QUERYMENU, the user should know which
> CID name
> is used in the videodev2.h, and anyway it can not be avoidable the user find out
> this name in it. :)

Yes, in many cases it is required to know the CID in advance, however it is not
mandatory for all applications.

> At least I've seen nobody makes the application just to open(), queryctrl(),
> querymenu(), and close(),
> only for scanning the specific control is existed or not.
> Until now, I have known these 2 ioctl is generally used for formating the UI
> componets like button, menu, and etc, on the screen.

Yup.

> So, it's safe to say that the user who knows that specific control is also know
> the CID name,
> the user knows such specific controls don't need even VIDIOC_QUERYCTRL and
> VIDIOC_QUERYMENU.

I respectfully disagree. Properly written applications must use VIDIOC_QUERYCTRL/
VIDIOC_QUERYMENU ioctls, as many v4l2 controls have now driver-specific value
range.

Please see this application for instance [1], it doesn't hard code any control
IDs in it, it only uses V4L2_CID_BASE, V4L2_CID_PRIVATE_BASE and V4L2_CID_LASTP1.

Yet, it can handle any control, as long as it supports the control's type.

> And IMHO, this is not related about pulling out the hidden(?) controls generally
> used in the camera,
> on the videodev2.h. I think it's only generic defined in videodev2.h.
> 
> I really had wondered why the controls I thought very general for camera is in
> hidden(?) the specific driver,
> not in the videodev2.h. It was just start to consider this issues.

I think you misunderstood me, I didn't mean to force anyone to use private
controls for common features. :)

> Regards,
> Heungjun Kim
> 
>> Perhaps it's uncommon in embedded systems though.

[1] http://sourceforge.net/projects/v4l2ucp/files/v4l2ucp/2.0/

-- 

Regards,
Sylwester