dma_alloc_coherent

[Ran Shalit]

On Mon, Feb 8, 2016 at 6:01 AM, Vishwas Srivastava
<vishu.kernel at gmail.com> wrote:
> 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.
>
>

Hi Vishwas,

That's fully clarify the questions about dma_alloc_coherent.

I also try to figure out what's the difference between
dma_alloc_coherent and dma_map_single.

I could not find and important difference between these two methods.
1. With both methods I stil need to program the dma controller with
the physical address and the start trigger.
2. I can still do the allocation whenever I want with both methods
(for example at the initialization of the driver),
3. Not sure what the actuall dma_map_single does (and if it really
necessary to use it), becuase it seems I could just translate the
virtual value from kmalloc into physical address and return it to the
dma controller.

Thank you for the time,
Ran