drm/i915: Fix conversion between clock ticks and nanoseconds

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 <[email protected]>
Reviewed-by: Ashutosh Dixit <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: unerlige

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)