Nouveau - Jul 2014 - [PATCH 09/17] drm/radeon: use common fence implementation for fences

Am 23.07.2014 10:07, schrieb Daniel Vetter:
> On Wed, Jul 23, 2014 at 9:58 AM, Christian K?nig
> <deathsimple at vodafone.de> wrote:
>> 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.
> But isn't that possible already without fences? X hangs radeon, user
> crashes X for unrelated reasons before radeon will notice the hang.
> Then no one uses radeon any longer and the hang stays undetected.
Yeah, especially with multimedia application. But I don't really care 
about this problem because the next time an application tries to use the 
block in question we actually do the reset and everything is fine.

In your example we would do the reset when the next X server starts, 
before that point nobody would care because nobody uses the hardware.

An additional problem here is that resets are something perfect normal 
for radeon. For example UVD can "crash" when you feed it with invalid 
bitstream data, (ok actually it send an interrupt and stops any 
processing for the driver to investigate). To continue processing you 
need to go through a rather complicated reset procedure.

Christian.
> -Daniel

op 23-07-14 10:20, Christian K?nig schreef:
> Am 23.07.2014 10:07, schrieb Daniel Vetter:
>> On Wed, Jul 23, 2014 at 9:58 AM, Christian K?nig
>> <deathsimple at vodafone.de> wrote:
>>> 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.
>> But isn't that possible already without fences? X hangs radeon,
user
>> crashes X for unrelated reasons before radeon will notice the hang.
>> Then no one uses radeon any longer and the hang stays undetected.
>
> Yeah, especially with multimedia application. But I don't really care
about this problem because the next time an application tries to use the block
in question we actually do the reset and everything is fine.
>
> In your example we would do the reset when the next X server starts, before
that point nobody would care because nobody uses the hardware.
>
> An additional problem here is that resets are something perfect normal for
radeon. For example UVD can "crash" when you feed it with invalid
bitstream data, (ok actually it send an interrupt and stops any processing for
the driver to investigate). To continue processing you need to go through a
rather complicated reset procedure.
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. :-)

~Maarten

On Wed, Jul 23, 2014 at 10:25 AM, Maarten Lankhorst
<maarten.lankhorst at canonical.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.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch