Mario Kleiner proposed a set of kernel patches that move latency-sensitive code of the VBlank time-stamping from DRM core and into the KMS drivers. Mario has tested this with the Intel and Radeon DRM drivers and for Intel KMS the changes have been of great improvement.
Since the Linux 3.11 kernel, vblank time-stamps for the Intel driver have been "too inaccurate" and also no longer works with a PREEMPT_RT kernel. These patches also restore support for handling DRM drivers with real-time Linux kernels.
The set of four kernel patches for improving vblank timestamps can be found via the dri-devel list. There's been some code review so far on the work but no word whether it will be ready for the Linux 3.13 kernel.
Mario noted that the time-stamp uncertainty for a low-end Atom netbook went from 3-8 micro-seconds with spikes greater than 20 micro-seconds to now typically being just one micro-second and then just occassionally 2 or 3 micro-seconds.