Irrespective of the backend for request submissions, busyness for an
engine with an active context is calculated using:

busyness = total + (current_time - context_switch_in_time)

In execlists mode of operation, the context switch events are handled
by the CPU. Context switch in/out time and current_time are captured
in CPU time domain using ktime_get().

In GuC mode of submission, context switch events are handled by GuC and
the times in the above formula are captured in GT clock domain. This
information is shared with the CPU through shared memory. This results
in 2 caveats:

1) The time taken between start of a batch and the time that CPU is able
to see the context_switch_in_time in shared memory is dependent on GuC
and memory bandwidth constraints.

2) Determining current_time requires an MMIO read that can take anywhere
between a few us to a couple ms. A reference CPU time is captured soon
after reading the MMIO so that the caller can compare the cpu delta
between 2 busyness samples. The issue here is that the CPU delta and the
busyness delta can be skewed because of the time taken to read the
register.

These 2 factors affect the accuracy of the selftest -
live_engine_busy_stats. For (1) the selftest waits until busyness stats
are visible to the CPU. The effects of (2) are more prominent for the
current busyness sample period of 100 us. Increase the busyness sample
period from 100 us to 10 ms to overccome (2).

v2: Fix checkpatch issues
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211115221640.30793-1-umesh.nerlige.ramappa@intel.com