Comment 36 for bug 1550371

OK, after quite a bit of research the bottom line is that this is a mali quirk.
There are 2 potential ways both are by specification allowed to sync with the driver - which resulted in a performance issue (covered by this bug) and a "correctness issue" covered by bug 1573014
The performance issue is now addressed by the quirk added to mir0.22 for all the devices with mali gpu.
Further work will continue on bug 1573014 as there are other suspicions from data collected here that the mali driver has other issues.