[MTD/NAND] Blackfin NFC driver DMA bug ?

[Bryan Wu]

On Fri, Feb 22, 2008 at 3:05 PM, David Woodhouse <dwmw2 at infradead.org> wrote:
>
>  On Fri, 2008-02-22 at 10:52 +0800, Bryan Wu wrote:
>  > Oh, I am not fully understand your concern.  The code is invalidating
>  > or flushing buf before DMA operation.
>  > And invalidate and flush operation is OK for buf which is not
>  > cache-aligned on Blackfin arch. it also should be
>  > OK for other arch.
>
>  The problem occurs when your buffer is not aligned at the beginning of a
>  cache line. If there is other data on the _same_ cache line as your
>  buffer, and you invalidate the cache, then you may cause data loss
>  outside the buffer.
>

Yes, I understand that. I prefer to make sure this buffer issue in the
MTD core API.

>  Also, in some systems you must make sure that allocations to be used for
>  DMA are from a certain memory pool.
>
>  As for the original question... I'm not sure. At the moment I don't
>  believe it's true that all such buffers are suitable for DMA. Perhaps it
>  would be sensible for us to redefine the MTD API so that it is required
>  (and fix the users).
>

Exactly, maybe xxx_nand_dma_read() and xxx_nand_dma_write().
The buffer for this DMA functions should be cache-aligned.

>  For a long time, flash I/O was always done by the CPU instead of DMA, so
>  it wasn't an interesting question. I did start wondering when I
>  implemented support for the CAFÉ controller on OLPC, but then that
>  turned out to need bounce buffers anyway so it escaped my attention
>  again.
>

We might need to add DMA support in the MTD NAND interface, because
more and more SoC support on-chip NAND controller with DMA support.

Thanks a lot
-Bryan Wu