Re: [RFC 4/4] drm: add support for raw monotonic vblank timestamps

[Imre Deak <imre.deak@intel.com>]

On Fri, 2012-10-05 at 16:14 +0200, Michel Dänzer wrote:
> On Fre, 2012-10-05 at 16:59 +0300, Imre Deak wrote: 
> > On Fri, 2012-10-05 at 15:55 +0200, Michel Dänzer wrote:
> > > On Fre, 2012-10-05 at 16:37 +0300, Imre Deak wrote: 
> > > > In practice we never want the timestamps for vblank and page flip events
> > > > to be affected by time adjustments, so in addition to the gettimeofday
> > > > timestamps we used so far add support for raw monotonic timestamps.
> > > > 
> > > > For backward compatibility use flags to select between the old and new
> > > > timestamp format.
> > > > 
> > > > Note that with this change we will save the timestamp in both formats,
> > > > for cases where multiple clients are expecting an event notification in
> > > > different time formats.
> > > 
> > > I wonder if all this trouble is really necessary. I honestly can't
> > > imagine any user of this API requiring non-monotonic timestamps and
> > > breaking with monotonic ones. I think it was simply a mistake that we
> > > didn't make them monotonic in the first place (or maybe it wasn't even
> > > possible when this API was first introduced).
> > 
> > Yea, I'd rather simply switch over to monotonic timestamps too. But that
> > would break apps that already compare against the wall time for whatever
> > purpose (for example A/V sync).
> 
> Are there actually any such apps in the real world?

Tbh, I haven't checked, but we can't be sure in any case.

> Do they work when the wall time jumps?

They will have a problem with that yes. But providing them with a
monotonic clock when they expect otherwise will probably make them
unusable, so I think it's better to avoid that.

--Imre


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