[libcamera-devel] Questions about a zero-copy RAW stream in the raspberrypi pipelinehandler

[Naushir Patuck]

Hi Niklas,

On Fri, 26 Jun 2020 at 13:03, Niklas Söderlund
<niklas.soderlund at ragnatech.se> wrote:
>
> Hi Naushir,
>
> I just merged a patch for the IPU3 pipeline handler making its RAW
> stream zero-copy [1]. I would like for something similar for the
> raspberrypi pipeline handler as that would allow us to drop
> FrameBuffer::copyFrom(). The need to copy out the RAW buffer was an idea
> we had when we wished to protect against applications re-using the RAW
> buffer before the request it belonged to had completed. In hindsight we
> think this was a bad idea and wish to reverse course and address the
> issue in a way that does not impact performance as much.
>
> I had a look at the raspberrypi pipeline. After understanding that you
> run the bayer and embeddeddata queues in lockstep and share the bayer
> buffer with the IPA (RPI_IPA_EVENT_SIGNAL_ISP_PREPARE) I realised it
> would not be a simple change as it was for the IPU3 pipeline. Would it
> be possible for you to have a look and provide some guidance on how we
> could move the pipeline in this direction? If you think getting rid of
> the copy operation is a bad idea that is of course also good feedback.
>
> Thanks and have a nice weekend.
>
> 1. 40148cfcafbac274 ("libcamera: ipu3: Allow zero-copy RAW stream capture")
>

I would be happy to assist with this.  We certainly do want to get rid
of the memcpy in our pipeline handler to improve performance.  I will
have a look at what changes you made to the IPU3 pipeline handler to
see what is involved.  Separate, could you perhaps summarise the
steps/changes you made in the IPU3 - this would help me understand the
code better, I have not looked at the IPU3 pipeline at all.  Perhaps
we can take this discussion private to avoid spamming the mailing list
as well?

Regards,
Naush