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

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Message-ID: <CAKMK7uE1G-EjbJ2RAXC3ZfkaCo=Gber5PJuRG623Yoz9D=FmGw@mail.gmail.com>
Date:	Wed, 23 Jul 2014 10:54:55 +0200
From:	Daniel Vetter <daniel.vetter@...ll.ch>
To:	Christian König <deathsimple@...afone.de>
Cc:	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	Christian König <christian.koenig@....com>,
	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

On Wed, Jul 23, 2014 at 10:46 AM, Christian König
<deathsimple@...afone.de> wrote:
> Am 23.07.2014 10:42, schrieb Daniel Vetter:
>
>> On Wed, Jul 23, 2014 at 10:25 AM, Maarten Lankhorst
>> <maarten.lankhorst@...onical.com> wrote:
>>>
>>> In this case if the sync was to i915 the i915 lockup procedure would take
>>> care of itself. It wouldn't fix radeon, but it would at least unblock your
>>> intel card again. I haven't specifically added a special case to attempt to
>>> unblock external fences, but I've considered it. :-)
>>
>> Actually the i915 reset stuff relies crucially on being able to kick
>> all waiters holding driver locks. Since the current fence code only
>> exposes an opaque wait function without exposing the underlying wait
>> queue we won't be able to sleep on both the fence queue and the reset
>> queue. So would pose a problem if we add fence_wait calls to our
>> driver.
>
>
> And apart from that I really think that I misunderstood Maarten. But his
> explanation sounds like i915 would do a reset because Radeon is locked up,
> right?
>
> Well if that's really the case then I would question the interface even
> more, cause that is really nonsense.

I disagree - the entire point of fences is that we can do multi-gpu
work asynchronously. So by the time we'll notice that radeon's dead we
have accepted the batch from userspace already. The only way to get
rid of it again is through our reset machinery, which also tells
userspace that we couldn't execute the batch. Whether we actually need
to do a hw reset depends upon whether we've committed the batch to the
hw already. Atm that's always the case, but the scheduler will change
that. So I have no issues with intel doing a reset when other drivers
don't signal fences.

Also this isn't a problem with the interface really, but with the
current implementation for radeon. And getting cross-driver reset
notifications right will require more work either way.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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