RFC: Change OML_sync_control UST to CLOCK_MONOTONIC

[Ian Romanick]

On 06/11/2012 03:02 AM, Michel Dänzer wrote:
> On Son, 2012-06-10 at 11:56 +0000, Joakim Plate wrote:
>> Hi,
>>
>> I'm currently trying to make use of OML_sync_control extension to schedule
>> presentation of video frames in xbmc.
>>
>> I've run into somewhat of a snag. It seem the spec doesn't specify what
>> time the UST clock really is, nor can i find any mention of it elsewhere
>> in docs.
>>
>> Code wise it seem to be using do_gettimeofday(), which seems like a rather
>> poor choice given that it can jump forward and back in time due to
>> settimeofday calls.
>>
>> We normally make use of clock_gettime(CLOCK_MONOTONIC) to timestamp display
>> of video frames, so to avoid major changes I'd need a way to convert to
>> gettimeofday (seem same as CLOCK_REALTIME).
>>
>> Currently i'm trying:
>>    struct timespec mon, rel;
>>    clock_gettime(CLOCK_MONOTONIC,&mon);
>>    clock_gettime(CLOCK_REALTIME ,&rel);
>>
>>    ticks += (rel.tv_sec  - mon.tv_sec)  * 1000000000;
>>    ticks += (rel.tv_nsec - mon.tv_nsec);
>>
>> To convert between the two, but that is quite a hack both in the
>> possibility of clock changes and scheduling errors.
>>
>> Is there a better way, or perhaps the DRI code should use CLOCK_MONOTONIC
>> in the first place?
>
>  From the GLX_OML_sync_control spec:
>
>          The Unadjusted System Time (or UST) is a 64-bit monotonically
>          increasing counter [...]
>
> So, without having thought a lot about potential ramifications, I'm
> inclined to say that CLOCK_MONOTONIC should indeed be used.

Maybe.  Isn't CLOCK_MONOTONIC per-process?  UST is supposed to be 
relatively consistent across the system.  Some relevant was elided above:

     "The Unadjusted System Time (or UST) is a 64-bit monotonically
     increasing counter that is available throughout the system. UST
     is not a resource that is controlled by OpenGL, so it is not
     defined further as part of this extension."

do_gettimeofday is definitely the wrong choice, but it was the best 
thing that I knew to use back in 2002(ish) when I wrote that code.  I'm 
just not sure CLOCK_MONOTONIC is quite right either.

What's behind door #3? :)