Messages in this thread | | | Date | Thu, 13 Nov 2014 17:24:02 -0800 | From | Laura Abbott <> | Subject | Re: Why does ion_cma_allocate need GFP_HIGHUSER | __GFP_ZERO flags? |
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On 11/12/2014 11:18 PM, Gioh Kim wrote: > Hi, > > I'm wondering why dma_alloc_coherent needs GFP_HIGHUSER | __GFP_ZERO flags in ion_cma_allocate(). > > I think dma_alloc_coherent can go to __dma_alloc if the system has CMA area. > If __GFP_WAIT flag is set, it allocates memory from atomic pool. > So I think calling dma_alloc_coherent with __GFP_WAIT is enough. > > And can I get zero-filled-memory from CMA if I set __GFP_ZERO? > >
If we are actually using CMA, the GFP flags don't make much of a difference. If we didn't have CMA, using GFP_HIGHUSER would still be appropriate. The memory is going to userspace so the user part is necessary and there should be no need for a lowmem mapping so highmem can be used. __GFP_ZERO might be redundant since the dma layer zeros memory anyway but it's probably safe.
I think the flags should be fine as is.
Thanks, Laura
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