drm/i915/selftests: Rearrange ktime_get to reduce latency against CS

In our tests where we measure the elapsed time on both the CPU and CS
using a udelay, our CS results match the udelay much more accurately
than the ktime (even when using ktime_get_fast_ns). With preemption
disabled, we can go one step lower than ktime and use local_clock.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2919Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210108204026.20682-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/selftest_engine_pm.c

@@ -112,11 +112,11 @@ static int __measure_timestamps(struct intel_context *ce,
 
 	/* Run the request for a 100us, sampling timestamps before/after */
 	preempt_disable();
-	*dt = ktime_get_raw_fast_ns();
+	*dt = local_clock();
 	write_semaphore(&sema[2], 0);
 	udelay(100);
+	*dt = local_clock() - *dt;
 	write_semaphore(&sema[2], 1);
-	*dt = ktime_get_raw_fast_ns() - *dt;
 	preempt_enable();
 
 	if (i915_request_wait(rq, 0, HZ / 2) < 0) {