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

[Dixit, Ashutosh]

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?

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.