Messages in this thread | | | Date | Tue, 25 Apr 2017 19:05:26 +0100 | From | Russell King - ARM Linux <> | Subject | Re: [RFC PATCH] drivers: dma-mapping: Do not attempt to create a scatterlist for from_coherent buffers |
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On Tue, Apr 25, 2017 at 06:57:39PM +0100, Catalin Marinas wrote: > Memory returned by dma_alloc_from_coherent() is not backed by struct > page and creating a scatterlist would use invalid page pointers. The > patch introduces the dma_vaddr_from_coherent() function and the > corresponding check in dma_get_sgtable_attrs(). > > Fixes: d2b7428eb0ca ("common: dma-mapping: introduce dma_get_sgtable() function") > Cc: Marek Szyprowski <m.szyprowski@samsung.com> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> > Cc: Russell King - ARM Linux <linux@arm.linux.org.uk> > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com> > --- > > In a recent discussion around the iommu DMA ops on arm64, Russell > pointed out that dma_get_sgtable is not safe since the coherent DMA > memory is not always backed by struct page. Russell has queued an > arm-specific patch checking for pfn_valid() but I thought I'd make a > more generic fix. This patch aims to bring the dma_get_sgtable() API in > line with the dma_alloc/mmap/free with respect to the from_coherent > memory.
Sorry, I don't think this is the correct approach.
You're assuming that 'vaddr' will always be a valid lowmem address. That isn't always the case - some dma coherent allocations provide remapped memory.
The reason for dma_get_sgtable() existing is to coerce the DMA coherent memory into a scatterlist so that it can be passed through the dma_buf API - that's where the problem lies. The dma_buf API needs fixing so that coherent memory can be sanely passed, and dma_get_sgtable() needs to be put out of its misery.
-- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net.
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