Messages in this thread | | | Date | Fri, 14 Nov 2014 10:38:37 +0900 | From | Gioh Kim <> | Subject | Re: Why does ion_cma_allocate need GFP_HIGHUSER | __GFP_ZERO flags? |
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2014-11-14 오전 10:24에 Laura Abbott 이(가) 쓴 글: > 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.
If cma-heap can be created even-if the system doesn't have CMA area, you're right.
But I think cma-heap should be created only if the system has CMA area. I think, in no CMA system, cma-heap creation and/or allocation from cma-heap should be failed, not allocate memory from lowmem or highmem as you say.
Thanks for your reply.
> > I think the flags should be fine as is. > > Thanks, > Laura > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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