svn commit: r239597 - head/sys/arm/arm

[John Baldwin]

On Wednesday, August 22, 2012 6:48:50 pm Oleksandr Tymoshenko wrote:
> Author: gonzo
> Date: Wed Aug 22 22:48:50 2012
> New Revision: 239597
> URL: http://svn.freebsd.org/changeset/base/239597
> 
> Log:
>   Do not change "cachable" attribute for DMA memory allocated with
>   BUS_DMA_COHERENT attribute
>   
>   The minimum unit for changing "cachable" attribute is page, so call
>   to pmap_change_attr effectively disable cache for all pages that newly
>   allocated DMA memory region spans on. The problem is that general-purpose
>   memory could reside on these pages too and disabling cache might affect
>   performance. Moreover ldrex/strex operators raise Data Abort exception
>   when accessing memory on page with "cachable" attribute off.
>   
>   BUS_DMA_COHERENT does nto require memory to be coherent. It just suggests
>   to do best effort for reducing synchronization overhead.

Alternatively you could force an allocation with BUS_DMA_COHERENT to always
allocate at least one page.  Also, it is probably better to match what I 
recently did in x86 and use kmem_alloc_contig() or kmem_alloc_attr() for
allocations with a custom memory attribute rather than using 
pmap_change_attr() directly.

-- 
John Baldwin