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

From: Daniel Vetter <daniel.vetter@ffwll.ch>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: "Oded Gabbay" <oded.gabbay@gmail.com>,
	"Greg KH" <gregkh@linuxfoundation.org>,
	"Oded Gabbay" <ogabbay@kernel.org>,
	linux-rdma <linux-rdma@vger.kernel.org>,
	"open list:DMA BUFFER SHARING FRAMEWORK"
	<linux-media@vger.kernel.org>,
	"Doug Ledford" <dledford@redhat.com>,
	"airlied@gmail.com" <airlied@gmail.com>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Sumit Semwal" <sumit.semwal@linaro.org>,
	"Christian König" <christian.koenig@amd.com>,
	"Gal Pressman" <galpress@amazon.com>,
	sleybo@amazon.com, dri-devel <dri-devel@lists.freedesktop.org>,
	"Tomer Tayar" <ttayar@habana.ai>,
	"moderated list:DMA BUFFER SHARING FRAMEWORK"
	<linaro-mm-sig@lists.linaro.org>,
	"amd-gfx list" <amd-gfx@lists.freedesktop.org>,
	"Alex Deucher" <alexander.deucher@amd.com>,
	"Leon Romanovsky" <leonro@nvidia.com>,
	"Christoph Hellwig" <hch@lst.de>
Subject: Re: [PATCH v3 1/2] habanalabs: define uAPI to export FD for DMA-BUF
Date: Mon, 21 Jun 2021 20:27:32 +0200	[thread overview]
Message-ID: <CAKMK7uEO1_B59DtM7N2g7kkH7pYtLM_WAkn+0f3FU3ps=XEjZQ@mail.gmail.com> (raw)
In-Reply-To: <20210621175511.GI1096940@ziepe.ca>

On Mon, Jun 21, 2021 at 7:55 PM Jason Gunthorpe <jgg@ziepe.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@ziepe.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