On Wed, Sep 19, 2012 at 07:59:45AM +0000, Arnd Bergmann wrote:

On Wednesday 19 September 2012, Hiroshi Doyu wrote:

quoted

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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