Re: [PATCH 5/5] media/Documentation: New flag EXECUTE_ON_WRITE

[Hans Verkuil <hverkuil@xs4all.nl>]

On 03/19/2015 04:21 PM, Ricardo Ribalda Delgado wrote:
> Document new flag V4L2_CTRL_FLAG_EXECUTE_ON_WRITE, and the new behavior
> of CH_VALUE event on VOLATILE controls.
> 
> Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
> ---
>  Documentation/DocBook/media/v4l/vidioc-dqevent.xml   |  7 ++++---
>  Documentation/DocBook/media/v4l/vidioc-queryctrl.xml | 15 +++++++++++++--
>  Documentation/video4linux/v4l2-controls.txt          |  4 +++-
>  3 files changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/DocBook/media/v4l/vidioc-dqevent.xml b/Documentation/DocBook/media/v4l/vidioc-dqevent.xml
> index b036f89..38d907e 100644
> --- a/Documentation/DocBook/media/v4l/vidioc-dqevent.xml
> +++ b/Documentation/DocBook/media/v4l/vidioc-dqevent.xml
> @@ -318,9 +318,10 @@
>  	    <entry><constant>V4L2_EVENT_CTRL_CH_VALUE</constant></entry>
>  	    <entry>0x0001</entry>
>  	    <entry>This control event was triggered because the value of the control
> -		changed. Special case: if a button control is pressed, then this
> -		event is sent as well, even though there is not explicit value
> -		associated with a button control.</entry>
> +		changed. Special cases: Volatile controls do no generate this event;
> +		If a button control is pressed, then this event is sent as well,

Not just a button control, but any non-volatile EXECUTE_ON_WRITE control.

> +		even though there is not explicit value associated with a button
> +		control.</entry>
>  	  </row>
>  	  <row>
>  	    <entry><constant>V4L2_EVENT_CTRL_CH_FLAGS</constant></entry>
> diff --git a/Documentation/DocBook/media/v4l/vidioc-queryctrl.xml b/Documentation/DocBook/media/v4l/vidioc-queryctrl.xml
> index 2bd98fd..d389292 100644
> --- a/Documentation/DocBook/media/v4l/vidioc-queryctrl.xml
> +++ b/Documentation/DocBook/media/v4l/vidioc-queryctrl.xml
> @@ -599,8 +599,11 @@ writing a value will cause the device to carry out a given action
>  	    <entry>This control is volatile, which means that the value of the control
>  changes continuously. A typical example would be the current gain value if the device
>  is in auto-gain mode. In such a case the hardware calculates the gain value based on
> -the lighting conditions which can change over time. Note that setting a new value for
> -a volatile control will have no effect. The new value will just be ignored.</entry>
> +the lighting conditions which can change over time. Another example would be an error
> +flag (missed trigger, invalid voltage on the sensor). In those situations the user
> +could write to the control to acknowledge the error, but that write will never
> +generate a <constant>V4L2_EVENT_CTRL_CH_VALUE</constant> event and the flag
> +<constant>V4L2_CTRL_FLAG_EXECUTE_ON_WRITE</constant> must be set.<entry>

I am not really happy with this change. In particular this dropped the "Note that setting
a new value for a volatile control will have no effect. The new value will just be ignored.".

This is still true as long as EXECUTE_ON_WRITE isn't set.

I would keep the original sentence, but add something like: "will have no effect and
no <constant>V4L2_EVENT_CTRL_CH_VALUE</constant> will be sent, unless
the <constant>V4L2_CTRL_FLAG_EXECUTE_ON_WRITE</constant> flag (see below) is also set.
Otherwise the new value will just be ignored."

And just skip the trigger example since that's already described in the EXECUTE_ON_WRITE
documentation.

>  	  </row>
>  	  <row>
>  	    <entry><constant>V4L2_CTRL_FLAG_HAS_PAYLOAD</constant></entry>
> @@ -610,6 +613,14 @@ using one of the pointer fields of &v4l2-ext-control;. This flag is set for cont
>  that are an array, string, or have a compound type. In all cases you have to set a
>  pointer to memory containing the payload of the control.</entry>
>  	  </row>
> +	  <row>
> +	    <entry><constant>V4L2_CTRL_FLAG_EXECUTE_ON_WRITE</constant></entry>
> +	    <entry>0x0200</entry>
> +	    <entry>The value provided to the control will be propagated to the driver
> +even if remains constant. This is required when the controls represents an action

s/controls/control/

> +on the hardware. For example: clearing an error flag or triggering the flash. All the
> +controls of the type <constant>V4L2_CTRL_TYPE_BUTTON</constant> have this flag set.</entry>
> +	  </row>
>  	</tbody>
>        </tgroup>
>      </table>
> diff --git a/Documentation/video4linux/v4l2-controls.txt b/Documentation/video4linux/v4l2-controls.txt
> index 0f84ce8..5517db6 100644
> --- a/Documentation/video4linux/v4l2-controls.txt
> +++ b/Documentation/video4linux/v4l2-controls.txt
> @@ -344,7 +344,9 @@ implement g_volatile_ctrl like this:
>  	}
>  
>  Note that you use the 'new value' union as well in g_volatile_ctrl. In general
> -controls that need to implement g_volatile_ctrl are read-only controls.
> +controls that need to implement g_volatile_ctrl are read-only controls. If they
> +are not, a V4L2_EVENT_CTRL_CH_VALUE will not be generated when the control
> +changes.
>  
>  To mark a control as volatile you have to set V4L2_CTRL_FLAG_VOLATILE:
>  
> 

Regards,

	Hans