On 08/09/2017 07:20 PM, Dan Williams wrote:
> On Wed, Aug 9, 2017 at 7:15 PM, Dan Williams <dan.j.willi...@intel.com> wrote:
>> On Mon, Aug 7, 2017 at 9:39 AM, Dave Jiang <dave.ji...@intel.com> wrote:
>>> In preparation of adding an API to perform SG to/from buffer for dmaengine,
>>> we will change DMA_SG to DMA_SG_SG in order to explicitly making clear what
>>> this op type is for.
>>>
>>> Signed-off-by: Dave Jiang <dave.ji...@intel.com>
>>> ---
>>>  Documentation/dmaengine/provider.txt |    2 +-
>>>  drivers/crypto/ccp/ccp-dmaengine.c   |    2 +-
>>>  drivers/dma/at_hdmac.c               |    8 ++++----
>>>  drivers/dma/dmaengine.c              |    2 +-
>>>  drivers/dma/dmatest.c                |   12 ++++++------
>>>  drivers/dma/fsldma.c                 |    2 +-
>>>  drivers/dma/mv_xor.c                 |    6 +++---
>>>  drivers/dma/nbpfaxi.c                |    2 +-
>>>  drivers/dma/ste_dma40.c              |    6 +++---
>>>  drivers/dma/xgene-dma.c              |    4 ++--
>>>  drivers/dma/xilinx/zynqmp_dma.c      |    2 +-
>>>  include/linux/dmaengine.h            |    2 +-
>>>  12 files changed, 25 insertions(+), 25 deletions(-)
>>
>> Given the prevalence and age of DMA_SG I think we should call the new
>> op DMA_SG_SINGLE, or something like that, so that we don't surprise
>> someone who was expecting the old command type.
> 
> Oh wait, you already call the new op DMA_MEMCPY_SG, so why does the
> old one need to change? ...and thinking about it further why do we
> need a new op at all? Couldn't we just pass in a single entry
> scatterlist that was setup on the stack with memcpy target address?

That would probably work if we can do dma_map_sg() before submit and
dma_unmap_page() on completion since we'll lose the sg entry. I also
have concerns with the DMA_SG function provided in the ioatdma driver
since it really doesn't do scatter gather and it's all software
abstracted. It's not a big deal to support a single entry SG, but it
wouldn't be supporting real SG to SG setup without more complex code. I
worry about the overhead and messiness of it. Of course if you are ok
with providing a DMA_SG function that really doesn't do exactly what
it's advertised to do and only cater to pmem usage.
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