Re: [PATCH] drm/i915: Fix conversion between clock ticks and nanoseconds

[Rodrigo Vivi <rodrigo.vivi@intel.com>]

On Fri, Oct 31, 2025 at 08:40:53AM +0000, Tvrtko Ursulin wrote:
> 
> On 29/10/2025 22:01, Umesh Nerlige Ramappa wrote:
> > On Wed, Oct 29, 2025 at 11:33:20AM +0000, Tvrtko Ursulin wrote:
> > > 
> > > On 16/10/2025 01:03, Umesh Nerlige Ramappa wrote:
> > > > When tick values are large, the multiplication by NSEC_PER_SEC is larger
> > > > than 64 bits and results in bad conversions.
> > > > 
> > > > The issue is seen in PMU busyness counters that look like they have
> > > > wrapped around due to bad conversion. i915 PMU implementation returns
> > > > monotonically increasing counters. If a count is lesser than previous
> > > > one, it will only return the larger value until the smaller value
> > > > catches up. The user will see this as zero delta between two
> > > > measurements even though the engines are busy.
> > > > 
> > > > Fix it by using mul_u64_u32_div()
> > > > 
> > > > Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/14955
> > > > Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
> > > 
> > > There was no need for Fixes: or cc: stable?
> > 
> > Oh, sorry, I missed that. Should have been both. Is it too late to do
> > that now?
> > 
> > Fixes: 77cdd054dd2c ("drm/i915/pmu: Connect engine busyness stats from
> > GuC to pmu")
> 
> Possibly not. Lets ask Rodrigo if he could manually cherry pick it for 6.18.

Done, and added the fixes tag while at it.

Thanks for the heads up.

> 
> Regards,
> 
> Tvrtko
> 
> > > > ---
> > > > v2:
> > > > - Fix divide by zero for Gen11 (Andi)
> > > > - Update commit message
> > > > 
> > > > v3:
> > > > - Drop GCD and use mul_u64_u32_div() instead (Ashutosh)
> > > > ---
> > > >  drivers/gpu/drm/i915/gt/intel_gt_clock_utils.c | 4 ++--
> > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/i915/gt/intel_gt_clock_utils.c b/
> > > > drivers/gpu/drm/i915/gt/intel_gt_clock_utils.c
> > > > index 88b147fa5cb1..c90b35881a26 100644
> > > > --- a/drivers/gpu/drm/i915/gt/intel_gt_clock_utils.c
> > > > +++ b/drivers/gpu/drm/i915/gt/intel_gt_clock_utils.c
> > > > @@ -205,7 +205,7 @@ static u64 div_u64_roundup(u64 nom, u32 den)
> > > >  u64 intel_gt_clock_interval_to_ns(const struct intel_gt *gt, u64 count)
> > > >  {
> > > > -    return div_u64_roundup(count * NSEC_PER_SEC, gt->clock_frequency);
> > > > +    return mul_u64_u32_div(count, NSEC_PER_SEC, gt->clock_frequency);
> > > >  }
> > > >  u64 intel_gt_pm_interval_to_ns(const struct intel_gt *gt, u64 count)
> > > > @@ -215,7 +215,7 @@ u64 intel_gt_pm_interval_to_ns(const struct
> > > > intel_gt *gt, u64 count)
> > > >  u64 intel_gt_ns_to_clock_interval(const struct intel_gt *gt, u64 ns)
> > > >  {
> > > > -    return div_u64_roundup(gt->clock_frequency * ns, NSEC_PER_SEC);
> > > > +    return mul_u64_u32_div(ns, gt->clock_frequency, NSEC_PER_SEC);
> > > >  }
> > > >  u64 intel_gt_ns_to_pm_interval(const struct intel_gt *gt, u64 ns)
> > > 
>