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? -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
