[Intel-gfx] [CI 1/2] drm/i915: Use ktime on wait_for

[Imre Deak]

On Mon, Apr 23, 2018 at 08:35:05AM -0700, Lucas De Marchi wrote:
> On Mon, Apr 23, 2018 at 4:37 AM, Mika Kuoppala
> <mika.kuoppala at linux.intel.com> 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.
> 
> Isn't this also the reason why we have *_to_jiffies_timeout() family
> of functions?

The reason for those is that you can sample jiffies between two clock tick
updates and calculate a short expiry time. So a different issue than missed
updates of jiffies due to some latency.

> Should those be converted as well?
> 
> Lucas De Marchi
> 
> > Discard jiffies and change to ktime to detect timeouts.
> >
> > v2: added bugzilla entry (Imre), added stable (Chris)
> >
> > Reported-by: Imre Deak <imre.deak at intel.com>
> > References: https://lkml.org/lkml/2018/4/18/798 [1]
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105771
> > 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>
> > Cc: <stable at vger.kernel.org>
> > Signed-off-by: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> > Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
> > ---
> >  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 44ed248f1fe9..33ff2638c92b 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)); \
> >         long wait__ = (Wmin); /* recommended min for usleep is 10 us */ \
> >         int ret__;                                                      \
> >         might_sleep();                                                  \
> >         for (;;) {                                                      \
> > -               bool expired__ = time_after(jiffies, timeout__);        \
> > +               const bool expired__ = ktime_after(ktime_get_raw(), end__); \
> >                 OP;                                                     \
> >                 if (COND) {                                             \
> >                         ret__ = 0;                                      \
> > --
> > 2.14.1
> >
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 
> 
> 
> -- 
> Lucas De Marchi
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