Re: [PATCH v4 1/4] Produce system time from correlated clocksource

John Stultz
2015-10-20T20:16:34Z

On Tue, Oct 20, 2015 at 12:11 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Tue, 20 Oct 2015, Richard Cochran wrote:
>> On Tue, Oct 20, 2015 at 01:51:13PM +0200, Richard Cochran wrote:
>> > You can, in fact, achieve "proper" correlation by sampling. As John
>> > said, the question is whether the method in the patch set "measurably
>> > improves the error" over using another, simpler method.
>>
>> Here is a short example to put some numbers on the expected error.
>> Let the driver sample at an interval of 1 ms. If the system time's
>> frequency hasn't changed between two samples, A and B, then the driver
>> may interpolate without introducing any error.
>
> Darn, we don't want to have that kind of sampling in every driver
> which has this kind of problem even if it looks like the simpler
> choice for this particular use case. This is going to be something
> which next generation chips will have on more than just the audio
> interface and we realy want to have a generic solution for this.

I sort of agree with Richard that the timekeeper history approach
doesn't seem like a generic solution here.

And again, you seem to be speaking with a bigger picture in mind that
at least I don't yet share (apologies for being thick headed here).
Being able to have various hardware sharing a time base is quite
useful, and methods for correlating timestamps together are useful.
But I don't yet really understand why its important that we can
translate a hardware timestamp from some time in the past to the
correct system time in the past without error.

thanks
-john