[libcamera-devel] [PATCH 3/3] pipeline: raspberrypi: Increase the V4L2BufferCache slot allocations

[Naushir Patuck]

Hi Kieran,

Thanks for all the reviews!

On Wed, 10 Nov 2021 at 10:33, Kieran Bingham <
kieran.bingham at ideasonboard.com> wrote:

> Quoting Naushir Patuck (2021-11-10 10:08:02)
> > If a stream is marked as external, double the number of V4L2BufferCache
> slots
> > that are allocated. This is to account for additional buffers that may be
> > allocated directly by the application.
> >
> > Signed-off-by: Naushir Patuck <naush at raspberrypi.com>
> > ---
> >  src/libcamera/pipeline/raspberrypi/rpi_stream.cpp | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/src/libcamera/pipeline/raspberrypi/rpi_stream.cpp
> b/src/libcamera/pipeline/raspberrypi/rpi_stream.cpp
> > index b3265d0e8aab..67901936d6b6 100644
> > --- a/src/libcamera/pipeline/raspberrypi/rpi_stream.cpp
> > +++ b/src/libcamera/pipeline/raspberrypi/rpi_stream.cpp
> > @@ -107,6 +107,14 @@ int Stream::prepareBuffers(unsigned int count)
> >                 count = bufferMap_.size();
> >         }
> >
> > +       /*
> > +        * If this is an external stream, we must allocate slots for
> buffers that
> > +        * might be externally allocated. We have no indication of how
> many buffers
> > +        * may be used, so this might overallocate slots in the buffer
> cache.
> > +        */
>
> Perhaps we'll need a better system here (not in this patch) to handle
> this ... but I think overallocating is cheap - and the whole point of
> the buffer cache is to try to re-use the same buffers where possible. So
> I /think/ overallocations is the right solution for now anyway.
>

I think this over allocation is the only change needed to "fix" Roman's
original
issue.  But I did mean to tidy up the buffer allocations for some time now,
so
why not :-)


>
> > +       if (isExternal())
> > +               count = count * 2;
>
> Of course it's hard to know /how/ far to overallocate ... But this will
> do until we figure it out ;-)
>

I wonder if we can perhaps allow the cache to grow in size dynamically
instead of relying on a fixed number of slots?

Naush


>
>
> Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
>
> > +
> >         return dev_->importBuffers(count);
> >  }
> >
> > --
> > 2.25.1
> >
>
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