Hi Sakari, Thanks for your patch. On 2017-06-29 09:38:53 +0300, Sakari Ailus wrote: > As we begin to add support for systems with media pipelines controlled by > more than one device driver, it is essential that we precisely define the > responsibilities of each component in the stream control and common > practices. > > Specifically, this patch documents two things: > > 1) streaming control is done per sub-device and sub-device drivers > themselves are responsible for streaming setup in upstream sub-devices and > > 2) broken frames should be tolerated at streaming stop. It is the > responsibility of the sub-device driver to stop the transmitter after > itself if it cannot handle broken frames (and it should be probably be > done anyway). > > Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx> > --- > Documentation/media/kapi/v4l2-subdev.rst | 36 ++++++++++++++++++++++++++++++++ > 1 file changed, 36 insertions(+) > > diff --git a/Documentation/media/kapi/v4l2-subdev.rst b/Documentation/media/kapi/v4l2-subdev.rst > index e1f0b72..27e371e 100644 > --- a/Documentation/media/kapi/v4l2-subdev.rst > +++ b/Documentation/media/kapi/v4l2-subdev.rst > @@ -262,6 +262,42 @@ is called. After all subdevices have been located the .complete() callback is > called. When a subdevice is removed from the system the .unbind() method is > called. All three callbacks are optional. > > +Streaming control > +^^^^^^^^^^^^^^^^^ > + > +Starting and stopping the stream are somewhat complex operations that > +often require walking the media graph to enable streaming on > +sub-devices which the pipeline consists of. This involves interaction > +between multiple drivers, sometimes more than two. > + > +The ``.s_stream()`` core op is responsible for starting and stopping .s_stream() is a video or audio op not a core op right? But at the same time it's all part of the v4l2 core :-) Apart from this nit-pick which I'm fine to leave as is at your leisure. Acked-by: Niklas Söderlund <niklas.soderlund+renesas@xxxxxxxxxxxx> > +the stream on the sub-device it is called on. Additionally, if there > +are sub-devices further up in the pipeline, i.e. connected to that > +sub-device's sink pads through enabled links, the sub-device driver > +must call the ``.s_stream()`` core op of all such sub-devices. The > +sub-device driver is thus in control of whether the upstream > +sub-devices start (or stop) streaming before or after the sub-device > +itself is set up for streaming. > + > +.. note:: > + > + As the ``.s_stream()`` callback is called recursively through the > + sub-devices along the pipeline, it is important to keep the > + recursion as short as possible. To this end, drivers are encouraged > + not to internally call ``.s_stream()`` recursively in order to make > + only a single additional recursion per driver in the pipeline. This > + greatly reduces stack usage. > + > +Stopping the transmitter > +^^^^^^^^^^^^^^^^^^^^^^^^ > + > +A transmitter stops sending the stream of images as a result of > +calling the ``.s_stream()`` callback. Some transmitters may stop the > +stream at a frame boundary whereas others stop immediately, > +effectively leaving the current frame unfinished. The receiver driver > +should not make assumptions either way, but function properly in both > +cases. > + > V4L2 sub-device userspace API > ----------------------------- > > -- > 2.7.4 > -- Regards, Niklas Söderlund
