Ran Shalit ranshalit at
Sat Feb 6 14:22:49 EST 2016

On Fri, Feb 5, 2016 at 11:15 AM, Denis Kirjanov <kirjanov at> wrote:
> On 2/5/16, Ran Shalit <ranshalit at> wrote:
>> Hello,
>> I read the readme about dma API, but still don't understand how it
>> should be used
>> It is said that dma_alloc_coherent is responsible for allocating the
>> buffer.
>> 1. But how to trigger the dma transaction to start ?
>> 2. Is there a callback when it is finished ?
> It's used for data transfer between IO device and system memory. You
> allocate a kernel buffer for DMA transaction (in this case dma from
> device to system memory) and setup your device for dma transfer. An IO
> device usually generates an interrupt when DMA transfer completes.
> Denis

If I understand correctly the full picture how to use dma is as
following (schematics):

buf = dma_alloc_coherent(); <<-- done once at the lifetime

every time we need to trigger dma:

//start transaction now
writeb(buf, DMA_ADDR) <<- juat an example, actually it is totally
depends on dma device
writeb(START_DMA, DMA_ADDR+2) <<- juat an example, actually it is
totally depends on dma device

//usually we also register on irq for callback on finishing the transaction.

 hope I got it all correct, if you have any comments please add.


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