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

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Date:	Tue, 20 Aug 2013 11:36:41 +0200
From:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
To:	Christian König <deathsimple@...afone.de>
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

Op 20-08-13 10:37, Christian König schreef:
> 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.
The call point (enable_signaling) only needs to know whether its own counter has passed or not. This prevents the race where the counter
has elapsed, but the irq was not yet enabled.

I don't really care if enable_signaling updates last_seq or not, it only needs to check if it's own fence has been signaled after enabling sw_irqs.

>>
>>>> [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.
I think it's not needed to leave the irq enabled, it's a leftover from when I was debugging the mac and no interrupt occurred at all.

> 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.
The fence takes up the role of a single shot workqueue here. Manually resetting the counter and calling wake_up_all would end up waking all active fences, there's no special handling needed inside radeon for this.
The fence api does provide a synchronous wait function, but this causes a stall of whomever waits on it. When I was testing this with intel I used the fence callback to poke a register in i915, this allowed it to not block until it hits the wait op in the command stream, and even then only if the callback was not called first.

It's documented that the callbacks can be called from any context and will be called with irqs disabled, so nothing scary should be done. The kernel provides enough debug mechanisms to find any violators.
PROVE_LOCKING and DEBUG_ATOMIC_SLEEP for example.

~Maarten

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