[Intel-gfx] [PATCH] drm/i915: use hrtimer in wait for vblank

[Chris Wilson]

On Mon, Mar 24, 2014 at 09:34:35AM +0000, Murthy, Arun R wrote:
> >> diff --git a/drivers/gpu/drm/i915/intel_drv.h 
> >> b/drivers/gpu/drm/i915/intel_drv.h
> >> index 44067bc..079280a 100644
> >> --- a/drivers/gpu/drm/i915/intel_drv.h
> >> +++ b/drivers/gpu/drm/i915/intel_drv.h
> >> @@ -52,7 +52,7 @@
> >>  			break;						\
> >>  		}							\
> >>  		if (W && drm_can_sleep())  {				\
> >> -			msleep(W);					\
> >> +			usleep_range(W * 1000, W * 2 * 1000);		\
> >>  		} else {						\
> >>  			cpu_relax();					\
> >>  		}							\
> >
> > Ok. But W is still just a random value we picked for being the mininum 
> > legal value for msleep(). So just usleep_range(500, 2000) or somesuch 
> > will be fine. We can rename W to CAN_SLEEP it that helps.
> 
> We do use _wait_for directly from intel_dp.c with W == 10 to not retry so many times on what's expected to be a long wait.

Hmm, missed that. We can fallback to the fallback plan of switching
based on W between msleep and usleep_range. Using -1, would be best.

 if (W && drm_can_sleep()) {
  if (__builtin_constant_p(W) && W < 0)
     usleep_range(500, 2000);
  else
     msleep(W);
 }
 
> Its not expected to be too long, we tend to get vblank every 16ms.  With using usleep_range
> with min and max as 1-2ms, we have observed that this function consuming 4ms to 17ms.

vs msleep()? That would be nice information to capture in the changelog.
We expect the average wait here to be 8ms, ofc.

> Having 10ms sleep timer, we might tend to be blocking for 6ms in some cases.(from the above data)
> Moreover 10ms usleep doesn't guarantee that we get a chance to execute after 10ms, it might
> get increased due to scheduling. So ideal solution would be to use useep_range which uses hrtimers.

Ideal for what? We only use the sleeping path where we do not care too
great about the latency, otherwise we use the busy-wait path. Improving
the latency of the sleep without adversely affecting overall system
performance/power is ideal.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre