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

[Jason Gunthorpe]

On Wed, Jun 23, 2021 at 10:39:48PM +0300, Oded Gabbay wrote:
> On Wed, Jun 23, 2021 at 10:34 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> >
> > On Wed, Jun 23, 2021 at 10:00:29PM +0300, Oded Gabbay wrote:
> > > On Wed, Jun 23, 2021 at 9:50 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > > >
> > > > On Wed, Jun 23, 2021 at 09:43:04PM +0300, Oded Gabbay wrote:
> > > >
> > > > > Can you please explain why it is so important to (allow) access them
> > > > > through the CPU ?
> > > >
> > > > It is not so much important, as it reflects significant design choices
> > > > that are already tightly baked into alot of our stacks.
> > > >
> > > > A SGL is CPU accessible by design - that is baked into this thing and
> > > > places all over the place assume it. Even in RDMA we have
> > > > RXE/SWI/HFI1/qib that might want to use the CPU side (grep for sg_page
> > > > to see)
> > > >
> > > > So, the thing at the top of the stack - in this case the gaudi driver
> > > > - simply can't assume what the rest of the stack is going to do and
> > > > omit the CPU side. It breaks everything.
> > > >
> > > > Logan's patch series is the most fully developed way out of this
> > > > predicament so far.
> > >
> > > I understand the argument and I agree that for the generic case, the
> > > top of the stack can't assume anything.
> > > Having said that, in this case the SGL is encapsulated inside a dma-buf object.
> > >
> > > Maybe its a stupid/over-simplified suggestion, but can't we add a
> > > property to the dma-buf object,
> > > that will be set by the exporter, which will "tell" the importer it
> > > can't use any CPU fallback ? Only "real" p2p ?
> >
> > The block stack has been trying to do something like this.
> >
> > The flag doesn't solve the DMA API/IOMMU problems though.
> hmm, I thought using dma_map_resource will solve the IOMMU issues,
> no ?

dma_map_resource() will configure the IOMMU but it is not the correct
API to use when building a SG list for DMA, that would be dma_map_sg
or sgtable.

So it works, but it is an API abuse to build things this way.

> If I use dma_map_resource to set the addresses inside the SGL before I
> export the dma-buf, and guarantee no one will use the SGL in the
> dma-buf for any other purpose than device p2p, what else is needed ?

You still have to check the p2p stuff to ensure that p2p is even
possible

And this approach is misusing all the APIs and has been NAK'd by
Christoph, so up to Greg if he wants to take it or insist you work
with Logan to get the proper generlized solution finished.

Jason