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From:   Shunsuke Mie <mie@igel.co.jp>
Date:   Wed, 8 Sep 2021 17:41:39 +0900
Message-ID: <CANXvt5rCCBku7LpAG5TV7LxkQ1bZnB6ACybKxJnTrRA1LE8e6Q@mail.gmail.com>
Subject: Re: [RFC PATCH 1/3] RDMA/umem: Change for rdma devices has not dma device
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Zhu Yanjun <zyjzyj2000@gmail.com>,
        "Christian K??nig" <christian.koenig@amd.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Doug Ledford <dledford@redhat.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Jianxin Xiong <jianxin.xiong@intel.com>,
        Leon Romanovsky <leon@kernel.org>,
        linux-kernel@vger.kernel.org, linux-rdma@vger.kernel.org,
        Damian Hobson-Garcia <dhobsong@igel.co.jp>,
        Takanari Hayama <taki@igel.co.jp>,
        Tomohito Esaki <etom@igel.co.jp>
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2021=E5=B9=B49=E6=9C=888=E6=97=A5(=E6=B0=B4) 16:20 Christoph Hellwig <hch@i=
nfradead.org>:
>
> On Wed, Sep 08, 2021 at 04:01:14PM +0900, Shunsuke Mie wrote:
> > Thank you for your comment.
> > >
> > > On Wed, Sep 08, 2021 at 03:16:09PM +0900, Shunsuke Mie wrote:
> > > > To share memory space using dma-buf, a API of the dma-buf requires =
dma
> > > > device, but devices such as rxe do not have a dma device. For those=
 case,
> > > > change to specify a device of struct ib instead of the dma device.
> > >
> > > So if dma-buf doesn't actually need a device to dma map why do we eve=
r
> > > pass the dma_device here?  Something does not add up.
> > As described in the dma-buf api guide [1], the dma_device is used by dm=
a-buf
> > exporter to know the device buffer constraints of importer.
> > [1] https://lwn.net/Articles/489703/
>
> Which means for rxe you'd also have to pass the one for the underlying
> net device.
I thought of that way too. In that case, the memory region is constrained b=
y the
net device, but rxe driver copies data using CPU. To avoid the constraints,=
 I
decided to use the ib device.

Thanks,