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

[Murthy, Arun R]

> > > 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);
>  }
>  

Seems to be a good approach, will take care of this in the next version of the patch.

> > 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.
> 

Sure, will add this in the changelog.

> > 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.

We've used it in a few places where we'd care a bit more about latency (like edid reads) but on most platforms those switched to interrupts now.
-Daniel

Thanks and Regards,
Arun R Murthy
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