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

[David Yang]

I have to use the linux-2.6.33-arm1 ,because I will use the android .

And ARM only offer the android patch for 2.6.33.

On Thu, Jul 15, 2010 at 8:25 PM, Catalin Marinas
<catalin.marinas at arm.com> wrote:
> On Thu, 2010-07-15 at 17:13 +0800, David Yang wrote:
>>        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.
>
> You need a wmb() before the writel() which tells the device to start the
> DMA transfer. We'd like to make this the default (in the write* macros)
> and there are patches for review on the list.
>
> Without the barrier, the card may transfer garbage since the date
> written in step (2) may have not reached the RAM.
>
>>                4,cpu captures the interrupt of reading completion
>> form the device.
> [...]
>>                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.
>
> Correct.
>
> Note that I haven't fixed this issue in the 2.6.33-arm1 kernel. My
> opinion for some time was that drivers should take care of memory
> ordering and use the correct barriers. But after discussion on LKML, I
> eventually changed my mind (and we now have a small drop in performance
> for PIO drivers on ARMv6/v7).
>
> Could you use a newer kernel? I can backport the I/O accessors patches
> to 2.6.33-arm1 but only after Russell is ok with the latest version (to
> avoid additional work on my side).
>
> --
> Catalin
>
>