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Importing dma-bufs via PRIME requires a DMA-capable device. Devices on peripheral busses, such as USB, often cannot perform DMA by themselves. Without DMA-capable device PRIME import fails. DRM drivers for USB devices already use a separate DMA device for dma-buf imports. Make the mechanism generally available. Besides the case of USB, there are embedded DRM devices without DMA capability. DMA is performed by a separate controller. DRM drivers should set this accordingly. Add the field dma_dev to struct drm_device to refer to the device's DMA device. For USB this should be the USB controller. Use dma_dev in the PRIME import helpers, if set. v2: - acquire internal reference on dma_dev (Jani) - add DMA-controller usecase to docs (Maxime) Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de> Reviewed-by: Jani Nikula <jani.nikula@intel.com> Reviewed-by: Maxime Ripard <mripard@kernel.org> Link: https://patchwork.freedesktop.org/patch/msgid/20250307080836.42848-2-tzimmermann@suse.de