Re: [GIT PULL for 4.14] Stream control documentation

[Mauro Carvalho Chehab <mchehab@s-opensource.com>]

Em Wed, 9 Aug 2017 11:03:40 +0300
Sakari Ailus <sakari.ailus@iki.fi> escreveu:

> Hi Mauro,
> 
> Add stream control documentation.
> 
> We have recently added support for hardware that makes it possible to have
> pipelines that are controlled by more than two drivers. This necessitates
> documenting V4L2 stream control behaviour for such pipelines.

Perhaps I missed this one, but I'm not seeing any e-mail with
	"docs-rst: media: Document s_stream() video op usage"

Please always submit patches via e-mail too, as it makes easier for
us to comment/review when needed.

In any case, I'm appending the patch contents here. I'll reply to it
on a next e-mail.

> From ef8e5d20b45b05290c56450d2130a0dc3c021c5a Mon Sep 17 00:00:00 2001
> From: Sakari Ailus <sakari.ailus@linux.intel.com>
> Date: Thu, 9 Mar 2017 15:07:57 +0200
> Subject: docs-rst: media: Document s_stream() video op usage
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> Cc: Linux Media Mailing List <linux-media@vger.kernel.org>,
>     Mauro Carvalho Chehab <mchehab@infradead.org>
> 
> 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@linux.intel.com>
> Acked-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
> ---
>  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 e1f0b726e438..100ffc783f72 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()`` op in :c:type:`v4l2_subdev_video_ops` is responsible
> +for starting and stopping 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()`` video 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
>  -----------------------------
>  
> -- 
> 2.13.3
> 


Thanks,
Mauro