Re: [RFC v2 0/8] dma-buf: Add support for mapping dmabufs via interconnects

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Oct 31, 2025 at 08:46:34AM +0100, Christian König wrote:
> On 10/31/25 06:15, Kasireddy, Vivek wrote:
> > Hi Jason,
> > 
> >> Subject: Re: [RFC v2 0/8] dma-buf: Add support for mapping dmabufs via
> >> interconnects
> >>
> >> On Thu, Oct 30, 2025 at 06:17:11AM +0000, Kasireddy, Vivek wrote:
> >>> It mostly looks OK to me but there are a few things that I want to discuss,
> >>> after briefly looking at the patches in your branch:
> >>> - I am wondering what is the benefit of the SGT compatibility stuff especially
> >>> when Christian suggested that he'd like to see SGT usage gone from
> >>> dma-buf
> >>
> >> I think to get rid of SGT we do need to put it in a little well
> >> defined box and then create alternatives and remove things using
> >> SGT. This is a long journey, and I think this is the first step.
> >>
> >> If SGT is some special case it will be harder to excise.
> >>
> >> So the next steps would be to make all the exporters directly declare
> >> a SGT and then remove the SGT related ops from dma_ops itself and
> >> remove the compat sgt in the attach logic. This is not hard, it is all
> >> simple mechanical work.
> > IMO, this SGT compatibility stuff should ideally be a separate follow-on
> > effort (and patch series) that would also probably include updates to
> > various drivers to add the SGT mapping type.
> 
> Nope, just the other way around. In other words the SGT
> compatibility is a pre-requisite.
> 
> We should first demonstrate with existing drivers that the new
> interface works and does what it promised to do and then extend it
> with new functionality.

Ok, so I think that is what my github is showing.

Everything interworks, non-mapping-type aware code simply acts exactly
as though it is using a SGT mapping type from the perspective of aware
code.

I see a fairly easy path to do some driver upgrades to make more
things more mapping aware and remove some of the compatibility parts.

Let me see if I can post a RFC version next week, got a big pile of
other stuff to do still..

Jason