With GuC handling scheduling, i915 is not aware of the time that a
context is scheduled in and out of the engine. Since i915 pmu relies on
this info to provide engine busyness to the user, GuC shares this info
with i915 for all engines using shared memory. For each engine, this
info contains:

- total busyness: total time that the context was running (total)
- id: id of the running context (id)
- start timestamp: timestamp when the context started running (start)

At the time (now) of sampling the engine busyness, if the id is valid
(!= ~0), and start is non-zero, then the context is considered to be
active and the engine busyness is calculated using the below equation

	engine busyness = total + (now - start)

All times are obtained from the gt clock base. For inactive contexts,
engine busyness is just equal to the total.

The start and total values provided by GuC are 32 bits and wrap around
in a few minutes. Since perf pmu provides busyness as 64 bit
monotonically increasing values, there is a need for this implementation
to account for overflows and extend the time to 64 bits before returning
busyness to the user. In order to do that, a worker runs periodically at
frequency = 1/8th the time it takes for the timestamp to wrap. As an
example, that would be once in 27 seconds for a gt clock frequency of
19.2 MHz.

Note:
There might be an over-accounting of busyness due to the fact that GuC
may be updating the total and start values while kmd is reading them.
(i.e kmd may read the updated total and the stale start). In such a
case, user may see higher busyness value followed by smaller ones which
would eventually catch up to the higher value.

v2: (Tvrtko)
- Include details in commit message
- Move intel engine busyness function into execlist code
- Use union inside engine->stats
- Use natural type for ping delay jiffies
- Drop active_work condition checks
- Use for_each_engine if iterating all engines
- Drop seq locking, use spinlock at GuC level to update engine stats
- Document worker specific details

v3: (Tvrtko/Umesh)
- Demarcate GuC and execlist stat objects with comments
- Document known over-accounting issue in commit
- Provide a consistent view of GuC state
- Add hooks to gt park/unpark for GuC busyness
- Stop/start worker in gt park/unpark path
- Drop inline
- Move spinlock and worker inits to GuC initialization
- Drop helpers that are called only once

v4: (Tvrtko/Matt/Umesh)
- Drop addressed opens from commit message
- Get runtime pm in ping, remove from the park path
- Use cancel_delayed_work_sync in disable_submission path
- Update stats during reset prepare
- Skip ping if reset in progress
- Explicitly name execlists and GuC stats objects
- Since disable_submission is called from many places, move resetting
  stats to intel_guc_submission_reset_prepare

v5: (Tvrtko)
- Add a trylock helper that does not sleep and synchronize PMU event
  callbacks and worker with gt reset

v6: (CI BAT failures)
- DUTs using execlist submission failed to boot since __gt_unpark is
  called during i915 load. This ends up calling the GuC busyness unpark
  hook and results in kick-starting an uninitialized worker. Let
  park/unpark hooks check if GuC submission has been initialized.
- drop cant_sleep() from trylock helper since rcu_read_lock takes care
  of that.

v7: (CI) Fix igt@i915_selftest@live@gt_engines
- For GuC mode of submission the engine busyness is derived from gt time
  domain. Use gt time elapsed as reference in the selftest.
- Increase busyness calculation to 10ms duration to ensure batch runs
  longer and falls within the busyness tolerances in selftest.

v8:
- Use ktime_get in selftest as before
- intel_reset_trylock_no_wait results in a lockdep splat that is not
  trivial to fix since the PMU callback runs in irq context and the
  reset paths are tightly knit into the driver. The test that uncovers
  this is igt@perf_pmu@faulting-read. Drop intel_reset_trylock_no_wait,
  instead use the reset_count to synchronize with gt reset during pmu
  callback. For the ping, continue to use intel_reset_trylock since ping
  is not run in irq context.

- GuC PM timestamp does not tick when GuC is idle. This can potentially
  result in wrong busyness values when a context is active on the
  engine, but GuC is idle. Use the RING TIMESTAMP as GPU timestamp to
  process the GuC busyness stats. This works since both GuC timestamp and
  RING timestamp are synced with the same clock.

- The busyness stats may get updated after the batch starts running.
  This delay causes the busyness reported for 100us duration to fall
  below 95% in the selftest. The only option at this time is to wait for
  GuC busyness to change from idle to active before we sample busyness
  over a 100us period.
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211027004821.66097-2-umesh.nerlige.ramappa@intel.com