Re: uvcvideo: logitech C920 resets controls during VIDIOC_STREAMON

[William Manley <will@williammanley.net>]

On 06/03/14 13:04, William Manley wrote:
> 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.

It would be really useful for me to hear definitively if this is
expected behaviour WRT the v4l2 API (in which case I'll fix the bug in
GStreamer) or a bug in the C920/uvcvideo driver (in which case I'll work
further to fix the bug in v4l).

To me it feels like it should be fixed here but I'd like to know so I
don't waste my time coming up with patches.

Thanks

Will