Re: [CI 16/20] drm/i915: Only query timestamp when measuring elapsed time

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On 19/05/16 12:32, Chris Wilson wrote:
Avoid the two calls to ktime_get_raw_ns() (at best it reads the TSC) as
we only need to compute the elapsed time for a timed wait.

v2: Eliminate the unused local variable reducing the function size by 64
bytes (using the storage space on the callers stack rather than adding
to our stack frame)

Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
  drivers/gpu/drm/i915/i915_gem.c | 14 +++++---------
  1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index b48a3b46e86f..2c254cf49c15 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1215,7 +1215,6 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
  	int state = interruptible ? TASK_INTERRUPTIBLE : TASK_UNINTERRUPTIBLE;
  	struct intel_wait wait;
  	unsigned long timeout_remain;
-	s64 before = 0; /* Only to silence a compiler warning. */
  	int ret = 0;

  	might_sleep();
@@ -1234,12 +1233,9 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
  		if (*timeout == 0)
  			return -ETIME;

+		/* Record current time in case interrupted, or wedged */
  		timeout_remain = nsecs_to_jiffies_timeout(*timeout);
-
-		/*
-		 * Record current time in case interrupted by signal, or wedged.
-		 */
-		before = ktime_get_raw_ns();
+		*timeout += ktime_get_raw_ns();
  	}

  	trace_i915_gem_request_wait_begin(req);
@@ -1296,9 +1292,9 @@ complete:
  	trace_i915_gem_request_wait_end(req);

  	if (timeout) {
-		s64 tres = *timeout - (ktime_get_raw_ns() - before);
-
-		*timeout = tres < 0 ? 0 : tres;
+		*timeout -= ktime_get_raw_ns();
+		if (*timeout < 0)
+			*timeout = 0;

  		/*
  		 * Apparently ktime isn't accurate enough and occasionally has a


I think this is bad, better have a local than play games with callers storage.

Regards,

Tvrtko

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