Hello Saumendra,

I would suggest you to go through the following link: 1) This will explain
need of DMA with practical scenario 2) ARM DMA Mapping with example

https://sanjeevsharmaengg.wordpress.com/2014/08/26/why-dma-required-in-linux-kernel/

http://linuxkernelhacker.blogspot.in/2014/07/arm-dma-mapping-explained.html

I hope these link will give you more confidence around understanding of DMA.

Regards
Sanjeev Sharma

On Mon, Feb 8, 2016 at 12:29 PM, Saumendra Dash [off-list ref] wrote:

quoted

quoted

Hi Ran,

             the api which you have mentioned...

quoted

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?

quoted

quoted

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 .

Hi Vishwas,

quoted

That's fully clarify the questions about dma_alloc_coherent.

quoted

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

and dma_map_single.

quoted

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.

quoted

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

quoted

seems I could just translate the virtual value from kmalloc into physical

address and return it to the dma controller.

DMA transfers are done either in BURST mode or CYCLE STEALING mode.
In Burst mode, the bus is captured for the entire duration of the
transfer  from the SRC to DST.  In this case, the bus will  be released
when the Xfer is complete, so obviously it is not an efficient way of doing
DMA.  DMA_ALLOC_COHERENT() does this way.
In Cycle Stealing mode, the DMA controller grab the bus when free, send a
byte  and then free the bus immediately. This process is repeated till the
Xfer is complete, it is very efficient sine the bus is not grabbed for the
entire transaction to complete.  DMA_MAP_SINGLE() does this way.

 Hope this helps.

Thanks,
Saumendra


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