Hello, On Friday, December 23, 2011 5:35 PM Matthew Wilcox wrote: > 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? It is about write-combining from the CPU point of view. Right now there are no devices with such advanced memory interface to do write combining on the DMA side, but I believe that they might appear at some point in the future as well. > > 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? On ARM architecture it is already implemented this way and a bunch of drivers use dma_mmap_coherent/dma_mmap_writecombine calls. We would like to standardize these calls across all architectures. > > 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. That is just a starting point for the discussion. I thought about this API a bit and came to conclusion that there is no much difference between a dma_alloc_coherent which creates a mapping in kernel virtual space and the one that does not. It is just a hint from the driver that it will not use that mapping at all. Of course this attribute makes sense only together with adding a dma_mmap_attrs() call, because otherwise drivers won't be able to get access to the buffer data. On coherent architectures where dma_alloc_coherent is just a simple wrapper around alloc_pages_exact() such attribute can be simply ignored without any impact on the drivers (that's the main idea behind dma attributes!). However such hint will help a lot on non-coherent architectures where additional work need to be done to provide a cohenent mapping in kernel address space. It also saves some precious kernel resources like vmalloc address range. Best regards -- Marek Szyprowski Samsung Poland R&D Center
