Re: [PATCH 14/15] drm/i915: Only query timestamp when measuring elapsed time

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

On 29/11/15 08:48, 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.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/i915_gem.c | 13 +++++--------
>   1 file changed, 5 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 2c3e36e19cb0..871201713c73 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1227,7 +1227,6 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
>   	int state = interruptible ? TASK_INTERRUPTIBLE : TASK_UNINTERRUPTIBLE;
>   	DEFINE_WAIT(wait);
>   	unsigned long timeout_remain;
> -	s64 before, now;
>   	int ret;
>
>   	if (list_empty(&req->list))
> @@ -1244,13 +1243,12 @@ 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);
> +		*timeout += ktime_get_raw_ns();

Don't really like this one, how you use the passed in pointer to store 
the intermediate local state.

It works etc but just feels too hacky.

Regards,

Tvrtko


>   	}
>
> -	/* Record current time in case interrupted by signal, or wedged */
>   	trace_i915_gem_request_wait_begin(req);
> -	before = ktime_get_raw_ns();
> -
>   	if (INTEL_INFO(req->i915)->gen >= 6)
>   		gen6_rps_boost(req->i915, rps, req->emitted_jiffies);
>
> @@ -1286,14 +1284,13 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
>   	}
>   	finish_wait(&req->wait, &wait);
>   out:
> -	now = ktime_get_raw_ns();
>   	intel_breadcrumbs_remove_waiter(req);
>   	trace_i915_gem_request_wait_end(req);
>
>   	if (timeout) {
> -		s64 tres = *timeout - (now - 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
>
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