Is it possible to call dma_unmap_single after dma_sync_single_for_cpu?

Question

I have a situation where I need to call dma_unmap_single after dma_sync_single_for_cpu . Below is my script.

• Allocate memory for buffer using kmalloc()
• Map the buffer to the device using dma_map_single with the direction DMA_FROM_DEVICE .
• After receiving the packet, obtain ownership of the buffer using dma_sync_single_for_cpu .
• After some time, if some other function in the kernel uses the same buffer, cancel the buffer from the device using dma_unmap_single with the direction DMA_FROM_DEVICE .

Are all of the above steps possible?

Answer 1

Buffer ownership (CPU or device) matters only to decide who is allowed access to the contents of the buffer.

dma_unmap_single() does not care about the content; it can be called in any state.

Note that after decoupling the DMA, the memory again “belongs” to the CPU. If you perform only one DMA transfer, you do not need to call dma_sync_single_for_cpu() ; you can just read the buffer after calling dma_unmap_single() .