dma cache coherency issue

[Russell King - ARM Linux]

On Tue, Mar 22, 2011 at 02:52:19AM +0800, Dongas wrote:
> Hi All,
> 
> Similar as stated in the following articles,
> http://lwn.net/Articles/2265/
> The issue is that i got a SDIO WiFi dirver which has the folliowing
> structure definitions:
>     struct iostruct {
>         ...
>         int ifield;
>         char dma_buffer[SMALL_SIZE];
>         ...
>     };
> And the ifield may share the same cache line with dma_buffer which is
> used for dma transfer.
> Consider the case that if CPU accesses ifield during the DMA transfer,
> then the CPU cache line may get the stale data from dma buffer.
> After DMA completes, in current kernel code(2.6.38), the dma_unmap_sg
> will finally call v7_dma_unmap_area and v7_dma_inv_range to clean and
> invalidate cache again for DMA_FROM_DEVICE operation.
> However, since the cache line already contains the stale data of dma
> buffer, the clean to write back the cache line to memory may cause
> data corruption.

Correct.

> My question is:
> Is it a bug of current linux kernel or still as above old article
> said, there's no better solutions for it?

It is a bug.

Another solution would be to allocate the dma buffer separately using
kmalloc, and store a pointer to it in the iostruct.  Don't forget to
free it after you've finished with it though.  That probably requires
the smallest overall change.