[Mesa-dev] [RFC] Linux Graphics Next: Explicit fences everywhere and no BO fences - initial proposal

[Marek Olšák]

On Wed, Apr 28, 2021 at 5:07 AM Michel Dänzer <michel at daenzer.net> wrote:

> On 2021-04-28 8:59 a.m., Christian König wrote:
> > Hi Dave,
> >
> > Am 27.04.21 um 21:23 schrieb Marek Olšák:
> >> Supporting interop with any device is always possible. It depends on
> which drivers we need to interoperate with and update them. We've already
> found the path forward for amdgpu. We just need to find out how many other
> drivers need to be updated and evaluate the cost/benefit aspect.
> >>
> >> Marek
> >>
> >> On Tue, Apr 27, 2021 at 2:38 PM Dave Airlie <airlied at gmail.com <mailto:
> airlied at gmail.com>> wrote:
> >>
> >>     On Tue, 27 Apr 2021 at 22:06, Christian König
> >>     <ckoenig.leichtzumerken at gmail.com <mailto:
> ckoenig.leichtzumerken at gmail.com>> wrote:
> >>     >
> >>     > Correct, we wouldn't have synchronization between device with and
> without user queues any more.
> >>     >
> >>     > That could only be a problem for A+I Laptops.
> >>
> >>     Since I think you mentioned you'd only be enabling this on newer
> >>     chipsets, won't it be a problem for A+A where one A is a generation
> >>     behind the other?
> >>
> >
> > Crap, that is a good point as well.
> >
> >>
> >>     I'm not really liking where this is going btw, seems like a ill
> >>     thought out concept, if AMD is really going down the road of
> designing
> >>     hw that is currently Linux incompatible, you are going to have to
> >>     accept a big part of the burden in bringing this support in to more
> >>     than just amd drivers for upcoming generations of gpu.
> >>
> >
> > Well we don't really like that either, but we have no other option as
> far as I can see.
>
> I don't really understand what "future hw may remove support for kernel
> queues" means exactly. While the per-context queues can be mapped to
> userspace directly, they don't *have* to be, do they? I.e. the kernel
> driver should be able to either intercept userspace access to the queues,
> or in the worst case do it all itself, and provide the existing
> synchronization semantics as needed?
>
> Surely there are resource limits for the per-context queues, so the kernel
> driver needs to do some kind of virtualization / multi-plexing anyway, or
> we'll get sad user faces when there's no queue available for <current hot
> game>.
>
> I'm probably missing something though, awaiting enlightenment. :)
>

The hw interface for userspace is that the ring buffer is mapped to the
process address space alongside a doorbell aperture (4K page) that isn't
real memory, but when the CPU writes into it, it tells the hw scheduler
that there are new GPU commands in the ring buffer. Userspace inserts all
the wait, draw, and signal commands into the ring buffer and then "rings"
the doorbell. It's my understanding that the ring buffer and the doorbell
are always mapped in the same GPU address space as the process, which makes
it very difficult to emulate the current protected ring buffers in the
kernel. The VMID of the ring buffer is also not changeable.

The hw scheduler doesn't do any synchronization and it doesn't see any
dependencies. It only chooses which queue to execute, so it's really just a
simple queue manager handling the virtualization aspect and not much else.

Marek
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