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

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Message-ID: <20210621141217.GE1096940@ziepe.ca>
Date:   Mon, 21 Jun 2021 11:12:17 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        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 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.

AFAIK the only viable in-tree other side is in mlx5 (look in
umem_dmabuf.c)

Though as we already talked habana has their own networking (out of
tree, presumably) so I suspect this is really to support some out of
tree stuff??

Jason