On Mon, Feb 8, 2016 at 6:01 AM, Vishwas Srivastava <vishu.kernel@xxxxxxxxx> wrote: > Hi Ran, > the api which you have mentioned... > > void * > dma_alloc_coherent(struct device *dev, size_t size, > dma_addr_t *dma_handle, gfp_t flag) > > is the kernel api to alloc consistent memory. > > DMA devices understand the physical addresses not the virtual addresses. > > which means that we must supply to the dma device, the physical address, to > read > > from or to write to. > > The second argument of this api is an input argument which is updated > by the kernel if this api returns a success (and contains the physical base > address of the allocated memory) and the returned value of this api is the > kernel virtual address. > > if the *CPU* has to operate on this memory (assume that the memory is dma'ed > by the dma device and cpu want to read it for further processing ) it should > use the virtual address, so the returned value of this api, as the base > address. > However, if the dma device has to operate on this memory (assume device want > to write to this memory), it should use the *dma_handle* , which is the > physical address (base) of the dma memory. > > Now the question is how the dma device knows about this *physical* address? > The answer is that the "dma controller" register would have a register to > accept this physical address. > > So the sequence of steps probably would be, in your case: > 1: allocate the dma memory > 2: programme the dma controller register with the physical address returned > by this api, plus the size of the transaction and may be some more registers > for setting some kind of flags (depends on your dma device) > 3: programme the dma controller's dma *start* bit. > > after this the dma starts and dma device starts writing to the memory . > > > I hope, this clarifies you. > > Hi Vishwas, That's fully clarify the questions about dma_alloc_coherent. I also try to figure out what's the difference between dma_alloc_coherent and dma_map_single. I could not find and important difference between these two methods. 1. With both methods I stil need to program the dma controller with the physical address and the start trigger. 2. I can still do the allocation whenever I want with both methods (for example at the initialization of the driver), 3. Not sure what the actuall dma_map_single does (and if it really necessary to use it), becuase it seems I could just translate the virtual value from kmalloc into physical address and return it to the dma controller. Thank you for the time, Ran _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
