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

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Date:	Wed, 23 Jul 2014 09:58:16 +0200
From:	Christian König <deathsimple@...afone.de>
To:	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	Christian König 
	<christian.koenig@....com>, Daniel Vetter <daniel.vetter@...ll.ch>
CC:	Thomas Hellstrom <thellstrom@...are.com>,
	nouveau <nouveau@...ts.freedesktop.org>,
	LKML <linux-kernel@...r.kernel.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	Ben Skeggs <bskeggs@...hat.com>,
	"Deucher, Alexander" <alexander.deucher@....com>
Subject: Re: [Nouveau] [PATCH 09/17] drm/radeon: use common fence implementation
 for fences

> Regardless of the fence implementation, why would it be a good idea to do a full lockup recovery when some other driver is
> calling your wait function? That doesn't seem to be a nice thing to do, so I think a timeout is the best error you could return here,
> other drivers have to deal with that anyway.
The problem is that we need to guarantee that the lockup will be 
resolved eventually.

Just imagine an application using prime is locking up Radeon and because 
of that gets killed by the user. Nothing else in the system would use 
the Radeon hardware any more and so radeon gets only called by another 
driver waiting patiently for radeon to finish rendering which never 
happens because the whole thing is locked up and we don't get a chance 
to recover.

Christian.

Am 23.07.2014 09:51, schrieb Maarten Lankhorst:
> op 23-07-14 09:37, Christian König schreef:
>> Am 23.07.2014 09:31, schrieb Daniel Vetter:
>>> On Wed, Jul 23, 2014 at 9:26 AM, Christian König
>>> <deathsimple@...afone.de> wrote:
>>>> It's not a locking problem I'm talking about here. Radeons lockup handling
>>>> kicks in when anything calls into the driver from the outside, if you have a
>>>> fence wait function that's called from the outside but doesn't handle
>>>> lockups you essentially rely on somebody else calling another radeon
>>>> function for the lockup to be resolved.
>>> So you don't have a timer in radeon that periodically checks whether
>>> progress is still being made? That's the approach we're using in i915,
>>> together with some tricks to kick any stuck waiters so that we can
>>> reliably step in and grab locks for the reset.
>> We tried this approach, but it didn't worked at all.
>>
>> I already considered trying it again because of the upcoming fence implementation, but reconsidering that when a driver is forced to change it's handling because of the fence implementation that's just another hint that there is something wrong here.
> As far as I can tell it wouldn't need to be reworked for the fence implementation currently, only the moment you want to allow callers outside of radeon. :-)
> Doing a GPU lockup recovery in the wait function would be messy even right now, you would hit a deadlock in ttm_bo_delayed_delete -> ttm_bo_cleanup_refs_and_unlock.
>
> Regardless of the fence implementation, why would it be a good idea to do a full lockup recovery when some other driver is
> calling your wait function? That doesn't seem to be a nice thing to do, so I think a timeout is the best error you could return here,
> other drivers have to deal with that anyway.
>
> ~Maarten
>

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