atmel_spi: DMA-imposed xfer buffer requirements

atmel_spi: DMA-imposed xfer buffer requirements

[Andrei Rylin]

Haavard,
thanks for clarification about cs_change.
I have another (hopefully last) question about atmel_spi driver.
atmel_spi driver DMA-maps the buffers pointed to by spi_transfer.
What are alignment and size requirements for the buffers ?
How to expressed them in a platform-independent way,
i.e. suitable for use with other SPI master drivers ?
Can the buffers be in stack ? Or do they always have to be
allocated from a non-paged pool ?
Thanks,
Andrei.

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Re: atmel_spi: DMA-imposed xfer buffer requirements

[Haavard Skinnemoen]

"Andrei Rylin" <port777-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> Haavard,
> thanks for clarification about cs_change.
> I have another (hopefully last) question about atmel_spi driver.
> atmel_spi driver DMA-maps the buffers pointed to by spi_transfer.
> What are alignment and size requirements for the buffers ?
> How to expressed them in a platform-independent way,
> i.e. suitable for use with other SPI master drivers ?

Please see Documentation/dma-mapping.txt, in particular the section
called "What memory is DMA'able?"

The actual size and alignment requirements are platform-dependent, but
in general, cache-aligned buffers are required. You can however use
fields embedded in other structs as long as you can guarantee that
  - The struct itself is properly aligned.
  - Any fields sharing a cacheline with the buffer must not be accessed
    while the transfer is ongoing.

The latter requirement usually boils down to "don't access any fields
at all" unless you know extremely well what you're doing.

> Can the buffers be in stack ? Or do they always have to be
> allocated from a non-paged pool ?

No, they can not be allocated on the stack. kmalloc() is ok, however.

Haavard

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Re: atmel_spi: DMA-imposed xfer buffer requirements

[Ben Dooks]

On Mon, Oct 20, 2008 at 08:56:41AM -0700, Andrei Rylin wrote:
> Haavard,
> thanks for clarification about cs_change.
> I have another (hopefully last) question about atmel_spi driver.
> atmel_spi driver DMA-maps the buffers pointed to by spi_transfer.
> What are alignment and size requirements for the buffers ?
> How to expressed them in a platform-independent way,
> i.e. suitable for use with other SPI master drivers ?
> Can the buffers be in stack ? Or do they always have to be
> allocated from a non-paged pool ?

no, I belive no linux arch allows dma to/from the stack.

dma_alloc_xxx() and friends are your best bet for allocating such
memory.

> Thanks,
> Andrei.
> 
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-- 
-- 
Ben

Q:      What's a light-year?
A:      One-third less calories than a regular year.


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