1) Support dma-buf memory management.
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In order to zero-copy import camera images into the 3D or display
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pipelines, we need to export our buffers through dma-buf so that the
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vc4 driver can import them. This may involve bringing in the VCSM
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driver (which allows long-term management of regions of memory in the
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space that the VPU reserved and Linux otherwise doesn't have access
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to), or building some new protocol that allows VCSM-style management
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of Linux's CMA memory.
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2) Avoid extra copies for padding of images.
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We expose V4L2_PIX_FMT_* formats that have a specified stride/height
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padding in the V4L2 spec, but that padding doesn't match what the
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hardware can do. If we exposed the native padding requirements
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through the V4L2 "multiplanar" formats, the firmware would have one
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less copy it needed to do.
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