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Subject: Re: [PATCH v3 4/9] media: vimc: Separate starting stream from
 pipeline initialisation
To:     Kaaira Gupta <kgupta@es.iitr.ac.in>
Cc:     Helen Koike <helen.koike@collabora.com>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        linux-media@vger.kernel.org, linux-kernel@vger.kernel.org,
        Kieran Bingham <kieran.bingham@ideasonboard.com>
References: <20200819180442.11630-1-kgupta@es.iitr.ac.in>
 <20200819180442.11630-5-kgupta@es.iitr.ac.in>
 <eb5d4259-fe77-b4f2-1e62-0f846420b7c2@collabora.com>
 <20200821210123.GA28410@kaaira-HP-Pavilion-Notebook>
From:   Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Message-ID: <b3e20f32-64b9-778b-fa2f-f17b0c1bcc72@collabora.com>
Date:   Fri, 28 Aug 2020 22:37:14 +0200
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Hi,

Am 21.08.20 um 23:01 schrieb Kaaira Gupta:
> Hi,
> 
> On Fri, Aug 21, 2020 at 05:11:23PM +0200, Dafna Hirschfeld wrote:
>>
>>
>> Am 19.08.20 um 20:04 schrieb Kaaira Gupta:
>>> Separate the process of initialising pipeline array from starting
>>> streaming for all entities in path of a stream. This is needed because
>>> multiple streams can stream, but only one pipeline object is needed.
>>>
>>> Process frames only for those entities in a pipeline which are
>>> streaming. This is known through their use counts.
>>>
>>> Signed-off-by: Kaaira Gupta <kgupta@es.iitr.ac.in>
>>> ---
>>>    .../media/test-drivers/vimc/vimc-streamer.c   | 95 ++++++++++++++++---
>>>    1 file changed, 83 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/drivers/media/test-drivers/vimc/vimc-streamer.c b/drivers/media/test-drivers/vimc/vimc-streamer.c
>>> index c1644d69686d..cc40ecabe95a 100644
>>> --- a/drivers/media/test-drivers/vimc/vimc-streamer.c
>>> +++ b/drivers/media/test-drivers/vimc/vimc-streamer.c
>>> @@ -40,33 +40,30 @@ static void vimc_streamer_pipeline_terminate(struct vimc_stream *stream)
>>>    }
>>>    /**
>>> - * vimc_streamer_pipeline_init - Initializes the stream structure
>>> + * vimc_streamer_stream_start - Starts streaming for all entities
>>> + * in a stream
>>>     *
>>> - * @stream: the pointer to the stream structure to be initialized
>>>     * @ved:    the pointer to the vimc entity initializing the stream
>>>     *
>>> - * Initializes the stream structure. Walks through the entity graph to
>>> - * construct the pipeline used later on the streamer thread.
>>> - * Calls vimc_streamer_s_stream() to enable stream in all entities of
>>> - * the pipeline.
>>> + * Walks through the entity graph to call vimc_streamer_s_stream()
>>> + * to enable stream in all entities in path of a stream.
>>>     *
>>>     * Return: 0 if success, error code otherwise.
>>>     */
>>> -static int vimc_streamer_pipeline_init(struct vimc_stream *stream,
>>> -				       struct vimc_ent_device *ved)
>>> +static int vimc_streamer_stream_start(struct vimc_stream *stream,
>>> +				      struct vimc_ent_device *ved)
>>>    {
>>>    	struct media_entity *entity;
>>>    	struct video_device *vdev;
>>>    	struct v4l2_subdev *sd;
>>> +	int stream_size = 0;
>>>    	int ret = 0;
>>> -	stream->pipe_size = 0;
>>> -	while (stream->pipe_size < VIMC_STREAMER_PIPELINE_MAX_SIZE) {
>>> +	while (stream_size < VIMC_STREAMER_PIPELINE_MAX_SIZE) {
>>>    		if (!ved) {
>>>    			vimc_streamer_pipeline_terminate(stream);
>>>    			return -EINVAL;
>>>    		}
>>> -		stream->ved_pipeline[stream->pipe_size++] = ved;
>>>    		if (is_media_entity_v4l2_subdev(ved->ent)) {
>>>    			sd = media_entity_to_v4l2_subdev(ved->ent);
>>> @@ -104,6 +101,73 @@ static int vimc_streamer_pipeline_init(struct vimc_stream *stream,
>>>    					    entity);
>>>    			ved = video_get_drvdata(vdev);
>>>    		}
>>> +		stream_size++;
>>> +	}
>>> +
>>> +	vimc_streamer_pipeline_terminate(stream);
>>> +	return -EINVAL;
>>> +}
>>> +
>>> +/**
>>> + * vimc_streamer_pipeline_init - Initialises pipeline and pipe size
>>> + *
>>> + * @stream: the pointer to the stream structure
>>> + * @ved:    the pointer to the vimc entity initializing the stream pipeline
>>> + *
>>> + * Walks through the entity graph to initialise ved_pipeline and updates
>>> + * pipe_size too.
>>> + *
>>> + * Return: 0 if success, error code otherwise.
>>> + */
>>> +static int vimc_streamer_pipeline_init(struct vimc_stream *stream,
>>> +				       struct vimc_ent_device *ved)
>>> +{
>>> +	struct media_entity *entity;
>>> +	struct media_device *mdev;
>>> +	struct media_graph graph;
>>> +	struct video_device *vdev;
>>> +	struct v4l2_subdev *sd;
>>> +	int ret;
>>> +
>>> +	entity = ved->ent;
>>> +	mdev = entity->graph_obj.mdev;
>>> +
>>> +	ret = media_graph_walk_init(&graph, mdev);
>>> +	if (ret)
>>> +		return ret;
>>> +
>>> +	media_graph_walk_start(&graph, entity);
>>> +
>>> +	/*
>>> +	 * Start pipeline array initialisation from RAW Capture only to get
>>> +	 * entities in the correct order of their frame processing.
>>> +	 */
>>> +	if (!strncmp(entity->name, "RGB", 3)) {
>>
>> I don't understand this condition, way is it good for?
> 
> I have explained that later in the reply
> 
>>
>> I think the function should be generic and not assume names of entities
>> or specific topology.
> 
> It doesn't assume the topology, rather it is in place just to make sure
> that the entities in ved_pipeline are in correct order.
> 
>>
>>
>>> +		entity = media_graph_walk_next(&graph);
>>> +		mdev = entity->graph_obj.mdev;
>>> +		media_graph_walk_cleanup(&graph);
>>> +
>>> +		ret = media_graph_walk_init(&graph, mdev);
>>> +		if (ret)
>>> +			return ret;
>>> +		media_graph_walk_start(&graph, entity);

Hi, can you explain what this code does?
Why does it start the search in the next entity?

>>> +	}
>>> +
>>> +	while (stream->pipe_size < VIMC_STREAMER_PIPELINE_MAX_SIZE) {
>>> +		if (is_media_entity_v4l2_subdev(entity)) {
>>> +			sd = media_entity_to_v4l2_subdev(entity);
>>> +			ved = v4l2_get_subdevdata(sd);
>>> +		} else {
>>> +			vdev = container_of(entity, struct video_device, entity);
>>> +			ved = video_get_drvdata(vdev);
>>> +		}
>>> +		stream->ved_pipeline[stream->pipe_size++] = ved;
>>> +		entity = media_graph_walk_next(&graph);
>>> +
>>> +		if (!strcmp(entity->name, stream->ved_pipeline[0]->ent->name)) {
>>
>> I also don't understand this condition
>>
>>> +			media_graph_walk_cleanup(&graph);
>>> +			return 0;
>>> +		}
>>>    	}
>>
>> It is not clear what this function does, it looks like it adds to 'ved_pipeline'
>> all entities that are connected to the video node, in addition to the entities
>> that where there from previous calls, so some entities appear several times.
> 
> This function returns all the entities in a pipe, weather they are
> streaming or not. For example, if only the RAW Capture 1 streams, or
> RGB/YUB capture streams, or both stream..in all three cases ved_pipeline
> will have the same entities, in exactly same order, and all will occur just once.
> Since media_graph_walk goes to each node of the graph, it returns back
> to the first one (as its a graph), hence the last condition, ie,
> 
> 	if (!strcmp(entity->name, stream->ved_pipeline[0]->ent->name)) {
> 
> makes sure that the first entity is not added again to the array.
> 
> First condition, ie
> 
> 	if (!strncmp(entity->name, "RGB", 3)) {
> 
> Just makes sure that the search starts at RGB capture only. This is
> because, in case of any other video node, the order of entities, as you
> have mentioned later in the mail, will not be desirable, ie it won't
> start at sensor and end at captures. So this condition just takes care
> that all the entities in ved_pipeline array are in correct order
> (starting at sensor, ending at captures).

It is better to compare to the whole name of the entity to make it more clear.
Also I think it is better to document that this function is called only upon the
first streaming node.

I still think this function should be independent of the topology.
Maybe you can use Helen's patch that allow walking a graph only opposite to the link direction: https://patchwork.kernel.org/patch/11564899/
This ensures that the Sensor will be first in the graph walk. Then the streaming thread
iterates the ved_pipeline from 0 upwards and not from 'pipe_size' downwards.

Thanks,
Dafna


> 
> Thanks,
> Kaaira
> 
>>
>> I think there is no need to use the graph walk here but to access the source entity
>> in each iteration, the way done in vimc_streamer_stream_start
>> also.
>> I think the code should iterate here until it reaches an entity that is already streaming,
>> this means that the entity is already in the `ved_pipeline`, also you should make sure
>> that the sensor is the first entity that process a frame, therefore the sensor should be
>> at the end/start of the list of entities. Generally each entity should appear exactly once
>> in the 'ved_pipeline' array and the entities should be ordered such that when calling 'process_frame'
>> on one entity should be after calling 'process_frame' on its source entity.
>> maybe it is easyer to implement if 'ved_pipeline' is a linked list.
>>
>> Thanks,
>> Dafna
>>
>>>    	vimc_streamer_pipeline_terminate(stream);
>>> @@ -138,8 +202,11 @@ static int vimc_streamer_thread(void *data)
>>>    		for (i = stream->pipe_size - 1; i >= 0; i--) {
>>>    			ved = stream->ved_pipeline[i];
>>> -			ret = ved->process_frame(ved);
>>> +			if (atomic_read(&ved->use_count) == 0)
>>> +				continue;
>>> +
>>> +			ret = ved->process_frame(ved);
>>>    			if (ret)
>>>    				break;
>>>    		}
>>> @@ -179,6 +246,10 @@ int vimc_streamer_s_stream(struct vimc_stream *stream,
>>>    		if (stream->kthread)
>>>    			return 0;
>>> +		ret = vimc_streamer_stream_start(stream, ved);
>>> +		if (ret)
>>> +			return ret;
>>> +
>>>    		ret = vimc_streamer_pipeline_init(stream, ved);
>>>    		if (ret)
>>>    			return ret;
>>>