Am 11.05.21 um 09:31 schrieb Daniel Vetter:
[SNIP]
And that's just the one ioctl I know is big trouble, I'm sure we'll find
more funny corner cases when we roll out explicit user fencing.
I think we can just ignore sync_file. As far as it concerns me that UAPI is
pretty much dead.
Uh that's rather bold. Android is built on it. Currently atomic kms is
built on it.
To be honest I don't think we care about Android at all.
What we should support is drm_syncobj, but that also only as an in-fence
since that's what our hardware supports.
Convince Android folks, minimally. Probably a lot more. Yes with hindsight
we should have just gone for drm_syncobj instead of the sync_file thing,
but hindsight and all that.
This is kinda why I don't think trying to support the existing uapi with
userspace fences underneath with some magic tricks is a good idea. It's
just a pile of work, plus it's not really architecturally clean.
Anotherone that looks very sketchy right now is buffer sharing between
different userspace drivers, like compute <-> media (if you have some
fancy AI pipeline in your media workload, as an example).
Yeah, we are certainly going to get that. But only inside the same driver,
so not much of a problem.
Why is this not much of a problem if it's just within one driver?
Because inside the same driver I can easily add the waits before
submitting the MM work as necessary.
Adding implicit synchronization on top of that is then rather trivial.
Well that's what I disagree with, since I already see some problems that I
don't think we can overcome (the atomic ioctl is one). And that's with us
only having a fairly theoretical understanding of the overall situation.
But how should we then ever support user fences with the atomic IOCTL?
We can't wait in user space since that will disable the support for waiting
in the hardware.
Well, figure it out :-)
This is exactly why I'm not seeing anything solved with just rolling a
function call to a bunch of places, because it's pretending all things are
solved when clearly that's not the case.
I really think what we need is to first figure out how to support
userspace fences as explicit entities across the stack, maybe with
something like this order:
1. enable them purely within a single userspace driver (like vk with
winsys disabled, or something else like that except not amd because
there's this amdkfd split for "real" compute)
1a. including atomic ioctl, e.g. for vk direct display support this can be
used without cross-process sharing, new winsys protocols and all that fun
2. figure out how to transport these userspace fences with something like
drm_syncobj
2a. figure out the compat story for drivers which dont do userspace fences
2b. figure out how to absorb the overhead if the winsys/compositor doesn't
support explicit sync
3. maybe figure out how to make this all happen magically with implicit
sync, if we really, really care
If we do 3 before we've nailed all these problems, we're just guaranteeing
we'll get the wrong solutions and so we'll then have 3 ways of doing
userspace fences
- the butchered implicit one that didn't quite work
- the explicit one
- the not-so-butchered implicit one with the lessons from the properly
done explicit one
The thing is, if you have no idea how to integrate userspace fences
explicitly into atomic ioctl, then you definitely have no idea how to do
it implicitly :-)
Well I agree on that. But the question is still how would you do
explicit with atomic?
Transporting fences between processes is not the fundamental problem
here, but rather the question how we represent all this in the kernel?
In other words I think what you outlined above is just approaching it
from the wrong side again. Instead of looking what the kernel needs to
support this you take a look at userspace and the requirements there.
Regards,
Christian.
And "just block" might be good enough for a quick demo, it still breaks
the contract. Same holds for a bunch of the winsys problems we'll have to
deal with here.
-Daniel
Regards,
Christian.
Like here at intel we have internal code for compute, and we're starting
to hit some interesting cases with interop with media already, but that's
it. Nothing even close to desktop/winsys/kms, and that's where I expect
will all the pain be at.
Cheers, Daniel
