[Intel-gfx] [PATCH 15/16] drm/i915: Mechanism to forward clock monotonic time in perf samples

[Chris Wilson]

On Mon, May 09, 2016 at 11:29:50AM +0530, sourab gupta wrote:
> On Sat, 2016-04-23 at 01:19 +0530, Chris Wilson wrote:
> > Have you looked at cross-timestamps?
> 
> I was looking at the cross-timestamp patch set
> (https://lkml.org/lkml/2016/1/4/541), but I'm not entirely sure the
> cross timestamp stuff is solving the same problem. The cross timestamp
> stuff is trying to "synchronously capture system/device timestamp(s)".
> While here, we have a log of many recorded device timestamps and want to
> correlate all of those with system timestamp (_CLOCK_MONOTONOC for now).
> As we aren't reading system/device timestamps together for all
> timestamps we have, our problem is more about deriving an accurate idea
> of timestamp frequency to improve the accuracy when correlating between
> the synchronization points. Maybe the cross timestamp stuff will help to
> periodically read tightly correlated timestamp pairs synchronously which
> can be used as basis for correlating the logged timestamps based on our
> derived timestamp frequency. This idea can be evaluated once the cross
> timestamp patches land in kernel.

My understanding was that the cross-timestamps was for providing the
tight coupling between clock sources. It is already available in
upstream or -nightly for use.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre