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

Alan Cox alc at rice.edu
Thu Aug 23 16:00:05 UTC 2012


On 08/23/2012 06:46, John Baldwin wrote:
> 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.
>

The arm pmap doesn't have the necessary support for memory attributes.  
pmap_page_{get,set}_memattr() are just stubs.

Alan



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