On Tuesday 03 February 2015 14:41:09 Russell King - ARM Linux wrote:

On Tue, Feb 03, 2015 at 03:17:27PM +0100, Arnd Bergmann wrote:

quoted

On Tuesday 03 February 2015 09:04:03 Rob Clark wrote:

quoted

Since I'm stuck w/ an iommu, instead of built in mmu, my plan was to
drop use of dma-mapping entirely (incl the current call to dma_map_sg,
which I just need until we can use drm_cflush on arm), and
attach/detach iommu domains directly to implement context switches.
At that point, dma_addr_t really has no sensible meaning for me.

I think what you see here is a quite common hardware setup and we really
lack the right abstraction for it at the moment. Everybody seems to
work around it with a mix of the dma-mapping API and the iommu API.
These are doing different things, and even though the dma-mapping API
can be implemented on top of the iommu API, they are not really compatible.

I'd go as far as saying that the "DMA API on top of IOMMU" is more
intended to be for a system IOMMU for the bus in question, rather
than a device-level IOMMU.

If an IOMMU is part of a device, then the device should handle it
(maybe via an abstraction) and not via the DMA API.  The DMA API should
be handing the bus addresses to the device driver which the device's
IOMMU would need to generate.  (In other words, in this circumstance,
the DMA API shouldn't give you the device internal address.)

Exactly. And the abstraction that people choose at the moment is the
iommu API, for better or worse. It makes a lot of sense to use this
API if the same iommu is used for other devices as well (which is
the case on Tegra and probably a lot of others). Unfortunately the
iommu API lacks support for cache management, and probably other things
as well, because this was not an issue for the original use case
(device assignment on KVM/x86).

This could be done by adding explicit or implied cache management
to the IOMMU mapping interfaces, or by extending the dma-mapping
interfaces in a way that covers the use case of the device managing
its own address space, in addition to the existing coherent and
streaming interfaces.

	Arnd