Re: [MAINTAINERS SUMMIT] Device Passthrough Considered Harmful?

Ricardo Ribalda Delgado
2024-07-26T08:05:07Z

Hi Laurent

On Thu, Jul 25, 2024 at 10:07 PM Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
>
> Hi Jason,
>
> On Thu, Jul 25, 2024 at 04:43:14PM -0300, Jason Gunthorpe wrote:
> > On Thu, Jul 25, 2024 at 10:31:25PM +0300, Laurent Pinchart wrote:
> >
> > > I don't think those are necessarily relevant examples, as far as device
> > > pass-through goes. Vendors have many times reverted to proprietary ways,
> > > and they still do, at least in the areas of the kernel I'm most active
> > > in. I've seen first hand a large SoC vendor very close to opening a
> > > significant part of their camera stack and changing their mind at the
> > > last minute when they heard they could possibly merge their code through
> > > a different subsystem with a pass-through blank cheque.
> >
> > If someone came with a fully open source framework for (say) some
> > camera,
>
> We have such a framework, it's called libcamera :-) Multiple vendors are
> already collaborating.
>
> > with a passthrough kernel driver design, would you reject it
> > soley because it is passthrough based and you are scared that
> > something else will use it to do something not open source?
>
> It depends what "passthrough kernel driver design" means. If it means
> accessing the PCI registers directly from userspace, yes. That's what X
> used to do before KMS, and I'm glad it's now a distant past.

Nobody has suggested giving PCI register access to userspace.

>
> If it means a kernel driver that takes the majority of its runtime
> parameters from a buffer blob assembled by userspace, while controlling
> clocks, power domains and performing basic validation in kernelspace,
> then I've already acked multiple drivers with such a design, exactly
> because they have open-source userspace that doesn't try to keep many
> device features proprietary and usable by closed-source userspace only.

If that was an option we would not be having this discussion.

Vendors cannot have vendor access in v4l2.
"""
It is not an option to upstream a driver that has support for
undocumented closed features. Basically maintainers can't put their
name on something that contains unverifiable (for them) and unusable
(by all except the vendor) features.
"""
https://linuxtv.org/news.php?entry=2022-11-14-1.hverkuil

>
> > I wouldn't agree with that position, I think denying users useful open
> > source solutions out of fear is not what Linux should be doing.
>
> --
> Regards,
>
> Laurent Pinchart
>

--
Ricardo Ribalda