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SubjectRE: [PATCH 3/7] ARM: Samsung: update/rewrite Samsung SYSMMU (IOMMU) driver

On Friday, March 11, 2011 12:51 PM Arnd Bergmann wrote:

> On Friday 11 March 2011, Marek Szyprowski wrote:
> >
> > We followed the style of iommu API for other mainline ARM platforms (both OMAP and MSM
> > also have custom API for their iommu modules). I've briefly checked include/linux/iommu.h
> > API and I've noticed that it has been designed mainly for KVM support. There is also
> > include/linux/intel-iommu.h interface, but I it is very specific to intel gfx chips.
> The MSM code actually uses the generic iommu.h code, using register_iommu, so
> the drivers can use the regular iommu_map.
> I believe the omap code predates the iommu API, and should really be changed
> to use that. At least it was added before I started reviewing the code.
> The iommu API is not really meant to be KVM specific, it's just that the
> in-tree users are basically limited to KVM at the moment. Another user that
> is coming up soon is the vmio device driver that can be used to transparently
> pass devices to user space. The idea behind the IOMMU API is that you can
> map arbitrary bus addresses to physical memory addresses, but it does not
> deal with allocating the bus addresses or providing buffer management such
> as cache flushes.

Yea, I've noticed this and this basically what we expect from iommu driver.
However the iommu.h API requires a separate call to map each single memory page.
This is quite ineffective approach and imho the API need to be extended to allow
mapping of the arbitrary set of pages.

> > Is there any example how include/linux/dma-mapping.h interface can be used for iommu
> > mappings?
> The dma-mapping API is the normal interface that you should use for IOMMUs
> that sit between DMA devices and kernel memory. The idea is that you
> completely abstract the concept of an IOMMU so the device driver uses
> the same code for talking to a device with an IOMMU and another device with
> a linear mapping or an swiotlb bounce buffer.
> This means that the user of the dma-mapping API does not get to choose the
> bus addresses, but instead you use the API to get a bus address for a
> chunk of memory, and then you can pass that address to a device.
> See arch/powerpc/kernel/iommu.c and arch/x86/kernel/amd_iommu.c for common
> examples of how this is implemented. The latter one actually implements
> both the iommu_ops for iommu.h and dma_map_ops for dma-mapping.h.

Thanks for your comments! We will check how is it suitable for our case.

Best regards
Marek Szyprowski
Samsung Poland R&D Center

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