Re: uvcvideo: logitech C920 resets controls during VIDIOC_STREAMON

[William Manley <will@williammanley.net>]

On 06/03/14 12:09, Paulo Assis wrote:
> Hi,
> 
> 2014-03-05 23:01 GMT+00:00 William Manley <will@williammanley.net>:
>> Hi All
>>
>> I've been attempting to use the Logitech C920 with the uvcvideo driver.
>>  I set the controls with v4l2-ctl but some of them change during
>> VIDIOC_STREAMON.  My understanding is that the values of controls should
>> be preserved.
>>

[snip]

>> The camera does
>> have a mode where it would change by itself but that is disabled
>> (exposure_auto=1).
> 
> This alone doesn't guarantee that exposure absolute is untouched.
> To do so you need to set V4L2_CID_EXPOSURE_AUTO_PRIORITY to 0 and
> V4L2_CID_EXPOSURE_AUTO to V4L2_EXPOSURE_MANUAL

The behaviour is the same whether exposure_auto_priority is set to 1 or
0.  I don't think I've explained myself clearly - the exposure time does
not change by itself, apart from during VIDIOC_STREAMON:

* When I'm streaming video from the camera it's constant.
* When no data is coming from the camera it's constant

It's only modified during STREAMON and when I explicitly set it with
v4l2-ctl.

This doesn't seem like correct behaviour as it breaks the use-case of
setting the controls as you want before starting streaming.  My
workaround is to reset all the controls after calling VIDIOC_STREAMON
but this is ugly and you get a few frames at the beginning of the video
stream where the settings are set correctly.

Thanks

Will