Messages in this thread | | | Subject | Re: block: DMA alignment of IO buffer allocated from slab | From | Bart Van Assche <> | Date | Mon, 24 Sep 2018 13:54:01 -0700 |
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On Mon, 2018-09-24 at 13:41 -0700, Matthew Wilcox wrote: > On Mon, Sep 24, 2018 at 12:56:18PM -0700, Bart Van Assche wrote: > > On Mon, 2018-09-24 at 11:57 -0700, Matthew Wilcox wrote: > > > You're not supposed to use kmalloc memory for DMA. This is why we have > > > dma_alloc_coherent() and friends. > > > > Are you claiming that all drivers that use DMA should use coherent DMA only? If > > coherent DMA is the only DMA style that should be used, why do the following > > function pointers exist in struct dma_map_ops? > > Good job snipping the part of my reply which addressed this. Go read > DMA-API.txt yourself. Carefully.
The snipped part did not contradict your claim that "You're not supposed to use kmalloc memory for DMA." In the DMA-API.txt document however there are multiple explicit statements that support allocating memory for DMA with kmalloc(). Here is one example from the DMA-API.txt section about dma_map_single():
Not all memory regions in a machine can be mapped by this API. Further, contiguous kernel virtual space may not be contiguous as physical memory. Since this API does not provide any scatter/gather capability, it will fail if the user tries to map a non-physically contiguous piece of memory. For this reason, memory to be mapped by this API should be obtained from sources which guarantee it to be physically contiguous (like kmalloc).
Bart.
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