[RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device memory management

Russell King - ARM Linux wrote:

On Wed, Jul 07, 2010 at 03:44:27PM -0700, Zach Pfeffer wrote:

quoted

The DMA API handles the allocation and use of DMA channels. It can
configure physical transfer settings, manage scatter-gather lists,
etc. 

You're confused about what the DMA API is.  You're talking about
the DMA engine subsystem (drivers/dma) not the DMA API (see
Documentation/DMA-API.txt, include/linux/dma-mapping.h, and
arch/arm/include/asm/dma-mapping.h)

Thanks for the clarification. 

quoted

The VCM allows all device buffers to be passed between all devices in
the system without passing those buffers through each domain's
API. This means that instead of writing code to interoperate between
DMA engines, IOMMU mapped spaces, CPUs and physically addressed
devices the user can simply target a device with a buffer using the
same API regardless of how that device maps or otherwise accesses the
buffer.

With the DMA API, if we have a SG list which refers to the physical
pages (as a struct page, offset, length tuple), the DMA API takes
care of dealing with CPU caches and IOMMUs to make the data in the
buffer visible to the target device.  It provides you with a set of
cookies referring to the SG lists, which may be coalesced if the
IOMMU can do so.

If you have a kernel virtual address, the DMA API has single buffer
mapping/unmapping functions to do the same thing, and provide you
with a cookie to pass to the device to refer to that buffer.

These cookies are whatever the device needs to be able to access
the buffer - for instance, if system SDRAM is located at 0xc0000000
virtual, 0x80000000 physical and 0x40000000 as far as the DMA device
is concerned, then the cookie for a buffer at 0xc0000000 virtual will
be 0x40000000 and not 0x80000000.

It sounds like I've got some work to do. I appreciate the feedback.

The problem I'm trying to solve boils down to this: map a set of
contiguous physical buffers to an aligned IOMMU address. I need to
allocate the set of physical buffers in a particular way: use 1 MB
contiguous physical memory, then 64 KB, then 4 KB, etc. and I need to
align the IOMMU address in a particular way. I also need to swap out the
IOMMU address spaces and map the buffers into the kernel.

I have this all solved, but it sounds like I'll need to migrate to the DMA
API to upstream it.

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