[Intel-gfx] [PATCH] drm/i915: Avoid potential division-by-zero in computing CS timestamp period

[Chris Wilson]

Quoting Lionel Landwerlin (2020-02-18 21:54:03)
> On 16/02/2020 18:17, Chris Wilson wrote:
> > Since we use a HW readback or estimation of the CS timestamp frequency,
> > sometimes it may result in 0. Avoid the division-by-zero in computing
> > its reciprocal, the timestamp period.
> >
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > ---
> >   drivers/gpu/drm/i915/intel_device_info.c | 18 +++++++++++-------
> >   1 file changed, 11 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_device_info.c b/drivers/gpu/drm/i915/intel_device_info.c
> > index a97437fac884..18d9de488593 100644
> > --- a/drivers/gpu/drm/i915/intel_device_info.c
> > +++ b/drivers/gpu/drm/i915/intel_device_info.c
> > @@ -1044,13 +1044,17 @@ void intel_device_info_runtime_init(struct drm_i915_private *dev_priv)
> >       }
> >   
> >       /* Initialize command stream timestamp frequency */
> > -     runtime->cs_timestamp_frequency_khz = read_timestamp_frequency(dev_priv);
> > -     runtime->cs_timestamp_period_ns =
> > -             div_u64(1e6, runtime->cs_timestamp_frequency_khz);
> > -     drm_dbg(&dev_priv->drm,
> > -             "CS timestamp wraparound in %lldms\n",
> > -             div_u64(mul_u32_u32(runtime->cs_timestamp_period_ns, S32_MAX),
> > -                     USEC_PER_SEC));
> > +     runtime->cs_timestamp_frequency_khz =
> > +             read_timestamp_frequency(dev_priv);
> > +     if (runtime->cs_timestamp_frequency_khz) {
> > +             runtime->cs_timestamp_period_ns =
> > +                     div_u64(1e6, runtime->cs_timestamp_frequency_khz);
> > +             drm_dbg(&dev_priv->drm,
> > +                     "CS timestamp wraparound in %lldms\n",
> > +                     div_u64(mul_u32_u32(runtime->cs_timestamp_period_ns,
> > +                                         S32_MAX),
> > +                             USEC_PER_SEC));
> > +     }
> 
> Arg this is used in i915-perf in at least one place as denominator too...

This was a quick fix for BAT. See
https://patchwork.freedesktop.org/patch/353455/?series=73507&rev=1
then there are only a handful of platforms for which we don't know the
frequency, none of which matter for i915-perf.
-Chris