[Intel-gfx] [PATCH] i915/query: Correlate engine and cpu timestamps with better accuracy

[Lionel Landwerlin]

On 03/03/2021 23:28, Umesh Nerlige Ramappa wrote:
> Perf measurements rely on CPU and engine timestamps to correlate
> events of interest across these time domains. Current mechanisms get
> these timestamps separately and the calculated delta between these
> timestamps lack enough accuracy.
>
> To improve the accuracy of these time measurements to within a few us,
> add a query that returns the engine and cpu timestamps captured as
> close to each other as possible.
>
> v2: (Tvrtko)
> - document clock reference used
> - return cpu timestamp always
> - capture cpu time just before lower dword of cs timestamp
>
> v3: (Chris)
> - use uncore-rpm
> - use __query_cs_timestamp helper
>
> v4: (Lionel)
> - Kernel perf subsytem allows users to specify the clock id to be used
>    in perf_event_open. This clock id is used by the perf subsystem to
>    return the appropriate cpu timestamp in perf events. Similarly, let
>    the user pass the clockid to this query so that cpu timestamp
>    corresponds to the clock id requested.
>
> v5: (Tvrtko)
> - Use normal ktime accessors instead of fast versions
> - Add more uApi documentation
>
> v6: (Lionel)
> - Move switch out of spinlock
>
> v7: (Chris)
> - cs_timestamp is a misnomer, use cs_cycles instead
> - return the cs cycle frequency as well in the query
>
> v8:
> - Add platform and engine specific checks
>
> v9: (Lionel)
> - Return 2 cpu timestamps in the query - captured before and after the
>    register read
>
> Signed-off-by: Umesh Nerlige Ramappa<umesh.nerlige.ramappa at intel.com>
> ---
>   drivers/gpu/drm/i915/i915_query.c | 144 ++++++++++++++++++++++++++++++

FYI, the MR for Mesa : 
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9407


-Lionel