On Fri, Dec 23, 2011 at 01:27:19PM +0100, Marek Szyprowski wrote: > The first issue we identified is the fact that on some platform (again, > mainly ARM) there are several functions for allocating DMA buffers: > dma_alloc_coherent, dma_alloc_writecombine and dma_alloc_noncoherent Is this write-combining from the point of view of the device (ie iommu), or from the point of view of the CPU, or both? > The next step in dma mapping framework update is the introduction of > dma_mmap/dma_mmap_attrs() function. There are a number of drivers > (mainly V4L2 and ALSA) that only exports the DMA buffers to user space. > Creating a userspace mapping with correct page attributes is not an easy > task for the driver. Also the DMA-mapping framework is the only place > where the complete information about the allocated pages is available, > especially if the implementation uses IOMMU controller to provide a > contiguous buffer in DMA address space which is scattered in physical > memory space. Surely we only need a helper which drivrs can call from their mmap routine to solve this? > Usually these drivers don't touch the buffer data at all, so the mapping > in kernel virtual address space is not needed. We can introduce > DMA_ATTRIB_NO_KERNEL_MAPPING attribute which lets kernel to skip/ignore > creation of kernel virtual mapping. This way we can save previous > vmalloc area and simply some mapping operation on a few architectures. I really think this wants to be a separate function. dma_alloc_coherent is for allocating memory to be shared between the kernel and a driver; we already have dma_map_sg for mapping userspace I/O as an alternative interface. This feels like it's something different again rather than an option to dma_alloc_coherent. -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step."
