On 09/29/2014 10:32 AM, Akinobu Mita wrote: > 2014-09-29 21:09 GMT+09:00 Peter Hurley <pe...@hurleysoftware.com>: >> On 09/27/2014 08:31 PM, Akinobu Mita wrote: >>> 2014-09-27 23:30 GMT+09:00 Peter Hurley <pe...@hurleysoftware.com>: >>>> On 04/15/2014 09:08 AM, Akinobu Mita wrote: >>>>> This patch set enhances the DMA Contiguous Memory Allocator on x86. >> >> [...] >> >>>> What this patchset does is restrict all iommu configurations which can >>>> map all of system memory to one _very_ small physical region, thus >>>> disabling >>>> the whole point of an iommu. >>>> >>>> Now I know why my GPU is causing paging to disk! And why my RAID controller >>>> stalls for ages when I do a git log at the same time as a kernel build! >>> >>> The solution I have for this is that instead of trying to >>> dma_alloc_from_contiguous() firstly, call alloc_pages() in >>> dma_alloc_coherent(). >>> dma_alloc_from_contiguous() should be called only when alloc_pages() is >>> failed >>> or DMA_ATTR_FORCE_CONTIGUOUS is specified in dma_attr. >> >> Why is all this extra complexity being added when there are no X86 users >> of DMA_ATTR_FORCE_CONTIGUOUS? > > I misunderstood DMA_ATTR_FORCE_CONTIGUOUS. It is specified to request > that underlaying DMA mapping span physically contiguous with IOMMU. > But current alloc_dma_coherent() for intel-iommu always returns > physically contiguous memory, so it is ignored on x86. > >>>> And the apparent goal of this patchset is to enable DMA allocation below >>>> 4GB, which is already supported in the existing page allocator with the >>>> GFP_DMA32 flag?! >>> >>> The goal of this patchset is to enable huge DMA allocation which >>> alloc_pages() can't (> MAX_ORDER) for the devices that require it. >> >> What x86 devices need > MAX_ORDER DMA allocation and why can't they allocate >> directly from dma_alloc_from_contiguous()? > > I need this for UFS unified memory extension which is apparently not in > mainline for now. > http://www.jedec.org/standards-documents/docs/jesd220-1 > http://www.jedec.org/sites/default/files/T_Fujisawa_MF_2013.pdf > > But there must be some other use cases on x86, too. Because I have > received several emails privately from developers who care its status. > > And allocating directly from dma_alloc_from_contiguous() in the driver > doesn't work with IOMMU, as it just returns memory regoin and doesn't > create DMA mapping.
I read the UFS Unified Memory Extension v1.0 (JESD220-1) specification and it is not clear to me that using DMA mapping is the right approach to supporting UM, at least on x86. And without a mainline user, the merits of this approach are not evident. I cannot even find a production x86 UFS controller, much less one that supports UME. The only PCI UFS controller I could find (and that mainline supports) is Samsung's x86 FPGA-based test unit for developing UFS devices in a x86 test environment, and not a production x86 design. Samsung's own roadmap (http://www.slideshare.net/linaroorg/next-gen-mobilestorageufs) mentions nothing about bringing UFS to x86 designs. Unless there's something else I've missed, I don't think these patches belong in mainline. Regards, Peter Hurley -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
