linux-kernel - Re: [PATCH for-next 2/4] RDMA/hns: Add IOMMU enable support in hip08

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Date:   Tue, 7 Nov 2017 08:48:38 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Wei Hu (Xavier)" <xavier.huwei@...wei.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Leon Romanovsky <leon@...nel.org>, shaobo.xu@...el.com,
        xavier.huwei@....com, lijun_nudt@....com, oulijun@...wei.com,
        linux-rdma@...r.kernel.org, charles.chenxin@...wei.com,
        linuxarm@...wei.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        dledford@...hat.com, liuyixian@...wei.com, zhangxiping3@...wei.com,
        shaoboxu@....com
Subject: Re: [PATCH for-next 2/4] RDMA/hns: Add IOMMU enable support in hip08

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