Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms

Russell King - ARM Linux
2015-02-03T15:22:30Z

On Tue, Feb 03, 2015 at 03:52:48PM +0100, Arnd Bergmann wrote:
> On Tuesday 03 February 2015 14:41:09 Russell King - ARM Linux wrote:
> > 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.

Don't we already have those in the DMA API? dma_sync_*() ?

dma_map_sg() - sets up the system MMU and deals with initial cache
coherency handling. Device IOMMU being the responsibility of the
GPU driver.

The GPU can then do dma_sync_*() on the scatterlist as is necessary
to synchronise the cache coherency (while respecting the ownership
rules - which are very important on ARM to follow as some sync()s are
destructive to any dirty data in the CPU cache.)

dma_unmap_sg() tears down the system MMU and deals with the final cache
handling.

Why do we need more DMA API interfaces?

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