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

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Wed, 15 Oct 2025 17:03:51 -0700, 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()

Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>

> Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/14955
> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
> ---
> 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)
> --
> 2.43.0
>