linux-kernel - Re: [PATCH v5 16/39] [media] v4l2: add a new-frame before end-of-frame event

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Message-ID: <72a06329-7f65-fef9-3153-573d9abb2689@xs4all.nl>
Date:   Fri, 10 Mar 2017 13:07:26 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Steve Longerbeam <slongerbeam@...il.com>, robh+dt@...nel.org,
        mark.rutland@....com, shawnguo@...nel.org, kernel@...gutronix.de,
        fabio.estevam@....com, linux@...linux.org.uk, mchehab@...nel.org,
        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
Cc:     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 v5 16/39] [media] v4l2: add a new-frame before
 end-of-frame event

On 10/03/17 05:52, Steve Longerbeam wrote:
> Add a NEW_FRAME_BEFORE_EOF event to signal that a video capture or
> output device has signaled a new frame is ready before a previous
> frame has completed reception or transmission. This usually indicates
> a DMA read/write channel is having trouble gaining bus access.

This too is a weird event. Based on what you describe this basically means
that the previous frame is incomplete, in which case you would typically
return the buffer with the V4L2_BUF_FLAG_ERROR bit set.

Using an event for this is not a good idea.

Regards,

	Hans

> Signed-off-by: Steve Longerbeam <steve_longerbeam@...tor.com>
> ---
>  Documentation/media/uapi/v4l/vidioc-dqevent.rst | 6 ++++++
>  Documentation/media/videodev2.h.rst.exceptions  | 1 +
>  include/uapi/linux/videodev2.h                  | 1 +
>  3 files changed, 8 insertions(+)
> 
> diff --git a/Documentation/media/uapi/v4l/vidioc-dqevent.rst b/Documentation/media/uapi/v4l/vidioc-dqevent.rst
> index dc77363..54bc7ae 100644
> --- a/Documentation/media/uapi/v4l/vidioc-dqevent.rst
> +++ b/Documentation/media/uapi/v4l/vidioc-dqevent.rst
> @@ -203,6 +203,12 @@ call.
>  	has measured an interval between the reception or transmit
>  	completion of two consecutive frames of video that is outside
>  	the nominal frame interval by some tolerance value.
> +    * - ``V4L2_EVENT_NEW_FRAME_BEFORE_EOF``
> +      - 8
> +      - This event is triggered when the video capture or output device
> +	has signaled a new frame is ready before a previous frame has
> +	completed reception or transmission. This usually indicates a
> +	DMA read/write channel is having trouble gaining bus access.
>      * - ``V4L2_EVENT_PRIVATE_START``
>        - 0x08000000
>        - Base event number for driver-private events.
> diff --git a/Documentation/media/videodev2.h.rst.exceptions b/Documentation/media/videodev2.h.rst.exceptions
> index c7d8fad..be6f332 100644
> --- a/Documentation/media/videodev2.h.rst.exceptions
> +++ b/Documentation/media/videodev2.h.rst.exceptions
> @@ -460,6 +460,7 @@ replace define V4L2_EVENT_FRAME_SYNC event-type
>  replace define V4L2_EVENT_SOURCE_CHANGE event-type
>  replace define V4L2_EVENT_MOTION_DET event-type
>  replace define V4L2_EVENT_FRAME_INTERVAL_ERROR event-type
> +replace define V4L2_EVENT_NEW_FRAME_BEFORE_EOF event-type
>  replace define V4L2_EVENT_PRIVATE_START event-type
>  
>  replace define V4L2_EVENT_CTRL_CH_VALUE ctrl-changes-flags
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index cf5a0d0..f54a82a 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -2132,6 +2132,7 @@ struct v4l2_streamparm {
>  #define V4L2_EVENT_SOURCE_CHANGE		5
>  #define V4L2_EVENT_MOTION_DET			6
>  #define V4L2_EVENT_FRAME_INTERVAL_ERROR		7
> +#define V4L2_EVENT_NEW_FRAME_BEFORE_EOF		8
>  #define V4L2_EVENT_PRIVATE_START		0x08000000
>  
>  /* Payload for V4L2_EVENT_VSYNC */
>