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

From: Christian KÃnig
Date: Wed Jul 23 2014 - 05:28:03 EST


Am 23.07.2014 10:54, schrieb Daniel Vetter:
On Wed, Jul 23, 2014 at 10:46 AM, Christian KÃnig
<deathsimple@xxxxxxxxxxx> wrote:
Am 23.07.2014 10:42, schrieb Daniel Vetter:

On Wed, Jul 23, 2014 at 10:25 AM, Maarten Lankhorst
<maarten.lankhorst@xxxxxxxxxxxxx> 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.

You submit a job to the hardware and then block the job to wait for radeon to be finished? Well than this would indeed require a hardware reset, but wouldn't that make the whole problem even worse?

I mean currently we block one userspace process to wait for other hardware to be finished with a buffer, but what you are describing here blocks the whole hardware to wait for other hardware which in the end blocks all userspace process accessing the hardware.

Talking about alternative approaches wouldn't it be simpler to just offload the waiting to a different kernel or userspace thread?

Christian.


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

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