linux-kernel - Re: [PATCHv4] DMAEngine: Define interleaved transfer request api

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Date:	Mon, 3 Oct 2011 10:15:16 -0700
From:	"Williams, Dan J" <dan.j.williams@...el.com>
To:	Jassi Brar <jaswinder.singh@...aro.org>
Cc:	Russell King <rmk@....linux.org.uk>,
	Barry Song <21cnbao@...il.com>,
	Vinod Koul <vinod.koul@...el.com>,
	linux-kernel@...r.kernel.org,
	DL-SHA-WorkGroupLinux <workgroup.linux@....com>,
	Dave Jiang <dave.jiang@...el.com>
Subject: Re: [PATCHv4] DMAEngine: Define interleaved transfer request api

On Mon, Oct 3, 2011 at 9:19 AM, Jassi Brar <jaswinder.singh@...aro.org> wrote:
> On 3 October 2011 21:43, Russell King <rmk@....linux.org.uk> wrote:
>> On Mon, Oct 03, 2011 at 11:54:23AM +0530, Jassi Brar wrote:
>>> On 2 October 2011 06:03, Barry Song <21cnbao@...il.com> wrote:
>>> > 2011/10/2 Jassi Brar <jaswinder.singh@...aro.org>
>>>
>>> >> > For example, it can't use
>>> >> > MEM_TO_MEM to map, it still need to know whether the memory is source
>>> >> > or dest.
>>> >> MEM_TO_MEM means "From Memory Source To Memory Destination"
>>> >>  Map Src buffer with DMA_TO_DEVICE and Dst buffer with DMA_FROM_DEVICE
>>> >>
>>> >> MEM_TO_DEV means "From Memory Source To FIFO Destination"
>>> >>  Map Src buffer with DMA_TO_DEVICE.
>>> >>
>>> >> DEV_TO_MEM means "From FIFO Source To Memory Destination"
>>> >>  Map Dst buffer with DMA_FROM_DEVICE
>>> >>
>>> >> DEV_TO_DEV means "From FIFO Source To FIFO Destination"
>>> >>
>>> >> What else would you want to know ?
>>> >
>>> > that is the problem. for example, drivers can't use MEM_TO_MEM as a
>>> > flag to do dma mapping. so xfer_direction can't cover all that
>>> > dma_data_direction can do.  that's why you need both
>>> > dma_data_direction and xfer_direction with some similar flags in them.
>>> >
>>> The client drivers map the src/dst buffers and the dmac driver unmaps
>>> them by default(!). For which, the dmac driver doesn't look at anything
>>> other than
>>>      DMA_COMPL_SKIP_SRC/DST_UNMAP
>>>      DMA_COMPL_SRC/DST_UNMAP_SINGLE
>>>   bits of 'enum dma_ctrl_flags'.
>>> For this unmap'ing purpose, the usage of dma_data_direction is already
>>> internal to the dmac driver.
>>
>> No.  Slave DMA engine drivers do *not* (and if they do, they should *not*)
>> honour the unmapping of submitted buffers.
>>
>> The unmapping of these buffers by the DMA engine driver is intended to be
>> done for the async_tx API and not slave DMA.
>>
> The proposed api is usable by both Slave as well as Async(Memcpy etc).
> So it *does* matter here.

I think the confusion is reduced if you don't try to use this api for
mem-to-mem transfers.  Then you can use DMA_NONE to indicate the
dev-to-dev case.  If a mem-to-mem user arrives we can revisit
xfer_direction, but as it stands it seems this is primarily useful for
slave-dma, i.e. I don't see async_tx_dma or net_dma switching to this
scheme anytime soon, if ever.  Are there other mem-to-mem use cases
that would use this?

Also in dmaxfer_template I do not understand the need for src_inc and
dst_inc.  Aren't those properties that the client would know about the
slave device?

--
Dan
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