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

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Message-ID: <52132AE0.4010702@vodafone.de>
Date:	Tue, 20 Aug 2013 10:37:52 +0200
From:	Christian König <deathsimple@...afone.de>
To:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
CC:	dri-devel@...ts.freedesktop.org, linux-arch@...r.kernel.org,
	linaro-mm-sig@...ts.linaro.org,
	Alex Deucher <alexander.deucher@....com>,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org
Subject: Re: [RFC PATCH] drm/radeon: rework to new fence interface

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.
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