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

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Message-ID: <YThj70ByPvZNQjgU@infradead.org>
Date:   Wed, 8 Sep 2021 08:19:11 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Shunsuke Mie <mie@...l.co.jp>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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

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.