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

[David Yang]

I think so ，but it looks like that it did not return uncacheble and
unbufferable memory.

what flushing command could I use in driver for  testing?

On Thu, Jul 15, 2010 at 5:31 PM, Ben Dooks <ben-linux at fluff.org> wrote:
> On Thu, Jul 15, 2010 at 05:13:57PM +0800, David Yang wrote:
>> hi ,everybody:
>>
>>        I encountered this problem when porting my ethernet driver from
>> linux-2.6.28 to linux-2.6.33-arm1.
>>
>>        In the linux-2.6.28, I used the dma_alloc_coherent to share the
>> informations between cpu and ethernet device.The program flow in the
>> function ndo_start_xmit:
>>                1,preparing the struct sk_buff->data for device internal DMA to
>> read,using the dma_map_single function.
>>                2,update the information in the memory which is allocated by
>> dma_alloc_coherent to tell the device DMA the data is readable.
>>                3,write the device register to inform the device DMA
>> to read the data.
>>                4,cpu captures the interrupt of reading completion
>> form the device.
>>
>>        In the linux-2.6.28,the whole program flow soon complete in order.
>>        But when the driver was ported to the linux-2.6.33-arm1,the problem
>> came:
>>                 I found when the cpu executed from step 1-3, the
>> device DMA alarm the
>> step 2 was not completed.As a result,the step 4 was not triggered.This
>> is different form linux-2.6.28.
>>
>>                After some tests, I think the problem comes from the
>> dma_alloc_coherent.It looks like ,in the linux-2.6.33-arm1, when the
>> memory allocated by dma_alloc_coherent is written,the data entry into
>> the ddr much slower than the same process in linux-2.6.28.Therefore,when
>> cpu has executed the step 3, the step 2 has not yet completed.So the DMA
>> can't get the correct information ,and the step 4 will never be reached.
>>
>>        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.
>
> --
> Ben
>
> Q:      What's a light-year?
> A:      One-third less calories than a regular year.
>
>