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[WHY] 1. Drivers using DRM_GEM_SHADOW_PLANE_HELPER_FUNCS and DRM_GEM_SHMEM_DRIVER_OPS (e.g., udl, ast) do not require sg_table import. They only need dma_buf_vmap() to access the shared buffer's kernel virtual address. 2. On certain Aspeed-based boards, a dma_mask of 0xffff_ffff may trigger SWIOTLB during dmabuf import. However, IO_TLB_SEGSIZE restricts the maximum DMA streaming mapping memory, resulting in errors like: ast 0000:07:00.0: swiotlb buffer is full (sz: 3145728 bytes), total 32768 (slots), used 0 (slots) [HOW] Provide a gem_prime_import implementation without sg_table mapping to avoid issues (e.g., "swiotlb buffer is full"). Drivers that do not require sg_table can adopt this. Signed-off-by: Shixiong Ou <oushixiong@kylinos.cn> Acked-by: Thomas Zimmermann <tzimmermann@suse.de> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de> Link: https://lore.kernel.org/r/20250522070714.439824-1-oushixiong1025@163.com