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From:   Kaaira Gupta <kgupta@es.iitr.ac.in>
Date:   Fri, 31 Jul 2020 22:52:21 +0530
To:     Dafna Hirschfeld <dafna.hirschfeld@collabora.com>,
        kieran.bingham@ideasonboard.com,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Hans Verkuil <hverkuil@xs4all.nl>,
        Helen Koike <helen.koike@collabora.com>
Cc:     Kaaira Gupta <kgupta@es.iitr.ac.in>,
        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
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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.

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).

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

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
> > > > > > > > > 
> > > > 
> >