Q: dma requires copying into dma buffer - so what's the benefit ?

[Ran Shalit]

Hello,

There is something I don't understand about dma,
when doing memory to memory dma, it requires to cllocate dma buffer
(for example with dma_alloc_coherent),
than for each transfer we need to copy the buffer to the allocated
memory and than trigger dma transaction.
So, if it requires additional memcpy for each transaction , what's the
benefit of using dma ?

without dma:
1. copy buffer (memcpy) from source to destination

with dma:
1. copy buffer (memcpy) from source to dma buffer
2. trigger dma transaction (which shall copy the buffer eventually to
destination buffer)

Regards,
Ran