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

[Ruben Safir]

On 09/06/2016 12:24 PM, Ran Shalit wrote:
> 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)

DMA hardware uses an expedited bus that frees the main memory to be
addressed for other processes.

> 
> Regards,
> Ran
> 
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