[Intel-gfx] [PATCH] drm/i915: Use ktime on wait_for

[Chris Wilson]

Quoting Sagar Arun Kamble (2018-04-20 11:23:50)
> 
> 
> On 4/20/2018 3:24 PM, Mika Kuoppala wrote:
> > We use jiffies to determine when wait expires. However
> > Imre did find out that jiffies can and will do a >1
> > increments on certain situations [1]. When this happens
> > in a wait_for loop, we return timeout errorneously
> > much earlier than what the real wallclock would say.
> >
> > We can't afford our waits to timeout prematurely.
> > Discard jiffies and change to ktime to detect timeouts.
> >
> > Reported-by: Imre Deak <imre.deak at intel.com>
> > References: https://lkml.org/lkml/2018/4/18/798 [1]
> > Cc: Imre Deak <imre.deak at intel.com>
> > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > Signed-off-by: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> > ---
> >   drivers/gpu/drm/i915/intel_drv.h | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
> > index 8b20824e806e..ac7565220aa3 100644
> > --- a/drivers/gpu/drm/i915/intel_drv.h
> > +++ b/drivers/gpu/drm/i915/intel_drv.h
> > @@ -49,12 +49,12 @@
> >    * check the condition before the timeout.
> >    */
> >   #define __wait_for(OP, COND, US, Wmin, Wmax) ({ \
> > -     unsigned long timeout__ = jiffies + usecs_to_jiffies(US) + 1;   \
> > +     const ktime_t end__ = ktime_add_ns(ktime_get_raw(), 1000ll * (US)); \
> Is ktime_get_raw() monotonic? Thomas suggested ktime_get()

It proclaims to be monotonic, without the clock drift calibration. For
the milliseconds we should be sleeping at most, I hope that is
immaterial.
-Chris