[PATCH v4 2/5] media: v4l2-common: add helper functions to call s_stream() callbacks

[Hans Verkuil]

On 22/05/2020 09:55, Dafna Hirschfeld wrote:
> From: Helen Koike <helen.koike at collabora.com>
> 
> Add v4l2_pipeline_stream_{enable,disable} helper functions to iterate
> through the subdevices in a given stream (i.e following links from sink
> to source) and call .s_stream() callback.
> 
> If .s_stream(true) fails, call .s_stream(false) in the reverse order.
> 
> Signed-off-by: Helen Koike <helen.koike at collabora.com>
> Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld at collabora.com>
> ---
>  drivers/media/v4l2-core/v4l2-common.c | 99 +++++++++++++++++++++++++++
>  include/media/v4l2-common.h           | 39 +++++++++++
>  2 files changed, 138 insertions(+)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-common.c b/drivers/media/v4l2-core/v4l2-common.c
> index 9e8eb45a5b03..734db2bf5ca9 100644
> --- a/drivers/media/v4l2-core/v4l2-common.c
> +++ b/drivers/media/v4l2-core/v4l2-common.c
> @@ -442,3 +442,102 @@ int v4l2_fill_pixfmt(struct v4l2_pix_format *pixfmt, u32 pixelformat,
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(v4l2_fill_pixfmt);
> +
> +#if defined(CONFIG_MEDIA_CONTROLLER)
> +
> +/*
> + * v4l2_pipeline_subdevs_get - Assemble a list of subdevices in a pipeline
> + * @subdevs: the array to be filled.
> + * @max_size: max number of elements that can fit in subdevs array.
> + *
> + * Walk from a video node, following links from sink to source and fill the
> + * array with subdevices in the path.
> + * The walker performs a depth-first traversal, which means that, in a topology
> + * sd1->sd2->sd3->vdev, sd1 will be the first element placed in the array.
> + *
> + * Return the number of subdevices filled in the array.
> + */
> +static int v4l2_pipeline_subdevs_get(struct video_device *vdev,
> +				     struct v4l2_subdev **subdevs,
> +				     unsigned int max_size)
> +{
> +	struct media_entity *entity = &vdev->entity;
> +	struct media_device *mdev = entity->graph_obj.mdev;
> +	struct media_graph graph;
> +	int idx = 0;
> +	int ret;
> +
> +	ret = media_graph_walk_init(&graph, mdev);
> +	if (ret)
> +		return ret;
> +
> +	media_graph_walk_start(&graph, entity);
> +
> +	while ((entity = media_graph_walk_next_stream(&graph))) {
> +		if (!is_media_entity_v4l2_subdev(entity))
> +			continue;
> +		if (WARN_ON(idx >= max_size))
> +			return -EINVAL;
> +		subdevs[idx++] = media_entity_to_v4l2_subdev(entity);
> +	}
> +
> +	media_graph_walk_cleanup(&graph);
> +
> +	return idx;
> +}
> +
> +static void v4l2_subdevs_stream_disable(struct v4l2_subdev **subdevs, int size)
> +{
> +	int i;
> +
> +	for (i = 0; i < size; i++) {
> +		struct v4l2_subdev *sd = subdevs[i];
> +
> +		dev_dbg(sd->dev,
> +			"disabling stream for '%s'\n", sd->entity.name);
> +		v4l2_subdev_call(sd, video, s_stream, false);
> +	}
> +}
> +
> +__must_check int v4l2_pipeline_stream_enable(struct video_device *vdev)
> +{
> +	struct media_device *mdev = vdev->entity.graph_obj.mdev;
> +	struct v4l2_subdev *subdevs[MEDIA_ENTITY_ENUM_MAX_DEPTH];
> +	struct v4l2_subdev *sd;
> +	int i, size, ret;
> +
> +	size = v4l2_pipeline_subdevs_get(vdev, subdevs, ARRAY_SIZE(subdevs));
> +	if (size <= 0)
> +		return size;
> +
> +	/* Call s_stream() in reverse order to enable sensors last */
> +	for (i = size - 1; i >= 0; i--) {
> +		sd = subdevs[i];
> +		dev_dbg(mdev->dev,
> +			"enabling stream for '%s'\n", sd->entity.name);
> +		ret = v4l2_subdev_call(sd, video, s_stream, true);
> +		if (ret && ret != -ENOIOCTLCMD) {
> +			v4l2_subdevs_stream_disable(subdevs + i + 1,
> +						    size - i - 1);
> +			return ret;
> +		}
> +	}
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(v4l2_pipeline_stream_enable);
> +
> +int v4l2_pipeline_stream_disable(struct video_device *vdev)
> +{
> +	struct v4l2_subdev *subdevs[MEDIA_ENTITY_ENUM_MAX_DEPTH];
> +	int size;
> +
> +	size = v4l2_pipeline_subdevs_get(vdev, subdevs, ARRAY_SIZE(subdevs));
> +	if (size < 0)
> +		return size;
> +
> +	v4l2_subdevs_stream_disable(subdevs, size);
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(v4l2_pipeline_stream_disable);
> +
> +#endif /* CONFIG_MEDIA_CONTROLLER */
> diff --git a/include/media/v4l2-common.h b/include/media/v4l2-common.h
> index 150ee16ebd81..a278bcf3c5bc 100644
> --- a/include/media/v4l2-common.h
> +++ b/include/media/v4l2-common.h
> @@ -539,4 +539,43 @@ static inline void v4l2_buffer_set_timestamp(struct v4l2_buffer *buf,
>  	buf->timestamp.tv_usec = ts.tv_nsec / NSEC_PER_USEC;
>  }
>  
> +#if defined(CONFIG_MEDIA_CONTROLLER)
> +
> +/**
> + * v4l2_pipeline_stream_enable - Call .s_stream(true) callbacks in the stream
> + * @vdev: Starting video device.
> + *
> + * Call .s_stream(true) callback in all the subdevices participating in the
> + * stream, i.e. following links from sink to source.
> + *
> + * .s_stream(true) is also called from sink to source, i.e. in a topology
> + * sd1->sd2->sd3->vdev, .s_stream(true) of sd3 is called first.
> + *
> + * Calls to this function can be nested, in which case the same number of

Since there is no refcounting (as opposed to v3) you cannot nest these calls,
so the comments for both these functions should be updated.

> + * v4l2_pipeline_stream_disable() calls will be required to disable streaming
> + * through subdevices in the pipeline.
> + * The  pipeline pointer must be identical for all nested calls to
> + * v4l2_pipeline_stream_enable().
> + */
> +__must_check int v4l2_pipeline_stream_enable(struct video_device *vdev);
> +
> +/**
> + * v4l2_pipeline_stream_disable - Call .s_stream(false) callbacks in the stream
> + * @vdev: Starting video device.
> + *
> + * Call .s_stream(false) callback in all the subdevices participating in the
> + * stream, i.e. following links from sink to source.
> + *
> + * s_stream(false) is called in a reverse order from
> + * v4l2_pipeline_stream_enable(), i.e. in a topology sd1->sd2->sd3->vdev,
> + * .s_stream(false) of sd1 is called first.
> + *
> + * If multiple calls to v4l2_pipeline_stream_enable() have been made, the same
> + * number of calls to this function are required to disable streaming through
> + * subdevices in the pipeline.
> + */
> +int v4l2_pipeline_stream_disable(struct video_device *vdev);

I'm inclined to make this a void function. Typically a 'disable' function should
always succeed because there really is not much you can do if it fails.

If v4l2_pipeline_subdevs_get() fails, then just WARN_ON. This really should not
happen.

Another problem (Tomasz described that in one of his replies) is with drivers like
adv748x that manage the rest of the pipeline on their own.

The problem is that (AFAIK) you can't see this when walking the graph. What you
would like to know is if an entity controls the upstream entities by having that
entity set a flag or something. Then these pipeline functions would stop at that
entity and let that entity do the work for the upstream entities.

If it is not too much work to support this, then it would be nice to have this.
If it is a lot of work, then it should be documented that these functions are
likely not suitable for such pipelines.

I was also wondering how this function relates to media_pipeline_start(). Which
function should be called first (I think media_pipeline_start() has to come before
v4l2_pipeline_stream_enable(), although patch 4/5 appears to do the opposite).
And I was also wondering if the stream_count (incremented by media_pipeline_start())
is something that v4l2_pipeline_stream_enable/disable could use: i.e. s_stream only
needs to be called if stream_count == 1.

In fact, perhaps v4l2_pipeline_stream_enable/disable should call __media_pipeline_start/stop?

Regards,

	Hans

> +
> +#endif /* CONFIG_MEDIA_CONTROLLER */
> +
>  #endif /* V4L2_COMMON_H_ */
>