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

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Message-ID: <20080130095641.39eaad13@dhcp-252-066.norway.atmel.com>
Date:	Wed, 30 Jan 2008 09:56:41 +0100
From:	Haavard Skinnemoen <hskinnemoen@...el.com>
To:	David Brownell <david-b@...bell.net>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	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 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().

> 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.

Haavard
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