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

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Date:	Fri, 30 Sep 2011 14:43:10 +0800
From:	Barry Song <21cnbao@...il.com>
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	Jassi Brar <jaswinder.singh@...aro.org>,
	linux-kernel@...r.kernel.org, dan.j.williams@...el.com,
	rmk@....linux.org.uk,
	DL-SHA-WorkGroupLinux <workgroup.linux@....com>
Subject: Re: [PATCHv4] DMAEngine: Define interleaved transfer request api

2011/9/29 Vinod Koul <vinod.koul@...el.com>:
> On Wed, 2011-09-28 at 20:45 +0530, Jassi Brar wrote:
>> >> +enum xfer_direction {
>> >> +     MEM_TO_MEM,
>> >> +     MEM_TO_DEV,
>> >> +     DEV_TO_MEM,
>> >> +     DEV_TO_DEV,
>> > Use/update dma_data_direction.
>> dma_data_direction is the mapping attribute of a buffer.
>> While that info is what some dmac driver might need ultimately, our
>> main aim here is to tell exactly if Src and Dst is Memory or a
>> device's FIFO.
>>
>> Mapping attribute of src/dst buffers could be very well deducted from
>> xfer_direction, but dma_data_direction isn't meant to tell if Src and
>> Dst is Mem or FIFO.
>> Also, for (SLAVE && !src_inc && !dst_inc) we need to disambiguate
>> three options  Mem->Fifo,  Fifo->Mem,  *Fifo->Fifo*(not impossible)
>> So while using dma_data_direction would work today, that sure is hacky
>> and not future-proof.
> That is why I said use/update, you missed the update part.
>
> One way would be to use direction field with new flag indicating if its
> memory or device transfer, otherwise you can expand this enum.
>
> Point is few things are already there so improve upon it rather than
> have two structures in kernel doing similar things...

i support we can update dma_data_direction to:   MEM_TO_MEM,
MEM_TO_DEV,  DEV_TO_MEM, DEV_TO_DEV.

Russell gave a good explain about the mapping decided by MEM_TO_DEV
and  DEV_TO_MEM. Then even a memory-to-memory DMA will require
MEM_TO_DEV/DEV_TO_MEM flag to do mapping because only
MEM_TO_DEV/DEV_TO_MEM can decide what dma_map_single() class functions
will operate on cache. But that is really a bad idea to users. To
users, i just want DMA_TO_DEV to be a DMA which transfers data from
memory to device. otherwise, it should have a different name. I think
how to map buffer is just a software detail. but the DMA_TO_DEV should
be a right description to the real world.

anyway, the update can be a seperate patch not in this one.

>
> --
> ~Vinod

-barry
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