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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

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@chris-wilson.co.uk>
> ---
>   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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