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

On Wed, Jul 23, 2014 at 9:06 AM, Maarten Lankhorst
<maarten.lankhorst at canonical.com> wrote:
>> Can we somehow avoid the need to call fence_signal() at all? The
interrupts at least on radeon are way to unreliable for such a thing. Can
enable_signalling fail? What's the reason for fence_signaled() in the first
place?
> It doesn't need to be completely reliable, or finish immediately.
>
> And any time wake_up_all(&rdev->fence_queue) is called all the
fences that were enabled will be rechecked.
I raised this already somewhere else, but should we have some common
infrastructure in the core fence code to recheck fences periodically?
radeon doesn't seem to be the only hw where this isn't reliable
enough. Of course timer-based rechecking would only work if the driver
provides the fence->signalled callback to recheck actual fence state.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

Am 23.07.2014 09:09, schrieb Daniel Vetter:
> On Wed, Jul 23, 2014 at 9:06 AM, Maarten Lankhorst
> <maarten.lankhorst at canonical.com> wrote:
>>> Can we somehow avoid the need to call fence_signal() at all? The
interrupts at least on radeon are way to unreliable for such a thing. Can
enable_signalling fail? What's the reason for fence_signaled() in the first
place?
>> It doesn't need to be completely reliable, or finish immediately.
>>
>> And any time wake_up_all(&rdev->fence_queue) is called all the
fences that were enabled will be rechecked.
> I raised this already somewhere else, but should we have some common
> infrastructure in the core fence code to recheck fences periodically?
> radeon doesn't seem to be the only hw where this isn't reliable
> enough. Of course timer-based rechecking would only work if the driver
> provides the fence->signalled callback to recheck actual fence state.
Yeah, agree. The proposal won't work reliable at all with radeon.

Interrupts are accumulated before sending them to the CPU, e.g. you can 
get one interrupt for multiple fences finished. If it's just the 
interrupt for the last fence submitted that gets lost you are completely 
screwed up because you won't get another interrupt.

I had that problem multiple times while working on UVD support, 
resulting in the driver thinking that it can't submit more jobs because 
non of the interrupts for the already submitted fence cam through.

Apart from that interrupts on Macs usually don't work at all, so we 
really need a solution where calling fence_signaled() is completely 
optional.

Christian.
> -Daniel

op 23-07-14 09:15, Christian K?nig schreef:
> Am 23.07.2014 09:09, schrieb Daniel Vetter:
>> On Wed, Jul 23, 2014 at 9:06 AM, Maarten Lankhorst
>> <maarten.lankhorst at canonical.com> wrote:
>>>> Can we somehow avoid the need to call fence_signal() at all?
The interrupts at least on radeon are way to unreliable for such a thing. Can
enable_signalling fail? What's the reason for fence_signaled() in the first
place?
>>> It doesn't need to be completely reliable, or finish
immediately.
>>>
>>> And any time wake_up_all(&rdev->fence_queue) is called all
the fences that were enabled will be rechecked.
>> I raised this already somewhere else, but should we have some common
>> infrastructure in the core fence code to recheck fences periodically?
>> radeon doesn't seem to be the only hw where this isn't reliable
>> enough. Of course timer-based rechecking would only work if the driver
>> provides the fence->signalled callback to recheck actual fence
state.
>
> Yeah, agree. The proposal won't work reliable at all with radeon.
>
> Interrupts are accumulated before sending them to the CPU, e.g. you can get
one interrupt for multiple fences finished. If it's just the interrupt for
the last fence submitted that gets lost you are completely screwed up because
you won't get another interrupt.
>
> I had that problem multiple times while working on UVD support, resulting
in the driver thinking that it can't submit more jobs because non of the
interrupts for the already submitted fence cam through.
Yeah but all the fences that have .enable_signaling will get signaled from a
single interrupt, or when any waiter calls radeon_fence_process.
> Apart from that interrupts on Macs usually don't work at all, so we
really need a solution where calling fence_signaled() is completely optional.
I haven't had a problem with interrupts on my mbp after
d1f9809ed1315c4cdc5760cf2f59626fd3276952, but it should be trivial to start a
timer that periodically does wake_up_all, and gets its timeout reset in a call
to radeon_fence_process. It could either be added as a work item, or as a normal
timer (disabled during gpu lockup recovery to prevent checks from fiddling with
things it shouldn't).

~Maarten