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videobuf-dma-contig sync question


I don't understand why __videobuf_sync in videobuf-dma-contig isn't a nop.

All the memory allocated by videobuf-dma-contig is coherent memory. And
Documentation/DMA-API.txt seems to imply that this memory is coherent
and doesn't need extra cache operation for synchronization.

Also calling dma_sync_single_for_cpu cause panic on arm for per-device
coherent memory, because the memory isn't in the main memory[1].

Why __videobuf_sync need dma_sync_single_for_cpu ?



void dma_cache_maint(const void *start, size_t size, int direction)
    const void *end = start + size;

    BUG_ON(!virt_addr_valid(start) || !virt_addr_valid(end - 1));

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