Re: [RFC 0/5] ARM: dma-mapping: New dma_map_ops to control IOVA more precisely

[Joerg Roedel <joerg.roedel@amd.com>]

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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