On R-Car Gen3, the DU uses a separate IP core named VSP to perform DMA
from memory and composition of planes. The DU hardware then only handles
the video timings and the interface with the encoders. This differs from
Gen2, where the DU included a composer with DMA engines.

When sourcing from the VSP, the DU hardware performs no memory access,
and thus has no requirements on imported dma-buf memory types. The GEM
CMA helpers however still create a DMA mapping to the DU device, which
isn't used. The mapping to the VSP is done when processing the atomic
commits, in the plane .prepare_fb() handler.

When the system uses an IOMMU, the VSP device is attached to it, which
enables the VSP to use non physically contiguous memory. The DU, as it
performs no memory access, isn't connected to the IOMMU. The GEM CMA
drm_gem_cma_prime_import_sg_table() helper will in that case fail to map
non-contiguous imported dma-bufs, as the DMA mapping to the DU device
will have multiple entries in its sgtable. The prevents using non
physically contiguous memory for display.

The DRM PRIME and GEM CMA helpers are designed to create the sgtable
when the dma-buf is imported. By default, the device referenced by the
drm_device is used to create the dma-buf attachment. Drivers can use a
different device by using the drm_gem_prime_import_dev() function. While
the DU has access to the VSP device, this won't help here, as different
CRTCs use different VSP instances, connected to different IOMMU
channels. The driver doesn't know at import time which CRTC a GEM object
will be used, and thus can't select the right VSP device to pass to
drm_gem_prime_import_dev().

To support non-contiguous memory, implement a custom
.gem_prime_import_sg_table() operation that accepts all imported dma-buf
regardless of the number of scatterlist entries. The sgtable will be
mapped to the VSP at .prepare_fb() time, which will reject the
framebuffer if the VSP isn't connected to an IOMMU.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>