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

[Christian König]

Am 22.07.2014 15:26, schrieb Daniel Vetter:
> On Tue, Jul 22, 2014 at 02:19:57PM +0200, Christian König wrote:
> > Am 22.07.2014 13:57, schrieb Daniel Vetter:
> > > On Tue, Jul 22, 2014 at 01:46:07PM +0200, Daniel Vetter wrote:
> > > > On Tue, Jul 22, 2014 at 10:43:13AM +0200, Christian König wrote:
> > > > > Am 22.07.2014 06:05, schrieb Dave Airlie:
> > > > > > On 9 July 2014 22:29, Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxx> wrote:
> > > > > > > Signed-off-by: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxx>
> > > > > > > ---
> > > > > > >   drivers/gpu/drm/radeon/radeon.h        |   15 +-
> > > > > > >   drivers/gpu/drm/radeon/radeon_device.c |   60 ++++++++-
> > > > > > >   drivers/gpu/drm/radeon/radeon_fence.c  |  223 ++++++++++++++++++++++++++------
> > > > > > >   3 files changed, 248 insertions(+), 50 deletions(-)
> > > > > >  From what I can see this is still suffering from the problem that we
> > > > > > need to find a proper solution to,
> > > > > >
> > > > > > My summary of the issues after talking to Jerome and Ben and
> > > > > > re-reading things is:
> > > > > >
> > > > > > We really need to work out a better interface into the drivers to be
> > > > > > able to avoid random atomic entrypoints,
> > > > > Which is exactly what I criticized from the very first beginning. Good to
> > > > > know that I'm not the only one thinking that this isn't such a good idea.
> > > > I guess I've lost context a bit, but which atomic entry point are we
> > > > talking about? Afaics the only one that's mandatory is the is
> > > > fence->signaled callback to check whether a fence really has been
> > > > signalled. It's used internally by the fence code to avoid spurious
> > > > wakeups. Afaik that should be doable already on any hardware. If that's
> > > > not the case then we can always track the signalled state in software and
> > > > double-check in a worker thread before updating the sw state. And wrap
> > > > this all up into a special fence class if there's more than one driver
> > > > needing this.
> > > One thing I've forgotten: The i915 scheduler that's floating around runs
> > > its bottom half from irq context. So I really want to be able to check
> > > fence state from irq context and I also want to make it possible
> > > (possible! not mandatory) to register callbacks which are run from any
> > > context asap after the fence is signalled.
> > NAK, that's just the bad design I've talked about. Checking fence state
> > inside the same driver from interrupt context is OK, because it's the
> > drivers interrupt that we are talking about here.
> >
> > Checking fence status from another drivers interrupt context is what really
> > concerns me here, cause your driver doesn't have the slightest idea if the
> > called driver is really capable of checking the fence right now.
> I guess my mail hasn't been clear then. If you don't like it we could add
> a bit of glue to insulate the madness and bad design i915 might do from
> radeon. That imo doesn't invalidate the overall fence interfaces.
>
> So what about the following:
> - fence->enabling_signaling is restricted to be called from process
>    context. We don't use any different yet, so would boild down to adding a
>    WARN_ON(in_interrupt) or so to fence_enable_sw_signalling.
>
> - Make fence->signaled optional (already the case) and don't implement it
>    in readon (i.e. reduce this patch here). Only downside is that radeon
>    needs to correctly (i.e. without races or so) call fence_signal. And the
>    cross-driver synchronization might be a bit less efficient. Note that
>    you can call fence_signal from wherever you want to, so hopefully that
>    doesn't restrict your implementation.
>
> End result: No one calls into radeon from interrupt context, and this is
> guaranteed.
>
> 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).

> 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.

Christian.
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