linux-kernel - Re: [RFC 0/5] ARM: dma-mapping: New dma_map

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Date:	Wed, 19 Sep 2012 14:50:20 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Hiroshi Doyu <hdoyu@...dia.com>,
	"m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"minchan@...nel.org" <minchan@...nel.org>,
	"chunsang.jeong@...aro.org" <chunsang.jeong@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"subashrp@...il.com" <subashrp@...il.com>,
	"linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	Krishna Reddy <vdumpa@...dia.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"kyungmin.park@...sung.com" <kyungmin.park@...sung.com>,
	"pullip.cho@...sung.com" <pullip.cho@...sung.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC 0/5] ARM: dma-mapping: New dma_map_ops to control IOVA more
 precisely

On Wed, Sep 19, 2012 at 07:59:45AM +0000, Arnd Bergmann wrote:
> On Wednesday 19 September 2012, Hiroshi Doyu wrote:
> > I guess that it would work. Originally I thought that using DMA-API
> > and IOMMU-API together in driver might be kind of layering violation
> > since IOMMU-API itself is used in DMA-API. Only DMA-API used in driver
> > might be cleaner. Considering that DMA API traditionally handling
> > anonymous {bus,iova} address only, introducing the concept of
> > specific address in DMA API may not be so encouraged, though.
> > 
> > It would be nice to listen how other SoCs have solved similar needs.
> 
> In general, I would recommend using only the IOMMU API when you have a device
> driver that needs to control the bus virtual address space and that manages
> a device that resides in its own IOMMU context. I would recommend using
> only the dma-mapping API when you have a device that lives in a shared
> bus virtual address space with other devices, and then never ask for
> a specific bus virtual address.
> 
> Can you explain what devices you see that don't fit in one of those two
> categories?

Well, I don't think that a driver should limit to one of these 2 APIs. A
driver can very well use the IOMMU-API during initialization (for
example to map the firmware to an address the device expects it to be)
and use the DMA-API later during normal operation to exchange data with
the device.

When a device driver would only use the IOMMU-API and needs small
DMA-able areas it has to re-implement something like the DMA-API
(basically an address allocator) for that. So I don't see a reason why
both can't be used in a device driver.

Regards,

	Joerg

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