Re: [PATCH v2 0/3] media: vimc: Allow multiple capture devices to use the same sensor

[Dafna Hirschfeld]

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 SÃderlund 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.

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@xxxxxxxxxxxxx/
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > > Changes since v1:
> > > > > > > > > > >  ÂÂÂÂÂ- All three patches rebased on latest media-tree.
> > > > > > > > > > >  ÂÂÂÂÂPatch 3:
> > > > > > > > > > >  ÂÂÂÂÂ- Search for an entity with a non-NULL pipe instead of
> > > > > > > > > > > searching
> > > > > > > > > > >  ÂÂÂÂÂÂ for sensor. This terminates the search at output itself.
> > > > > > > > > > >
> > > > > > > > > > > Kaaira Gupta (3):
> > > > > > > > > > >  ÂÂÂ media: vimc: Add usage count to subdevices
> > > > > > > > > > >  ÂÂÂ media: vimc: Serialize vimc_streamer_s_stream()
> > > > > > > > > > >  ÂÂÂ media: vimc: Join pipeline if one already exists
> > > > > > > > > > >
> > > > > > > > > > >  ÂÂ .../media/test-drivers/vimc/vimc-capture.cÂÂÂ | 35
> > > > > > > > > > > ++++++++++++++++++-
> > > > > > > > > > >  ÂÂ .../media/test-drivers/vimc/vimc-debayer.cÂÂÂ |Â 8 +++++
> > > > > > > > > > >  ÂÂ drivers/media/test-drivers/vimc/vimc-scaler.c |Â 8 +++++
> > > > > > > > > > >  ÂÂ drivers/media/test-drivers/vimc/vimc-sensor.c |Â 9 ++++-
> > > > > > > > > > >  ÂÂ .../media/test-drivers/vimc/vimc-streamer.cÂÂ | 23
> > > > > > > > > > > +++++++-----
> > > > > > > > > > >  ÂÂ 5 files changed, 73 insertions(+), 10 deletions(-)
> > > > > > > > > > >
> > > > > > > > > > > --
> > > > > > > > > > > 2.17.1
> > > > > > > > > >
> > > > > > > > > > --
> > > > > > > > > > Regards,
> > > > > > > > > > Niklas SÃderlund