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Date: Tue, 7 Nov 2017 08:48:38 -0700
From: Jason Gunthorpe <jgg@ziepe.ca>
Subject: Re: [PATCH for-next 2/4] RDMA/hns: Add IOMMU enable support in hip08
Message-ID: <20171107154838.GC21466@ziepe.ca>
References: <1506763741-81429-1-git-send-email-xavier.huwei@huawei.com>
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To: "Wei Hu (Xavier)" <xavier.huwei@huawei.com>
Cc: Robin Murphy <robin.murphy@arm.com>, Leon Romanovsky <leon@kernel.org>, shaobo.xu@intel.com, xavier.huwei@tom.com, lijun_nudt@163.com, oulijun@huawei.com, linux-rdma@vger.kernel.org, charles.chenxin@huawei.com, linuxarm@huawei.com, iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, dledford@redhat.com, liuyixian@huawei.com, zhangxiping3@huawei.com, shaoboxu@tom.com

On Tue, Nov 07, 2017 at 10:45:29AM +0800, Wei Hu (Xavier) wrote:

>     We reconstruct the code as below:

>             It replaces dma_alloc_coherent with __get_free_pages and
> dma_map_single functions. So, we can vmap serveral ptrs returned by
> __get_free_pages, right?

Can't you just use vmalloc and dma_map that? Other drivers follow that
approach..

However, dma_alloc_coherent and dma_map_single are not the same
thing. You can't touch the vmap memory once you call dma_map unless
the driver also includes dma cache flushing calls in all the right
places.

The difference is that alloc_coherent will return non-cachable memory
if necessary, while get_free_pages does not.

Jason

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