linux-kernel - Re: [RFC PATCH 1/3] RDMA/umem: Change for rdma devices has not dma device

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Message-ID: <CANXvt5rCCBku7LpAG5TV7LxkQ1bZnB6ACybKxJnTrRA1LE8e6Q@mail.gmail.com>
Date:   Wed, 8 Sep 2021 17:41:39 +0900
From:   Shunsuke Mie <mie@...l.co.jp>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Zhu Yanjun <zyjzyj2000@...il.com>,
        "Christian K??nig" <christian.koenig@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jianxin Xiong <jianxin.xiong@...el.com>,
        Leon Romanovsky <leon@...nel.org>,
        linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
        Damian Hobson-Garcia <dhobsong@...l.co.jp>,
        Takanari Hayama <taki@...l.co.jp>,
        Tomohito Esaki <etom@...l.co.jp>
Subject: Re: [RFC PATCH 1/3] RDMA/umem: Change for rdma devices has not dma device

2021年9月8日(水) 16:20 Christoph Hellwig <hch@...radead.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 ever
> > > 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 dma-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 by the
net device, but rxe driver copies data using CPU. To avoid the constraints, I
decided to use the ib device.

Thanks,