On 8 February 2016 09:55:08 GMT+00:00, Lars-Peter Clausen <lars@xxxxxxxxxx> wrote: >On 02/06/2016 07:33 PM, Jonathan Cameron wrote: >> On 03/02/16 11:22, Lars-Peter Clausen wrote: >>> On 02/03/2016 11:55 AM, Gregor Boirie wrote: >>>> Dear all, >>>> >>>> Our application relies on precise and monotonic timestamping of IMU >samples >>>> (and other sensors). >>>> I am wondering what reasons / use cases led to the choice of >realtime clock >>>> to implement >>>> iio_get_time_ns (not to mention time gaps that may be seen after >wake up >>>> from sleep states). >>> >>> It's more of an oversight than a deliberate design decision. I >noticed this >>> problem as well a while ago and wanted to re-write things to use the >>> monotonic clock, but then realized that this would be a ABI change >so >>> dropped it and forgot about it again. >> There are certainly cases where the other clock would make sense (for >slow >> sampling device where being as correct as possible is the most >important >> thing). > >I'm not sure I understand what you are trying to say, maybe we are not >on >the same page. As far as I know both clocks have the same precession, >but >their absolute value differs. > >What iio_get_time_ns() currently returns is the system clock, which >changes >whenever the time is changed (e.g. to compensate for drift, or daylight >saving, etc.). This is obviously not so great if that happens in the >middle >of the capture since what you care about is the relative distance >between >the samples and if your time base changes you have no idea what that is >anymore. > >So the monotonic clock which just keeps going at a fixed interval would >be >the better choice. In general I agree. My thought was that there are plausible usecases where a capture is every n minutes over days or years where a timestamp that tracks with 'corrections' would make sense as intervals are not always the most important thing. Most cases if course the monotonic clock is the best one. I chose badly a long time ago! Risk is someone is relying on it for some reason. J -- Sent from my Android device with K-9 Mail. Please excuse my brevity. -- To unsubscribe from this list: send the line "unsubscribe linux-iio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
