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

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Date:   Mon, 21 Jun 2021 20:27:32 +0200
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Oded Gabbay <oded.gabbay@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Oded Gabbay <ogabbay@...nel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>, Doug Ledford <dledford@...hat.com>,
        "airlied@...il.com" <airlied@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Gal Pressman <galpress@...zon.com>, sleybo@...zon.com,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Tomer Tayar <ttayar@...ana.ai>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Alex Deucher <alexander.deucher@....com>,
        Leon Romanovsky <leonro@...dia.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v3 1/2] habanalabs: define uAPI to export FD for DMA-BUF

On Mon, Jun 21, 2021 at 7:55 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
> On Mon, Jun 21, 2021 at 07:26:14PM +0300, Oded Gabbay wrote:
> > On Mon, Jun 21, 2021 at 5:12 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
> > >
> > > On Mon, Jun 21, 2021 at 03:02:10PM +0200, Greg KH wrote:
> > > > On Mon, Jun 21, 2021 at 02:28:48PM +0200, Daniel Vetter wrote:
> > >
> > > > > Also I'm wondering which is the other driver that we share buffers
> > > > > with. The gaudi stuff doesn't have real struct pages as backing
> > > > > storage, it only fills out the dma_addr_t. That tends to blow up with
> > > > > other drivers, and the only place where this is guaranteed to work is
> > > > > if you have a dynamic importer which sets the allow_peer2peer flag.
> > > > > Adding maintainers from other subsystems who might want to chime in
> > > > > here. So even aside of the big question as-is this is broken.
> > > >
> > > > From what I can tell this driver is sending the buffers to other
> > > > instances of the same hardware,
> > >
> > > A dmabuf is consumed by something else in the kernel calling
> > > dma_buf_map_attachment() on the FD.
> > >
> > > What is the other side of this? I don't see any
> > > dma_buf_map_attachment() calls in drivers/misc, or added in this patch
> > > set.
> >
> > This patch-set is only to enable the support for the exporter side.
> > The "other side" is any generic RDMA networking device that will want
> > to perform p2p communication over PCIe with our GAUDI accelerator.
> > An example is indeed the mlnx5 card which has already integrated
> > support for being an "importer".
>
> It raises the question of how you are testing this if you aren't using
> it with the only intree driver: mlx5.

For p2p dma-buf there's also amdgpu as a possible in-tree candiate
driver, that's why I added amdgpu folks. Otoh I'm not aware of AI+GPU
combos being much in use, at least with upstream gpu drivers (nvidia
blob is a different story ofc, but I don't care what they do in their
own world).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch