Re: [PATCH v4 0/2] Add p2p via dmabuf to habanalabs

Jason Gunthorpe
2021-07-06T15:25:49Z

On Tue, Jul 06, 2021 at 04:39:19PM +0200, Daniel Vetter wrote:
> On Tue, Jul 6, 2021 at 4:23 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> >
> > On Tue, Jul 06, 2021 at 12:36:51PM +0200, Daniel Vetter wrote:
> >
> > > If that means AI companies don't want to open our their hw specs
> > > enough to allow that, so be it - all you get in that case is
> > > offloading the kernel side of the stack for convenience, with zero
> > > long term prospects to ever make this into a cross vendor subsystem
> > > stack that does something useful.
> >
> > I don't think this is true at all - nouveau is probably the best
> > example.
> >
> > nouveau reverse engineered a userspace stack for one of these devices.
> >
> > How much further ahead would they have been by now if they had a
> > vendor supported, fully featured, open kernel driver to build the
> > userspace upon?
>
> There is actually tons of example here, most of the arm socs have
> fully open kernel drivers, supported by the vendor (out of tree).

I choose nouveau because of this:

$ git ls-files drivers/gpu/drm/arm/ | xargs wc -l
15039 total
$ git ls-files drivers/gpu/drm/nouveau/ | xargs wc -l
204198 total

At 13x the size of mali this is not just some easy to wire up memory
manager and command submission. And after all that typing it still
isn't very good. The fully supported AMD vendor driver is over 3
million lines, so nouveau probably needs to grow several times.

My argument is that an in-tree open kernel driver is a big help to
reverse engineering an open userspace. Having the vendors
collaboration to build that monstrous thing can only help the end goal
of an end to end open stack.

For instance a vendor with an in-tree driver has a strong incentive to
sort out their FW licensing issues so it can be redistributed.

I'm not sure about this all or nothing approach. AFAIK DRM has the
worst problems with out of tree drivers right now.

> Where it would have helped is if this open driver would come with
> redistributable firmware, because that is right now the thing making
> nouveau reverse-engineering painful enough to be non-feasible. Well
> not the reverse-engineering, but the "shipping the result as a working
> driver stack".

I don't think much of the out of tree but open drivers. The goal must
be to get vendors in tree.

I would applaud Habana for getting an intree driver at least, even if
the userspace is not what we'd all want to see.

> I don't think the facts on the ground support your claim here, aside
> from the practical problem that nvidia is unwilling to even create an
> open driver to begin with. So there isn't anything to merge.

The internet tells me there is nvgpu, it doesn't seem to have helped.

Jason