Vishwas Srivastava vishu.kernel at
Sun Feb 7 23:01:40 EST 2016

Hi Ran,
             the api which you have mentioned...

void *
dma_alloc_coherent(struct device *dev, size_t size,
			     dma_addr_t *dma_handle, gfp_t flag)

is the kernel api to alloc consistent memory.

DMA devices understand the physical addresses not the virtual addresses.

which means that we must supply to the dma device, the physical address, to read

from or to write to.

The second argument of this api is an input argument which is updated
by the kernel if this api returns a success (and contains the physical base
address of the allocated memory) and the returned value of this api is the
kernel virtual address.

if the *CPU* has to operate on this memory (assume that the memory is
dma'ed by the dma device and cpu want to read it for further processing )
it should use the virtual address, so the returned value of this api, as
the base address.
However, if the dma device has to operate on this memory (assume device
want to write to this memory), it should use the *dma_handle* , which is
the physical address (base) of the dma memory.

Now the question is how the dma device knows about this *physical* address?
The answer is that the "dma controller" register would have a register to
accept this physical address.

So the sequence of steps probably would be, in your case:
1: allocate the dma memory
2: programme the dma controller register with the physical address returned
by this api, plus the size of the transaction and may be some more
registers for setting some kind of flags (depends on your dma device)
3: programme the dma controller's dma *start* bit.

after this the dma starts and dma device starts writing to the memory .

I hope, this clarifies you.

On Sun, Feb 7, 2016 at 10:30 PM, <kernelnewbies-request at>

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> Today's Topics:
>    1. Re: dma_alloc_coherent (Ran Shalit)
> ----------------------------------------------------------------------
> Message: 1
> Date: Sat, 6 Feb 2016 21:22:49 +0200
> From: Ran Shalit <ranshalit at>
> Subject: Re: dma_alloc_coherent
> To: Denis Kirjanov <kirjanov at>
> Cc: kernelnewbies <kernelnewbies at>
> Message-ID:
>         <
> Content-Type: text/plain; charset=UTF-8
> 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.
> Thanks,
> Ran
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