When do i need to allocate DMA buffer

[sjoyeau@wanadoo.fr]

Hi Wael,

1. The DMA engine source and destination registers are based on
physical memory addresses, whatever is the bus connected on, thus you
need to provide it physical addresses.
It looks like you are a bit confused about the terminology "DMA
buffer": a DMA buffer is either the source and/or destination buffer,
but not a intermediate buffer that you could bypass. Perhaps anybody
speaking english more fluently can explain that better than me.

2. Know nothing about Linux DMA API, but more thant a bit about
MPC8555 DMA, and I can say it's quite easy to program, directly
(through DMA registers) if your application doesn't require
portability across platforms.
AFAIK, the better way to allocate source and/or destination RAM buffer
in the kernel side is dma_alloc_coherent(): that's one seems rather
portable !

Best regards.
--
sj

2009/1/12 wael showair <showair2003 at yahoo.com>:
>
> Hi All,
> i have MPC8555 processor, also i have linux-kernel 2.6.19-rc5 on my board.i
> want to use the DMA on my board, just a simple example is enough for me but
> during my reading on how to use the DMA i have some questions:
> 1. The DMA buffer is always needed for all kind of buses or not? (I think
> this is valid in the ISA bus but not in the PCI bus, plz correct me if i m
> wrong?)
> Note that my target is to transfer data from memory location to another
> memory location & it will be an overhead if i copy the data from the src to
> the DMA buffer then from the DMA buffer to the Destination.
> isn't there any other way to avoid using that DMA buffer & to copy data from
> src to dest directly?
> i read the documentation of my board & it tells me that it contain DMA
> engine which "is capable of transferring blocks of data from any legal
> address range to any
> other legal address range."
>
> 2. i have read DMA-API.txt in the linux Documentation folder but i m little
> confused about the APIs that exist in this file. Actually, it is stated in
> this file the following:
> "This document describes the DMA API.This API is split into two pieces.
> Part I describes the API and the corresponding pci_ API".
>
> so if the DMA controller is connected to the PCI bus, am i obligated to use
> the pci_API & for any other case (like ISA bus or any other bus types) shall
> i use this generic API?
> also what r the benefits of these APIs over the normal kmalloc? specially
> that i have read the following section in the DMA-mapping.txt:
> "If you acquired your memory via the page allocator
> (i.e. __get_free_page*()) or the generic memory allocators
> (i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
> that memory using the addresses returned from those routines."
>
> thanks in advance for ur help
> --
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-- 
------------------
Sylvain JOYEAU
Freelance Engineer
Software RT-OS R&D
sylvain.joyeau at gmail.com
"A good idea is one side of the coin. The other side is the practical
usefulness". J. Liedke.