linux-kernel - Re: [Linaro-mm-sig] [PATCH v3 1/2] habanalabs: define uAPI to export FD for DMA-BUF

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Date:   Wed, 23 Jun 2021 22:39:48 +0300
From:   Oded Gabbay <oded.gabbay@...il.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Christian König <ckoenig.leichtzumerken@...il.com>,
        Christian König <christian.koenig@....com>,
        Gal Pressman <galpress@...zon.com>, sleybo@...zon.com,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Oded Gabbay <ogabbay@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Doug Ledford <dledford@...hat.com>,
        Tomer Tayar <ttayar@...ana.ai>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Alex Deucher <alexander.deucher@....com>,
        Leon Romanovsky <leonro@...dia.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [Linaro-mm-sig] [PATCH v3 1/2] habanalabs: define uAPI to export
 FD for DMA-BUF

On Wed, Jun 23, 2021 at 10:34 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Wed, Jun 23, 2021 at 10:00:29PM +0300, Oded Gabbay wrote:
> > On Wed, Jun 23, 2021 at 9:50 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
> > >
> > > On Wed, Jun 23, 2021 at 09:43:04PM +0300, Oded Gabbay wrote:
> > >
> > > > Can you please explain why it is so important to (allow) access them
> > > > through the CPU ?
> > >
> > > It is not so much important, as it reflects significant design choices
> > > that are already tightly baked into alot of our stacks.
> > >
> > > A SGL is CPU accessible by design - that is baked into this thing and
> > > places all over the place assume it. Even in RDMA we have
> > > RXE/SWI/HFI1/qib that might want to use the CPU side (grep for sg_page
> > > to see)
> > >
> > > So, the thing at the top of the stack - in this case the gaudi driver
> > > - simply can't assume what the rest of the stack is going to do and
> > > omit the CPU side. It breaks everything.
> > >
> > > Logan's patch series is the most fully developed way out of this
> > > predicament so far.
> >
> > I understand the argument and I agree that for the generic case, the
> > top of the stack can't assume anything.
> > Having said that, in this case the SGL is encapsulated inside a dma-buf object.
> >
> > Maybe its a stupid/over-simplified suggestion, but can't we add a
> > property to the dma-buf object,
> > that will be set by the exporter, which will "tell" the importer it
> > can't use any CPU fallback ? Only "real" p2p ?
>
> The block stack has been trying to do something like this.
>
> The flag doesn't solve the DMA API/IOMMU problems though.
hmm, I thought using dma_map_resource will solve the IOMMU issues, no ?
We talked about it yesterday, and you said that it will "work"
(although I noticed a tone of reluctance when you said that).

If I use dma_map_resource to set the addresses inside the SGL before I
export the dma-buf, and guarantee no one will use the SGL in the
dma-buf for any other purpose than device p2p, what else is needed ?

Oded

>
> Jason