linux-kernel - Re: [RFC v2 0/5] dmaengine: Slave DMA interface and example users

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Date:	Wed, 30 Jan 2008 10:39:47 -0700
From:	"Dan Williams" <dan.j.williams@...el.com>
To:	"Haavard Skinnemoen" <hskinnemoen@...el.com>
Cc:	"David Brownell" <david-b@...bell.net>,
	linux-kernel@...r.kernel.org,
	"Shannon Nelson" <shannon.nelson@...el.com>, kernel@...32linux.org,
	"Francis Moreau" <francis.moro@...il.com>,
	"Paul Mundt" <lethal@...ux-sh.org>,
	"Vladimir A. Barinov" <vbarinov@...mvista.com>,
	"Pierre Ossman" <drzeus-list@...eus.cx>
Subject: Re: [RFC v2 0/5] dmaengine: Slave DMA interface and example users

On Jan 30, 2008 1:56 AM, Haavard Skinnemoen <hskinnemoen@...el.com> wrote:
> On Tue, 29 Jan 2008 22:56:14 -0800
> David Brownell <david-b@...bell.net> wrote:
>
> > On Tuesday 29 January 2008, Haavard Skinnemoen wrote:
> > >
> > > Btw, there's one issue I forgot to mention: I believe the DMA Engine
> > > framework is currently misusing the DMA mapping API, and this patchset
> > > makes things worse.
> > >
> > > Currently, the async_tx bits of the API do the required calls to
> > > dma_map_single() and/or dma_map_page(), but they rely on the driver to
> > > do the unmapping. This is problematic ...
> > >
> > > How do we solve this?
> >
> > How about:  for peripheral DMA, don't let the engine see anything
> > except dma_addr_t values.
>
> I don't think it does, but the dma_addr_t value is enough to call
> dma_unmap_single() and dma_unmap_page().

Right, dma_addr_t values are all the driver sees in the current scheme.

>
> > The engine needs to be able to dma_alloc_coherent() memory too,
> > which is pre-mapped.
>
> Right, which is another argument for not doing any unmapping in the DMA
> engine driver. We really need to push this responsibility to the client.
>

Agreed, the issue is how to do this without requiring an
interrupt+callback sequence for each transaction or requiring the
client to carry per transaction unmap-data.  For example NET_DMA never
sees a dma_addr_t and assumes that all it needs to care about is the
last transaction in a sequence.  Since it is alive for the duration of
a transaction, we could put unmap data in dma_async_tx_descriptor
along with an unmap function pointer since dma_unmap* routines have an
equal number of parameters.  But I just got through making this
structure smaller so maybe there is a better way.

--
Dan
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