On Mon, 2018-09-24 at 13:41 -0700, Matthew Wilcox wrote: +AD4 On Mon, Sep 24, 2018 at 12:56:18PM -0700, Bart Van Assche wrote: +AD4 +AD4 On Mon, 2018-09-24 at 11:57 -0700, Matthew Wilcox wrote: +AD4 +AD4 +AD4 You're not supposed to use kmalloc memory for DMA. This is why we have +AD4 +AD4 +AD4 dma+AF8-alloc+AF8-coherent() and friends. +AD4 +AD4 +AD4 +AD4 Are you claiming that all drivers that use DMA should use coherent DMA only? If +AD4 +AD4 coherent DMA is the only DMA style that should be used, why do the following +AD4 +AD4 function pointers exist in struct dma+AF8-map+AF8-ops? +AD4 +AD4 Good job snipping the part of my reply which addressed this. Go read +AD4 DMA-API.txt yourself. Carefully. The snipped part did not contradict your claim that +ACI-You're not supposed to use kmalloc memory for DMA.+ACI 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+AF8-map+AF8-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.
