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/2919
Signed-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

diff --git a/drivers/gpu/drm/i915/gt/selftest_engine_pm.c b/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
index ca08044..c3d9652 100644 (file)
--- a/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
+++ b/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) {