proxy allocation query to fix appsink zero-copy issue

[mksafavi]

  > 1. Are you modifying the buffers in-place?
yes. I memcpy the buffer data to a mmapped region, in order to send the
buffer with axidma to the accelerator. the returned data is scaled
down(right now I'm only testing it on 1080 to 720 on NV12).then I wrap it on
a GstMem and push the buffer to appsrc.
(we're trying to avoid the memcpy by setting up SMMU using VFIO driver)

  > 2. If not, do you have the ability to allocated DMABuf memory?
I'd like to allocate DMABUF but I couldn't make it work with Xilinx VCU
decoder.
the Xilinx changelog states that it's possible to get DMABUF form decoder. 
I remember there was /iomode=1/ option for v4l2src that enabled dmabuf on
decoder.
but couldn't find anything for my pipeline
filesrc->decoder->appsink->scaler->appsrc->encoder->filesink

  > 3. What do you know about video buffer alignment?
I'm not really familiar with buffer alignment. I just know that it affects
the throughput of memcpy. but don't know why exactly 





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