Re: [Nouveau] [PATCH 09/17] drm/radeon: use common fence implementation for fences

[Christian KÃnig]

Am 22.07.2014 17:17, schrieb Daniel Vetter:
> On Tue, Jul 22, 2014 at 3:45 PM, Christian KÃnig
> <deathsimple@xxxxxxxxxxx> wrote:
> > > Would that be something you can agree to?
> >
> > No, the whole enable_signaling stuff should go away. No callback from the
> > driver into the fence code, only the other way around.
> >
> > fence->signaled as well as fence->wait should become mandatory and only
> > called from process context without holding any locks, neither atomic nor
> > any mutex/semaphore (rcu might be ok).
> So for the enable_signaling, that's optional already. It's only for
> drivers that don't want to keep interrupts enabled all the time. You
> can opt out of that easily.
>
> Wrt holding no locks at all while calling into any fence functions,
> that's just not going to work out. The point here is to make different
> drivers work together and we can rework all the ttm and i915 code to
> work locklessly in all cases where they need to wait for someone to
> complete rendering. Or at least I don't think that's feasible. So if
> you insist that no one might call into radeon code then we simply need
> to exclude radeon from participating in any shared fencing. But that's
> a bit pointless.
>
> > > Like I've said I think restricting the insanity other people are willing
> > > to live with just because you don't like it isn't right. But it is
> > > certainly right for you to insist on not being forced into any such
> > > design. I think the above would achieve this.
> >
> > I don't think so. If it's just me I would say that I'm just to cautious and
> > the idea is still save to apply to the whole kernel.
> >
> > But since Dave, Jerome and Ben seems to have similar concerns I think we
> > need to agree to a minimum and save interface for all drivers.
> Well I haven't yet seen a proposal that actually works.

How about this:

Drivers exporting fences need to provide a fence->signaled and a 
fence->wait function, everything else like fence->enable_signaling or 
calling fence_signaled() from the driver is optional.

Drivers wanting to use exported fences don't call fence->signaled or 
fence->wait in atomic or interrupt context, and not with holding any 
global locking primitives (like mmap_sem etc...). Holding locking 
primitives local to the driver is ok, as long as they don't conflict 
with anything possible used by their own fence implementation.

Christian.

>   From an intel
> pov I don't care that much since we don't care about desktop prime, so
> if radeon/nouveau don't want to do that, meh. Imo the design as-is is
> fairly sound, and as simple as it can get given the requirements. I
> haven't heard an  argument convincing me otherwise, so I guess we
> won't have prime support on linux that actually works, ever.
> -Daniel

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