[PATCH 1/1] i915/query: Correlate engine and cpu timestamps with better accuracy

[Dixit, Ashutosh]

On Fri, 30 Apr 2021 16:00:46 -0700, Dixit, Ashutosh wrote:
>
> On Fri, 30 Apr 2021 15:26:09 -0700, Umesh Nerlige Ramappa wrote:
> >
> > Looks like the engine can be dropped since all timestamps are in sync. I
> > just have one more question here. The timestamp itself is 36 bits.  Should
> > the uapi also report the timestamp width to the user OR should I just
> > return the lower 32 bits of the timestamp?
>
> How would exposing only the lower 32 bits of the timestamp work?

It would work I guess but overflow every few seconds. So if the counters
are sampled at a low frequency (once every few seconds) it would yield
misleading timestamps.

> The way to avoid exposing the width would be to expose the timestamp as a
> regular 64 bit value. In the kernel engine state, have a variable for the
> counter and keep on accumulating that (on each query) to full 64 bits in
> spite of the 36 bit HW counter overflow.
>
> So not exposing the width (or exposing a 64 bit timestamp) is a cleaner
> interface but also more work in the kernel.