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

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Message-ID: <CAFCwf12tW_WawFfAfrC8bgVhTRnDA7DuM+0V8w3JsUZpA2j84w@mail.gmail.com>
Date:   Wed, 23 Jun 2021 22:00:29 +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 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 ?
Won't that solve the problem by eliminating the unsupported access methods ?

Oded

>
> > The whole purpose is that the other device accesses my device,
> > bypassing the CPU.
>
> Sure, but you don't know that will happen, or if it is even possible
> in any given system configuration. The purpose is to allow for that
> optimization when possible, not exclude CPU based approaches.
>
> Jason