linux-kernel - Re: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

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Message-Id: <201104280840.09206.arnd@arndb.de>
Date:	Thu, 28 Apr 2011 08:40:08 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linaro-mm-sig@...ts.linaro.org
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"Russell King - ARM Linux" <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

On Wednesday 27 April 2011 23:37:51 Benjamin Herrenschmidt wrote:
> On Wed, 2011-04-27 at 08:35 +0100, Russell King - ARM Linux wrote:
> > On Thu, Apr 21, 2011 at 09:29:16PM +0200, Arnd Bergmann wrote:
> > > 2. Implement dma_alloc_noncoherent on ARM. Marek pointed out
> > >    that this is needed, and it currently is not implemented, with
> > >    an outdated comment explaining why it used to not be possible
> > >    to do it.
> > 
> > dma_alloc_noncoherent is an entirely pointless API afaics.
> 
> I was about to ask what the point is ... (what is the expected
> semantic ? Memory that is reachable but not necessarily cache
> coherent ?)

Drivers use this when they explicitly want to manage the caching
themselves. I think this is most interesting on big NUMA systems,
where you really want to use fast (local cached) memory and
then flush it explicitly to do dma. Very few drivers use this:

arnd@...rfel:~/linux-2.6$ git grep dma_alloc_noncoherent drivers/
drivers/base/dma-mapping.c:     vaddr = dma_alloc_noncoherent(dev, size, dma_handle, gfp);
drivers/net/au1000_eth.c:       aup->vaddr = (u32)dma_alloc_noncoherent(NULL, MAX_BUF_SIZE *
drivers/net/lasi_82596.c:#define DMA_ALLOC                        dma_alloc_noncoherent
drivers/net/sgiseeq.c:  sr = dma_alloc_noncoherent(&pdev->dev, sizeof(*sp->srings),
drivers/scsi/53c700.c:  memory = dma_alloc_noncoherent(hostdata->dev, TOTAL_MEM_SIZE,
drivers/scsi/sgiwd93.c: hdata->cpu = dma_alloc_noncoherent(&pdev->dev, HPC_DMA_SIZE,
drivers/tty/serial/mpsc.c:              } else if ((pi->dma_region = dma_alloc_noncoherent(pi->port.dev,
drivers/video/au1200fb.c:               fbdev->fb_mem = dma_alloc_noncoherent(&dev->dev,

> > So you can't have a dma_map_ops for the cache handling bits, a dma_map_ops
> > for IOMMU, and a dma_map_ops for the dmabounce stuff.  It just doesn't
> > work like that.
> 
> Well, the dmabounce and cache handling is one implementation that's just
> on/off with parameters no ?. iommu is different implementations. So the
> ops should be for the iommu backends. The dmabounce & cache handling is
> then done by those backends based on flags you stick in struct device
> for example.

Well, what we are currently discussing is to have a common implementation
for IOMMUs that provide the generic iommu_ops that the KVM people introduced.
Once we get there, we only need a single dma_map_ops structure for all
IOMMUs.

	Arnd
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