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Subject: Re: [RFC] Make use of non-dynamic dmabuf in RDMA
To:     =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Doug Ledford <dledford@redhat.com>,
        Jason Gunthorpe <jgg@ziepe.ca>
CC:     <linux-media@vger.kernel.org>, <dri-devel@lists.freedesktop.org>,
        <linux-kernel@vger.kernel.org>, <linux-rdma@vger.kernel.org>,
        Oded Gabbay <ogabbay@habana.ai>,
        Tomer Tayar <ttayar@habana.ai>,
        Yossi Leybovich <sleybo@amazon.com>,
        Alexander Matushevsky <matua@amazon.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Jianxin Xiong <jianxin.xiong@intel.com>
References: <20210818074352.29950-1-galpress@amazon.com>
 <3abaef49-2733-8b5e-3eaa-662a2a57b96e@amd.com>
From:   Gal Pressman <galpress@amazon.com>
Message-ID: <01573693-cf7a-a160-2f60-5049dfba1ecd@amazon.com>
Date:   Wed, 18 Aug 2021 11:37:31 +0300
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On 18/08/2021 11:00, Christian König wrote:
> Am 18.08.21 um 09:43 schrieb Gal Pressman:
>> Hey all,
>>
>> Currently, the RDMA subsystem can only work with dynamic dmabuf
>> attachments, which requires the RDMA device to support on-demand-paging
>> (ODP) which is not common on most devices (only supported by mlx5).
>>
>> While the dynamic requirement makes sense for certain GPUs, some devices
>> (such as habanalabs) have device memory that is always "pinned" and do
>> not need/use the move_notify operation.
>>
>> The motivation of this RFC is to use habanalabs as the dmabuf exporter,
>> and EFA as the importer to allow for peer2peer access through libibverbs.
>>
>> This draft patch changes the dmabuf driver to differentiate between
>> static/dynamic attachments by looking at the move_notify op instead of
>> the importer_ops struct, and allowing the peer2peer flag to be enabled
>> in case of a static exporter.
> 
> Well NAK to the general approach, this can be solved much easier.
> 
> If you can't support dynamic moves while using the buffer then just pin all
> buffers during import/export.
> 
> This avoids the move notification and the framework/exporter can still correctly
> account for pinned buffers.
> 
> But please note that at least amdgpu never uses P2P support for pinned buffers
> since we want to avoid that unmoveable buffers clutter video memory and create
> conflicts with V4L and scanout.
> 
> If you don't have such concerns in habanalabs then you can implement the pinning
> there while keeping P2P still enabled.
Thanks Christian!

Are you suggesting to pass an empty move_notify callback instead of passing NULL?
Also, doesn't the pin operation move the memory from the device to host memory?