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From:   Kaaira Gupta <kgupta@es.iitr.ac.in>
Date:   Wed, 5 Aug 2020 00:22:59 +0530
To:     Kieran Bingham <kieran.bingham@ideasonboard.com>
Cc:     Dafna Hirschfeld <dafna.hirschfeld@collabora.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Hans Verkuil <hverkuil@xs4all.nl>,
        Helen Koike <helen.koike@collabora.com>,
        Niklas =?iso-8859-1?Q?S=F6derlund?= 
        <niklas.soderlund@ragnatech.se>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        linux-media@vger.kernel.org, linux-kernel@vger.kernel.org,
        Ezequiel Garcia <ezequiel@collabora.com>
Subject: Re: [PATCH v2 0/3] media: vimc: Allow multiple capture devices to
 use the same sensor
Message-ID: <20200804185259.GB13319@kaaira-HP-Pavilion-Notebook>
References: <20200728113959.GA6350@kaaira-HP-Pavilion-Notebook>
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 <b5fd3811-2f0e-7563-13fa-bb1e32189814@collabora.com>
 <f2fc6d81-16a6-206b-6bb2-fb99d5a84f65@ideasonboard.com>
 <cd1d18ef-d4ff-8240-e9dd-2d9f9c7c5a45@collabora.com>
 <a2ae2dba-6076-0abc-f620-9047a52b513c@ideasonboard.com>
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On Wed, Aug 05, 2020 at 12:19:52AM +0530, Kaaira Gupta wrote:
> Hi
> 
> On Tue, Aug 04, 2020 at 11:24:56AM +0100, Kieran Bingham wrote:
> > 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 S?derlund 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?
> 
> Yes, but actually, the current implementation of the thread does not
> walk through the entire pipeline, rather ved_pipeline, which is the
> portions of the pipeline which come in the streaming path of a stream
> which calls the thread. This also answers your doubt why the pipeline is
> decreamenting (which you have asked later in the mail). ved_pipeline is
> an array of the entities in the path of the stream starting from
> capture. Hence my suggestion was if I should make a data structure (a 
> graph) which holds all the entities in one or more stream path.
> 
> > 
> > 
> > > 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...
> 
> I meant that to start a thread for the driver, rather than each stream,
> maybe instead of creating a graph (as in the first option), maybe the
> druver could know the number of steams alsready running, and hence when
> s_stream is called for another, it knows how many and what type of
> streams are running, which can then /know/ which entity's output of
> process_frame(previous stream) to give on to the current stream. But I
> wasn't sure about the implementation of the /knwing/part without
> hardcoding it to take output from the sensor. But since that is the only
> topology (hardcoded)we have currently, maybe that can be a solution,
> hence I asked.

The idea is actually almost same as the pseudo code/implementation you wrote
below :D

> 
> > 
> > 
> > > 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 S?derlund
> > >>>>>>>>>>
> > >>>>>
> > >>>
> > 
> > -- 
> > Regards
> > --
> > Kieran