Message-ID: <afcdaa96-11b0-bb36-3edb-47505aa28004@ideasonboard.com>
Date:   Tue, 4 Aug 2020 11:24:56 +0100
From:   Kieran Bingham <kieran.bingham@...asonboard.com>
To:     Kaaira Gupta <kgupta@...iitr.ac.in>,
        Dafna Hirschfeld <dafna.hirschfeld@...labora.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Hans Verkuil <hverkuil@...all.nl>,
        Helen Koike <helen.koike@...labora.com>
Cc:     Niklas Söderlund <niklas.soderlund@...natech.se>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ezequiel Garcia <ezequiel@...labora.com>
Subject: Re: [PATCH v2 0/3] media: vimc: Allow multiple capture devices to use
 the same sensor

Hi Kaaira,

On 31/07/2020 18:22, Kaaira Gupta wrote:
> Hi everyone,
> 
> On Wed, Jul 29, 2020 at 05:24:25PM +0200, Dafna Hirschfeld wrote:
>>
>>
>> On 29.07.20 15:27, Kieran Bingham wrote:
>>> Hi Dafna, Kaaira,
>>>
>>> On 29/07/2020 14:16, Dafna Hirschfeld wrote:
>>>>
>>>>
>>>> On 29.07.20 15:05, Kieran Bingham wrote:
>>>>> Hi Dafna,
>>>>>
>>>>> On 28/07/2020 15:00, Dafna Hirschfeld wrote:
>>>>>>
>>>>>>
>>>>>> On 28.07.20 14:07, Dafna Hirschfeld wrote:
>>>>>>> Hi
>>>>>>>
>>>>>>> On 28.07.20 13:39, Kaaira Gupta wrote:
>>>>>>>> On Mon, Jul 27, 2020 at 02:54:30PM -0300, Helen Koike wrote:
>>>>>>>>> Hi,
>>>>>>>>>
>>>>>>>>> On 7/27/20 11:31 AM, Kieran Bingham wrote:
>>>>>>>>>> Hi all,
>>>>>>>>>>
>>>>>>>>>> +Dafna for the thread discussion, as she's missed from the to/cc
>>>>>>>>>> list.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On 24/07/2020 13:21, Kaaira Gupta wrote:
>>>>>>>>>>> On Fri, Jul 24, 2020 at 02:15:21PM +0200, Niklas Söderlund wrote:
>>>>>>>>>>> Hi,
>>>>>>>>>>>
>>>>>>>>>>>> Hi Kaaira,
>>>>>>>>>>>>
>>>>>>>>>>>> Thanks for your work.
>>>>>>>>>>>
>>>>>>>>>>> Thanks for yours :D
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> On 2020-07-24 17:32:10 +0530, Kaaira Gupta wrote:
>>>>>>>>>>>>> This is version 2 of the patch series posted by Niklas for
>>>>>>>>>>>>> allowing
>>>>>>>>>>>>> multiple streams in VIMC.
>>>>>>>>>>>>> The original series can be found here:
>>>>>>>>>>>>> https://patchwork.kernel.org/cover/10948831/
>>>>>>>>>>>>>
>>>>>>>>>>>>> This series adds support for two (or more) capture devices to be
>>>>>>>>>>>>> connected to the same sensors and run simultaneously. Each
>>>>>>>>>>>>> capture device
>>>>>>>>>>>>> can be started and stopped independent of each other.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Patch 1/3 and 2/3 deals with solving the issues that arises once
>>>>>>>>>>>>> two
>>>>>>>>>>>>> capture devices can be part of the same pipeline. While 3/3
>>>>>>>>>>>>> allows for
>>>>>>>>>>>>> two capture devices to be part of the same pipeline and thus
>>>>>>>>>>>>> allows for
>>>>>>>>>>>>> simultaneously use.
>>>>>>>
>>>>>>> I wonder if these two patches are enough, since each vimc entity also
>>>>>>> have
>>>>>>> a 'process_frame' callback, but only one allocated frame. That means
>>>>>>> that the 'process_frame' can be called concurrently by two different
>>>>>>> streams
>>>>>>> on the same frame and cause corruption.
>>>>>>>
>>>>>>
>>>>>> I think we should somehow change the vimc-stream.c code so that we have
>>>>>> only
>>>>>> one stream process per pipe. So if one capture is already streaming,
>>>>>> then the new
>>>>>> capture that wants to stream uses the same thread so we don't have two
>>>>>> threads
>>>>>> both calling 'process_frame'.
>>>>>
>>>>>
>>>>> Yes, I think it looks and sounds like there are two threads running when
>>>>> there are two streams.
>>>>>
>>>>> so in effect, although they 'share a pipe', aren't they in effect just
>>>>> sending two separate buffers through their stream-path?
>>>>>
>>>>> If that's the case, then I don't think there's any frame corruption,
>>>>> because they would both have grabbed their own frame separately.
>>>>
>>>> But each entity allocates just one buffer. So the same buffer is used for
>>>> both stream.
>>>
>>> Aha, ok, I hadn't realised there was only a single buffer available in
>>> the pipeline for each entity. Indeed there is a risk of corruption in
>>> that case.
>>>
>>>> What for example can happen is that the debayer of one stream can read the
>>>> sensor's buffer while the sensor itself writes to the buffer for the other
>>>> stream.
>>>
>>>
>>> So, In that case, we have currently got a scenario where each 'stream'
>>> really is operating it's own pipe (even though all components are reused).
>>>
>>> Two questions:
>>>
>>> Is this acceptable, and we should just use a mutex to ensure the buffers
>>> are not corrupted, but essentially each stream is a separate temporal
>>> capture?
>>>
>>>
>>> Or B:
>>>
>>> Should we refactor to make sure that there is a single thread, and the
>>> code which calls process_frame on each entity should become aware of the
>>> potential for multiple paths at the point of the sensor.
>>>
>>>
>>> I suspect option B is really the 'right' path to take, but it is more
>>> complicated of course.
>>
>> I also think option B is preferable.
>>
>> Maybe we can add a bool field 'is_streaming' to struct 'vimc_ent_device'
>> The stream thread can do a BFS scan from the sensor up to the captures
>> and call the 'process_frame' for each entity if 'is_streaming == true'.
>> When a new capture wants to stream it sets 'is_streaming = true'
>> on the entities on his streaming path.
> 
> It is s_stream(enable) that initialises a streaming pipeline, ie the one with
> those components of the pipeline which are in stream path and then runs a
> thread which calls process_frame on each and passes the frame to the
> next entity in streaming pipeline. So currently, one thread is for one
> "streaming pipeline". So there are two options I can think of if a
> single thread is required,
> 
> 1. Not creating a streaming pipeline, rather create a graph(?) which
> connects both say Raw capture 1 and debayer B to sensor B if two streams
> are asked for, and only one of them if one stream is asked..that will
> not be a property of streamer, so I am not sure where it should be kept.
> Then I could move creating a thread out of s_stream. Creating the thread
> should wait for entire pipeline to be created, ie s_stream(enable) to
> must be called by both the captures, and a graph made of all pipeline
> components before thread initialisation starts. I am not sure how this
> should be implemented.

The graph already exists, and can be walked through the media controller
right?


> 2. Another option is to check if a stream already exists (by creating it
> a property of vimc to keep a track of no. of streams maybe?), if it is
> already present I could take the previous output of sensor (but
> then it will have to be stored, so i don't think this is a nice idea),
> and use it further (but thread will be different in this case).

I don't think I understand this one...


> What can be a better design for VIMC to have a single thread if two
> streams are asked (apart/of the options I mentioned)?

How about adding a count in s_stream so that the thread only gets
started when the use count is > 0, and stopped when the usage < 1.

That handles making sure that only one thread is available.

All calls into s_stream() will need to take a lock/mutex to protect /
prevent any action from occurring while the thread is performing a
process of the pipeline.


static int vimc_streamer_thread(void *data)
{
	struct vimc_stream *stream = data;
	u8 *frame = NULL;
	int i;

	set_freezable();

	for (;;) {
		try_to_freeze();
		if (kthread_should_stop())
			break;

+		/* take lock shared with s_stream */

		for (i = stream->pipe_size - 1; i >= 0; i--) {
			frame = stream->ved_pipeline[i]->process_frame(
					stream->ved_pipeline[i], frame);
			if (!frame || IS_ERR(frame))
				break;
		}

+		/* Release lock/mutex shared with s_stream

		//wait for 60hz
		set_current_state(TASK_UNINTERRUPTIBLE);
		schedule_timeout(HZ / 60);
	}

	return 0;
}



And you'll need to make the code which processes the pipeline aware of
the fact that there may be two pipelines to fulfil:

Pseudo patch/code:

static int vimc_streamer_thread(void *data)
{
-	struct vimc_stream *stream = data;
+	/* Something which knows about the whole device */
+	struct xxxxx *yyy = data;

+	u8 *raw;
	u8 *frame = NULL;
	int i;

	set_freezable();

	for (;;) {
		try_to_freeze();
		if (kthread_should_stop())
			break;

		/* take lock shared with s_stream */

+		/* Process the sensor first */
+		raw = stream->ved_pipeline[sensor]->process_frame(..);
+		error check;

+		/* (If connected) Process stream 1 */
+		if (raw)
+		frame = stream->ved_pipeline[raw]->process_frame();
+		error check;

+		/* If connected process the rest of the pipe */
+		for (i = after sensor; end_entity; i++) {
			frame = stream->ved_pipeline[i]->process_frame(
					stream->ved_pipeline[i], frame);
			if (!frame || IS_ERR(frame))
				break;
		}

		/* Release lock/mutex shared with s_stream

		//wait for 60hz
		set_current_state(TASK_UNINTERRUPTIBLE);
		schedule_timeout(HZ / 60);
	}

	return 0;
}



I may have missed something as the original loop was decrementing and
going backwards through the entities in stream->ved_pipeline.

I guess splitting that all out so instead it starts at the sensor, and
just walks the graph (handling any running/connected fork to two
entities appropriately) in a neater way would be another option rather
than hardcoding it, but either way the thread needs to operate at the
device level rather than the stream level.



> Thanks
> Kaaira
> 
>>
>> Thanks,
>> Dafna
>>
>>
>>>
>>> --
>>> Kieran
>>>
>>>
>>>
>>>
>>>> Thanks,
>>>> Dafna
>>>>
>>>>>
>>>>>
>>>>> I don't think that's a good example of the hardware though, as that
>>>>> doesn't reflect what 'should' happen where the TPG runs once to generate
>>>>> a frame at the sensor, which is then read by both the debayer entity and
>>>>> the RAW capture device when there are two streams...
>>>>>
>>>>>
>>>>> So I suspect trying to move to a single thread is desirable, but that
>>>>> might be a fair bit of work also.
>>>>>
>>>>> -- 
>>>>> Kieran
>>>>>
>>>>>
>>>>>
>>>>>> The second capture that wants to stream should iterate the topology
>>>>>> downwards until
>>>>>> reaching an entity that already belong to the stream path of the other
>>>>>> streaming capture
>>>>>> and tell the streamer it wants to read the frames this entity
>>>>>> produces.
>>>>>>
>>>>>> Thanks,
>>>>>> Dafna
>>>>>>
>>>>>>> Thanks,
>>>>>>> Dafna
>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> I'm just curious if you are aware of this series? It would
>>>>>>>>>>>> replace the
>>>>>>>>>>>> need for 1/3 and 2/3 of this series right?
>>>>>>>>>>>
>>>>>>>>>>> v3 of this series replaces the need for 1/3, but not the current
>>>>>>>>>>> version
>>>>>>>>>>> (ie v4). v4 of patch 2/5 removes the stream_counter that is
>>>>>>>>>>> needed to
>>>>>>>>>>> keep count of the calls to s_stream. Hence 1/3 becomes relevant
>>>>>>>>>>> again.
>>>>>>>>>>
>>>>>>>>>> So the question really is, how do we best make use of the two
>>>>>>>>>> current
>>>>>>>>>> series, to achieve our goal of supporting multiple streams.
>>>>>>>>>>
>>>>>>>>>> Having not parsed Dafna's series yet, do we need to combine
>>>>>>>>>> elements of
>>>>>>>>>> both ? Or should we work towards starting with this series and get
>>>>>>>>>> dafna's patches built on top ?
>>>>>>>>>>
>>>>>>>>>> Or should patch 1/3 and 3/3 of this series be on top of Dafna's v4 ?
>>>>>>>>>>
>>>>>>>>>> (It might be noteworthy to say that Kaaira has reported successful
>>>>>>>>>> multiple stream operation from /this/ series and her development
>>>>>>>>>> branch
>>>>>>>>>> on libcamera).
>>>>>>>>>
>>>>>>>>> Dafna's patch seems still under discussion, but I don't want to
>>>>>>>>> block progress in Vimc either.
>>>>>>>>>
>>>>>>>>> So I was wondering if we can move forward with Vimc support for
>>>>>>>>> multistreaming,
>>>>>>>>> without considering Dafna's patchset, and we can do the clean up
>>>>>>>>> later once we solve that.
>>>>>>>>>
>>>>>>>>> What do you think?
>>>>>>>>
>>>>>>>> I agree with supporting multiple streams with VIMC with this patchset,
>>>>>>>> and then we can refactor the counters for s_stream in VIMC later (over
>>>>>>>> this series) if dafna includes them in subsequent version of her
>>>>>>>> patchset.
>>>>>>>>
>>>>>>>
>>>>>>> I also think that adding support in the code will take much longer and
>>>>>>> should not
>>>>>>> stop us from supporting vimc independently.
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Dafna
>>>>>>>
>>>>>>>>>
>>>>>>>>> Regards,
>>>>>>>>> Helen
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>>> 1.
>>>>>>>>>>>> https://lore.kernel.org/linux-media/20200522075522.6190-1-dafna.hirschfeld@collabora.com/
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> Changes since v1:
>>>>>>>>>>>>>       - All three patches rebased on latest media-tree.
>>>>>>>>>>>>>       Patch 3:
>>>>>>>>>>>>>       - Search for an entity with a non-NULL pipe instead of
>>>>>>>>>>>>> searching
>>>>>>>>>>>>>         for sensor. This terminates the search at output itself.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Kaaira Gupta (3):
>>>>>>>>>>>>>      media: vimc: Add usage count to subdevices
>>>>>>>>>>>>>      media: vimc: Serialize vimc_streamer_s_stream()
>>>>>>>>>>>>>      media: vimc: Join pipeline if one already exists
>>>>>>>>>>>>>
>>>>>>>>>>>>>     .../media/test-drivers/vimc/vimc-capture.c    | 35
>>>>>>>>>>>>> ++++++++++++++++++-
>>>>>>>>>>>>>     .../media/test-drivers/vimc/vimc-debayer.c    |  8 +++++
>>>>>>>>>>>>>     drivers/media/test-drivers/vimc/vimc-scaler.c |  8 +++++
>>>>>>>>>>>>>     drivers/media/test-drivers/vimc/vimc-sensor.c |  9 ++++-
>>>>>>>>>>>>>     .../media/test-drivers/vimc/vimc-streamer.c   | 23
>>>>>>>>>>>>> +++++++-----
>>>>>>>>>>>>>     5 files changed, 73 insertions(+), 10 deletions(-)
>>>>>>>>>>>>>
>>>>>>>>>>>>> -- 
>>>>>>>>>>>>> 2.17.1
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> -- 
>>>>>>>>>>>> Regards,
>>>>>>>>>>>> Niklas Söderlund
>>>>>>>>>>
>>>>>
>>>

-- 
Regards
--
Kieran

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