Re: [PATCH v5.1 3/7] v4l: Add timestamp source flags, mask and document them

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Sakari,

On Friday 21 February 2014 15:04:48 Sakari Ailus wrote:
> Hi Laurent,
> 
> On Fri, Feb 21, 2014 at 12:58:58PM +0100, Laurent Pinchart wrote:
> ...
> 
> > > It's possible to calculate it (decrementing the readout time + exposure
> > > time from the end of frame timestamp) and that's what the devices
> > > supposedly do. The pre-frame exposure time isn't available to the host,
> > > so the end of frame timestamp cannot be calculated by the host from the
> > > camera generated timestamp.
> > > 
> > > However the link to the host is USB which has a lot more latency than
> > > almost anything else which makes even hardware generated timestamps a
> > > little imprecise.
> > 
> > Why so ? There will be a jitter in frame arrival, but the hardware
> > timestamp should be accurate (at least if properly generated by the
> > camera firmware).
>
> Yes, the hardware timestamp should be accurate on its own, but as there's
> delay and jitter converting that into something that's relevant on the host
> adds some uncertainty. AFAIR the accuracy of the camera generated timestamp
> was still much better than that of the driver generated one, right?

Yes it is, as the conversion mechanism is stats-based and takes jitter and 
delays into account.

-- 
Regards,

Laurent Pinchart