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

[ben-linux@fluff.org (Ben Dooks)]

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.