Re: [RFC PATCH] drm/radeon: rework to new fence interface

[Christian König]

Am 19.08.2013 21:37, schrieb Maarten Lankhorst:
> Op 19-08-13 14:35, Christian König schreef:
> > Am 19.08.2013 12:17, schrieb Maarten Lankhorst:
> > > [SNIP]
> > > @@ -190,25 +225,24 @@ void radeon_fence_process(struct radeon_device *rdev, int ring)
> > >            }
> > >        } while (atomic64_xchg(&rdev->fence_drv[ring].last_seq, seq) > seq);
> > >    -    if (wake) {
> > > +    if (wake)
> > >            rdev->fence_drv[ring].last_activity = jiffies;
> > > -        wake_up_all(&rdev->fence_queue);
> > > -    }
> > > +    return wake;
> > >    }
> > Very bad idea, when sequence numbers change, you always want to wake up the whole fence queue here.
> Yes, and the callers of this function call wake_up_all or wake_up_all_locked themselves, based on the return value..

And as I said that's a very bad idea. The fence processing shouldn't be 
called with any locks held and should be self responsible for activating 
any waiters.

> > > [SNIP]
> > > +/**
> > > + * radeon_fence_enable_signaling - enable signalling on fence
> > > + * @fence: fence
> > > + *
> > > + * This function is called with fence_queue lock held, and adds a callback
> > > + * to fence_queue that checks if this fence is signaled, and if so it
> > > + * signals the fence and removes itself.
> > > + */
> > > +static bool radeon_fence_enable_signaling(struct fence *f)
> > > +{
> > > +    struct radeon_fence *fence = to_radeon_fence(f);
> > > +
> > > +    if (atomic64_read(&fence->rdev->fence_drv[fence->ring].last_seq) >= fence->seq ||
> > > +        !fence->rdev->ddev->irq_enabled)
> > > +        return false;
> > > +
> > Do I get that right that you rely on IRQs to be enabled and working here? Cause that would be a quite bad idea from the conceptual side.
> For cross-device synchronization it would be nice to have working irqs, it allows signalling fences faster,
> and it allows for callbacks on completion to be called. For internal usage it's no more required than it was before.

That's a big NAK.

The fence processing is actually very fine tuned to avoid IRQs and as 
far as I can see you just leave them enabled by decrementing the atomic 
from IRQ context. Additional to that we need allot of special handling 
in case of a hardware lockup here, which isn't done if you abuse the 
fence interface like this.

Also your approach of leaking the IRQ context outside of the driver is a 
very bad idea from the conceptual side. Please don't modify the fence 
interface at all and instead use the wait functions already exposed by 
radeon_fence.c. If you need some kind of signaling mechanism then wait 
inside a workqueue instead.

Christian.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/