Re: [MAINTAINER SUMMIT] User-space requirements for accelerator drivers

[Greg KH <greg@kroah.com>]

On Sun, Sep 12, 2021 at 11:29:45AM +0300, Leon Romanovsky wrote:
> On Sun, Sep 12, 2021 at 09:26:57AM +0200, Greg KH wrote:
> > On Sun, Sep 12, 2021 at 07:27:55AM +0300, Leon Romanovsky wrote:
> > > On Sun, Sep 12, 2021 at 01:04:01AM +0300, Laurent Pinchart wrote:
> > > > Hi Leon,
> > > > 
> > > > On Sat, Sep 11, 2021 at 03:04:07PM +0300, Leon Romanovsky wrote:
> > > > > On Sat, Sep 11, 2021 at 02:41:52PM +0300, Laurent Pinchart wrote:
> > > > > > On Sat, Sep 11, 2021 at 01:31:02PM +0300, Leon Romanovsky wrote:
> > > > > > > On Sat, Sep 11, 2021 at 01:55:16AM +0200, Thomas Gleixner wrote:
> > > > > > > > On Fri, Sep 10 2021 at 16:45, Josh Triplett wrote:
> > > > > > > > 
> > > > > > > > > On Sat, Sep 11, 2021 at 12:52:14AM +0200, Mauro Carvalho Chehab wrote:
> > > > > > > > >> On media, enforcing userspace to always be open source would
> > > > > > > > >> have been very bad, as it would prevent several videoconferencing 
> > > > > > > > >> software to exist on Linux.
> > > > > > > > >
> > > > > > > > > I don't think we should enforce that all userspace users of an interface
> > > > > > > > > be Open Source. I do think we should enforce that *some* userspace user
> > > > > > > > > of an interface be Open Source before we add the interface.
> > > > > > > > 
> > > > > > > > The real question is whether the interface is documented in a way that
> > > > > > > > an Open Source implementation is possible. It does not matter whether it
> > > > > > > > exists at that point in time or not. Even if it exists there is no
> > > > > > > > guarantee that it is feature complete.
> > > > > > > > 
> > > > > > > > Freely accessible documentation is really the key.
> > > > > > > 
> > > > > > > I have more radical view than you and think that documentation is far
> > > > > > > from being enough. I would like to see any userspace API used (or to be
> > > > > > > used) in any package which exists in Debiam/Fedora/SuSE.
> > > > > > 
> > > > > > We probably need to add Android AOSP to that list, as we have
> > > > > > Android-specific APIs (not that I believe we *should* have
> > > > > > Android-specific APIs, there's been lots of efforts over the past years
> > > > > > to develop standard APIs for use cases that stem from Android, slowly
> > > > > > replacing Android-specific APIs in some area, but I don't believe we can
> > > > > > realisticly bridge that gap completely overnight, if ever).
> > > > > 
> > > > > Maybe.
> > > > > 
> > > > > > > Only this will give us some sort of confidence that API and device are usable
> > > > > > > to some level. As a side note, we will be able to estimate possible API
> > > > > > > deprecation/fix/extension based on simple search in package databases.
> > > > > > 
> > > > > > Linux supports devices from very diverse markets, from very tiny
> > > > > > embedded devices to supercomputers. We have drivers for devices that
> > > > > > exist in data centres of a single company only, or for which only a
> > > > > > handful of units exist through the world. The set of rules that we'll
> > > > > > decide on, if any, should take this into account.
> > > > > 
> > > > > I'm part of that group (RDMA) who cares about enterprise, cloud and supercomputers. :)
> > > > > So for us, working out-of-the box (distro packages and not github code drops) is
> > > > > the key to the scalability.
> > > > 
> > > > What if we're dealing with a device that only exists in a handful of
> > > > machines though ? Would distributions accept the burden of packaging
> > > > corresponding userspace code, and maintaining the packages, when only a
> > > > handful of people in the world will use it ? It's a genuine question.
> > > 
> > > Fedora, Debian and OpenSuSE are volunteer based distributions, they
> > > accept new packages, which need to be prepared (or asked to be
> > > prepared) by such vendors.
> > > 
> > > There is no "accept the burden of packaging corresponding userspace code,
> > > and maintaining the packages", it is on package maintainer who can or
> > > can't be associated with distribution.
> > > 
> > > > 
> > > > > Regarding "embedded devices", I remind that we are talking about
> > > > > userspace API and most likely busybox will be used for them, which is
> > > > > also part of larger distro anyway, so fails under category "exists in
> > > > > Debian/Fedora/SuSE".
> > > > 
> > > > We're talking about APIs exposed by drivers, for devices such as GPUs,
> > > > cameras or AI/ML accelerators. I don't think busybox will exercise those
> > > > :-) We have Masa for GPUs, libcamera for cameras, and other frameworks
> > > > I'm less familiar with for AI/ML accelerators, and I expect those to be
> > > > packaged by distributions. There are however other kind of devices that
> > > > don't fall in existing well-defined categories.
> > > 
> > > I'm a little bit confused here. IMHO, you are trying to find an universal
> > > solution for a problem that doesn't exist.
> > > 
> > > Above you asked how to deal with niche devices? Here you talk about mass
> > > products devices for the enterprise while before you mentioned "embedded
> > > devices".
> > > 
> > > 1. Niche devices - continue to do as they do it now, by supplying
> > > out-of-tree solutions for their customers. Such devices and companies
> > > rarely need upstream linux kernel support, because the burden to
> > > upstream it is very high. We don't want them in the tree either, because
> > > once they upstream it, the maintenance burden will be on us.
> > 
> > {sigh}
> > 
> > No, that is NOT our rule at all.
> > 
> > These devices and companies need to be upstream more than anything else
> > as that way they become part of our community and are responsible for
> > maintaining their code in the tree.  To force them to remain outside is
> > to go against everything that many of us have been saying for _decades_
> > now.
> > 
> > And how are you going to judge what is, and is not, a "niche" device?
> 
> I will leave to that company to decide. Again this is exactly how they
> operate now, there is nothing new here. Every company calculates ROI
> for working with upstream and small companies with niche devices are not
> different here.
> 
> The main idea that I want to see working userspace stack, and being in
> distro sets a certain quality level, am I asking too much?

Define "working userspace stack" and "distro" please.  Like others have
said, many distros will not take userspace code unless it's already in
the kernel tree first, as that ensures that the abi will not break.

> > > 2. Devices that hits the certain level of adoption - need to be
> > > integrated into certain userspace stack, which needs to be part of
> > > distro.
> > 
> > Distros are a very odd rule to rely on given that they are by far the
> > minority of the usage in raw numbers for Linux in the world.
> 
> You can count Android as another distro, it is just semantics.

But how do you define Android's userspace?  Just one vendor?  2 vendors?
10 vendors?  There is major userspace fragmentation in Android userspace
in many places, the user/kernel boundry being one of the big ones as
many of us have found out over the past years.  And many of us are
working to resolve this, but it's not so simple at times, and I have
many examples if you want specifics.

> > > And AI/ML is no different here, someone just need to start build such
> > > stack. Otherwise, we will continue to see more free riders like HabanaLabs
> > > which don't have any real benefit to the community.
> > 
> > Everyone contributes to Linux in a selfish manner, that's just how the
> > community works.  The work that companies like habanalabs is NOT being a
> > "free rider" at all, they have worked with us and done the hard work of
> > actually getting their code merged into the tree.
> 
> I perfectly remember them trying to bypass netdev and RDMA communities
> by pretending "misc" device.
> 
> https://lore.kernel.org/linux-rdma/20200915133556.21268811@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com/
> https://lore.kernel.org/linux-rdma/20200917171833.GJ8409@ziepe.ca/
> 
> Or DRM
> https://lore.kernel.org/linux-rdma/CAKMK7uFOfoxbD2Z5mb-qHFnUe5rObGKQ6Ygh--HSH9M=9bziGg@mail.gmail.com/
> 
> So I can agree with the statement "worked hard", but not with the
> relevant communities.

I point at these as doing exactly what we want vendors to be doing!
Thank you for finding the good examples.  This is a vendor submitting
patches and saying, "here is what we want to do, with a first cut at
doing it."  It's up to us as a community to tell them if they are doing
it the right way or not.

If we just let them all go their own ways, they will come up with
horrible apis and interfaces, we have all seen that before.

So by working together, we both can learn from, and work together to
solve the issue.  And that is what these driver authors and company has
been doing!  They are part of our community, why are you saying they
should now just go do their own thing away from us?

And as for "bypassing", that feels very mean.  We have had accelerator
code in the char/misc and other parts of the kernel tree since at least
2018 if not earlier (I didn't look all that hard.)  Just because someone
wanted to use the in-kernel apis that are there (why is dma-buf some
magic thing?) does not mean that they suddenly need to move to a
different subsystem.

We get at least 1-2 new subsystems and major drivers that get added to
the kernel tree that do things that have never been done before with
custom user/kernel apis every kernel release.  Not everything can be a
standard api no matter how much I, and others, wish it were.

As examples, what about the hyperv blob api that was submitted recently
going around the block layer?  What about the new Intel accelerator that
added yet-another-set-of-custom-ioctls?  What about the rpi drivers?
What about the virtualbox drivers?  Should all of those just live
outside of the kernel for forever?

Of course not.

thanks,

greg k-h