RE: [RFC PATCH 0/3] RDMA: Add dma-buf support

["Xiong, Jianxin" <jianxin.xiong@intel.com>]

> > > >
> > > > Right, the GPU driver needs to cooperate to get the thing to work
> > > > as expected. The "p2p" flag and related GPU driver changes
> > > > proposed in other threads would ensure VRAM is really used.
> > > > Alternatively, a GPU driver can have a working mode that assumes
> > > > p2p mapping capability of the client. Either way, the patches to
> > > > the RDMA driver would be mostly identical except for adding the use of the "p2p"
> > > > flag.
> > >
> > > I think the other thread has explained this would not be "mostly
> > > identical" but here is significant work to rip out the scatter list from the umem.
> > >
> >
> > Probably we are referring to different threads here. Could you kindly
> > refer me to the thread you mentioned? I was referring to the thread
> > about patching dma-buf and GPU driver:
> > https://www.spinics.net/lists/amd-gfx/msg47022.html
> 
> https://lore.kernel.org/linux-media/20200311152838.GA24280@infradead.org/
> 

Thanks. We are actually looking at the same series but somehow I skipped the details of the single patch that looks simplest which turns out to have most complication. I agree if scatter list is not to be used, there are going to be significant work involved.

> > > So, I'm back to my original ask, can you justify adding this if it
> > > cannot do VRAM? What is the use case?
> >
> > Working with VRAM is the goal. This patch has been tested with a
> > modified GPU driver that has dma_buf_ops set up to not migrate the
> > buffer to system memory when attached. The GPU drivers and the RDMA
> > drivers can be improved independently and it doesn't hurt to have the
> > RDMA driver ready before the GPU drivers.
> 
> Well, if there is no other use case then this series will have to wait until someone does all the other work to make P2P work upstream.
> 
> Jason