Problem with dma_alloc_coherent at linux-2.6.33-arm1 , with RealView platform,board PBX-A9 and armv7 instructions.

[David Yang]

I have explicitly added the barrier() in my driver, but it did't work.

I will go on testing until solve the problem. I need more advice.

Thanks!

On Thu, Jul 15, 2010 at 5:38 PM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Thu, Jul 15, 2010 at 10:31:07AM +0100, Ben Dooks wrote:
>> On Thu, Jul 15, 2010 at 05:13:57PM +0800, David Yang wrote:
>> >        I don't know the reason until now.I guess the memory allocated by the
>> > dma_alloc_coherent may be cached....if not , why it is so slowly?
>>
>> The dma_alloc_coherent coherent should return uncachable and un-bufferable
>> memory, otherwise you meed explicit flushing commands when changing between
>> the HW and CPU ownership.
>
> It's a result of the weakly ordered memory model - writes to memory
> are now delayed and can be re-ordered, but not across the appropriate
> barrier instructions.
>
> Lack of barrier instructions makes that these writes can arrive in
> unexpected orders, which will cause exactly these kinds of problems.
>
> Catalin has been tracking down where these barriers need to be added,
> so we're getting these issues solved - but we can only find them
> through people using the kernel, reporting problems like this, and
> fixing the missing places.
>