linux-kernel - Re: [PATCH] dmaengine: at_hdmac: add DMA slave transfers

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Message-Id: <20090731.125852.229722391.anemo@mba.ocn.ne.jp>
Date:	Fri, 31 Jul 2009 12:58:52 +0900 (JST)
From:	Atsushi Nemoto <anemo@....ocn.ne.jp>
To:	dan.j.williams@...el.com
Cc:	maciej.sosnowski@...el.com, haavard.skinnemoen@...el.com,
	nicolas.ferre@...el.com, linux-arm-kernel@...ts.arm.linux.org.uk,
	patrice.vilchez@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dmaengine: at_hdmac: add DMA slave transfers

On Wed, 29 Jul 2009 07:27:59 -0700, Dan Williams <dan.j.williams@...el.com> wrote:
> > Then, what should dma driver do when client driver did not set these
> > flags?  If it should call dma_unmap_sg(), the dma driver should keep
> > sg and direction somewhere...
> > 
> > Also, calling dma_map_sg() in its prep_slave_sg function will not fit
> > for sound drivers, which use DMA buffers prepared in its framework.
> > 
> > For slave DMA, doing all mapping/unmapping in DMA client is better,
> > isn't it?
> 
> Yes it is.  The whole point of the dma driver doing its own unmapping is 
> specifically for clients that use the async_tx api, i.e. where they do 
> not know if the transaction is carried out in hardware and have no way 
> of tracking the details necessary to perform the unmap.  DMA-slave 
> clients request specific channels and know the hardware details at a low 
> level, so it should not be too high an expectation to push dma mapping 
> responsibility to the client.

OK, I will send a patch to move all map_sg/unmap_sg for slave channel
to its client.  Affected drivers are at_hdmac, dw_dmac and atmel-mci.

I do not have any of them, I hope someone who have them can test it.
Thank you.

---
Atsushi Nemoto
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