[PATCH weston 1/1] compositor: Abort on bad page flip timestamps

[Mario Kleiner]

On 06/11/14 17:42, Pekka Paalanen wrote:
> On Thu, 06 Nov 2014 16:08:56 +0900
> Michel Dänzer <michel at daenzer.net> wrote:
>
>> On 06.11.2014 03:06, Frederic Plourde wrote:
>>> Many features, like animations, hardly depend on page flip timestamps
>>> to work properly, but some DRM drivers do not correctly support page flip
>>> timestamps (or not at all) and in that case, things start to go wrong.
>>>
>>> This patch adds sanity check to weston_output_finish_frame. By solely
>>> verifying that page flip timestamps are monotonically increasing, we
>>> make sure that :
>>>
>>> 1) Underlying driver is not throwing zeroed-out timestamp series at us.
>>> 2) We have not mistakenly jumped backwards because of integer overflow.
>>>
>>> If a pathological case is detected, we gracefully exit Weston
>>> with an appropriate exit code to help developers debug their drivers.
>>
>> That seems a bit harsh. IIRC, zero can be returned for the timestamp
>> intermittently if no accurate timestamp value can be determined, e.g.
>> because the CRTC is disabled. At the very least, I'd recommend
>> double-checking this with Mario Kleiner (Cc'd) and the dri-devel mailing
>> list.
>
> Can that really happen if we are not doing stupid things like
> attempting to flip on a disabled crtc or output?
>

I don't think it could happen for non-stupid regular use and would 
indicate a driver bug.

> Or can it happen, if we schedule a flip, then disable the crtc
> before the flip completes? Or maybe when an output is hot-unplugged?
>

On kernels <= 3.17 disabling vblank irqs will clear the cached timestamp 
to zero, so a waitvblank ioctl() for a pure query of msc/ust could 
return zero as a signal of "invalid/undefined timestamp" on some drivers 
under some circumstances. Basically a -EAGAIN error code.

But that shouldn't ever happen for kms-pageflip completion timestamps, 
because vblank irqs don't get disabled while a pageflip is pending, as 
the pending pageflip keeps the vblank reference count > 0.

There are also some new patches into Linux 3.18rc which should prevent 
returning zero timestamps even for pure waitvblank ioctl() queries if 
vblank irqs get disabled for one reason or the other, cfe.

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/drivers/gpu/drm/drm_irq.c?id=844b03f27739135fe1fed2fef06da0ffc4c7a081

kms drivers should usually use the drm_send_vblank_event() helper in 
their pageflip completion routine. That helper will ideally send out the 
cached vblank count and high precision timestamps which were computed 
during the most recent vblank interrupt for the crtc that pageflipped. 
There's also some fallback there which, if no crtc is specified (crtc == 
-1), will simply return a msc of zero and the current system time as 
timestamp. The fallback is for gpus with weird flip completion 
behaviour. Currently nouveau uses it for old < NV-50 gpu's where we 
couldn't find a better way to make pageflip completion behave 
properly/reliably due to some hardware weirdness.

> Is zero a special timestamp that simply cannot be produced during
> normal operations, like due to clock wrap-around?
>

The timestamps are CLOCK_MONOTONIC uint32 seconds.microseconds since 
bootup, so a wraparound would ony happen after 2^32 seconds or 136 
years, so normal operation shouldn't cause a zero timestamp.

So i think observing zero pageflip timestamps would be a sign that the 
kms-driver needs fixing.

-mario

>
> Thanks,
> pq
>