Re: [PATCH 1/4] block: use ARCH_KMALLOC_MINALIGN as the default dma pad mask

[FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>]

On Wed, 21 May 2008 09:26:22 +0800
Herbert Xu <herbert@gondor.apana.org.au> wrote:

> On Wed, May 21, 2008 at 01:09:41AM +0900, FUJITA Tomonori wrote:
> >
> > Then, you don't need to use ARCH_KMALLOC_MINALIGN. 8 bytes align works
> > for you on all the architectures.
> 
> DMA isn't the only thing that requires alignment.  The CPU needs
> it too.  Also using a constant like 8 is broken because if we used
> a value larger than the alignment guaranteed by kmalloc then the
> context may end up unaligned.

ARCH_KMALLOC_MINALIGN represents DMA alignment since we guarantee
kmalloced buffers can be used for DMA.

Only non coherent architecutures defines ARCH_KMALLOC_MINALIGN to the
cache line size since an DMA'able object within a structure isn't
sharing a cache line with some other object (note that it not about
only alignment).

For your case, the alignment requirement for a pointer is appropriate
(it's about the CPU alignment requirement that you talk about. 8 bytes
alignment always works).

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