Russell King - ARM Linux wrote: > On Wed, Jul 07, 2010 at 03:44:27PM -0700, Zach Pfeffer wrote: >> 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. > >> 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. -- Sent by an employee of the Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum. -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
