The cycle register is always u32, so cycles_to_ns() can take a u32
instead of a u64. With that narrowing, cycles * NSEC_PER_SEC is at most
u32::MAX * 1e9 (~4.3e18), which fits in u64 without overflow. The
saturating arithmetic is therefore no longer needed, and the ceiling
division can use Rust's u64::div_ceil() directly instead of the
open-coded numerator/denominator form.

This also drops the TODO referring to a future
mul_u64_u64_div_u64_roundup kernel helper, which is no longer required.

Reviewed-by: Michal Wilczynski <m.wilczynski@samsung.com>
Signed-off-by: Maurice Hieronymus <mhi@mailbox.org>
---
Changes in v2:
- Rebase on pwm/for-next
- Add Reviewed-by trailers
- Link to v1: https://lore.kernel.org/r/20260521-pwm-th1520-fix-v1-1-df9cd956dab4@mailbox.org
---
 drivers/pwm/pwm_th1520.rs | 16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/drivers/pwm/pwm_th1520.rs b/drivers/pwm/pwm_th1520.rs
index ddd44a5ce497..933c1ec59c2a 100644
--- a/drivers/pwm/pwm_th1520.rs
+++ b/drivers/pwm/pwm_th1520.rs
@@ -67,16 +67,10 @@ fn ns_to_cycles(ns: u64, rate_hz: u64) -> u64 {
     ns.saturating_mul(rate_hz) / NSEC_PER_SEC_U64
 }
 
-fn cycles_to_ns(cycles: u64, rate_hz: u64) -> u64 {
+fn cycles_to_ns(cycles: u32, rate_hz: u64) -> u64 {
     const NSEC_PER_SEC_U64: u64 = time::NSEC_PER_SEC as u64;
 
-    // TODO: Replace with a kernel helper like `mul_u64_u64_div_u64_roundup`
-    // once available in Rust.
-    let numerator = cycles
-        .saturating_mul(NSEC_PER_SEC_U64)
-        .saturating_add(rate_hz - 1);
-
-    numerator / rate_hz
+    (u64::from(cycles) * NSEC_PER_SEC_U64).div_ceil(rate_hz)
 }
 
 /// Hardware-specific waveform representation for TH1520.
@@ -192,15 +186,15 @@ fn round_waveform_fromhw(
             return Ok(());
         }
 
-        wf.period_length_ns = cycles_to_ns(u64::from(wfhw.period_cycles), rate_hz);
+        wf.period_length_ns = cycles_to_ns(wfhw.period_cycles, rate_hz);
 
-        let duty_cycles = u64::from(wfhw.duty_cycles);
+        let duty_cycles = wfhw.duty_cycles;
 
         if (wfhw.ctrl_val & TH1520_PWM_FPOUT) != 0 {
             wf.duty_length_ns = cycles_to_ns(duty_cycles, rate_hz);
             wf.duty_offset_ns = 0;
         } else {
-            let period_cycles = u64::from(wfhw.period_cycles);
+            let period_cycles = wfhw.period_cycles;
             let original_duty_cycles = period_cycles.saturating_sub(duty_cycles);
 
             // For an inverted signal, `duty_length_ns` is the high time (period - low_time).

---
base-commit: 5b5e33c44a491d5b5e019f527e028bd567a226fa
change-id: 20260521-pwm-th1520-fix-8e45558bbd31

Best regards,
-- 
Maurice Hieronymus <mhi@mailbox.org>

Hello,

On Fri, Jun 05, 2026 at 09:03:59AM +0200, Maurice Hieronymus wrote:
> The cycle register is always u32, so cycles_to_ns() can take a u32
> instead of a u64. With that narrowing, cycles * NSEC_PER_SEC is at most
> u32::MAX * 1e9 (~4.3e18), which fits in u64 without overflow. The
> saturating arithmetic is therefore no longer needed, and the ceiling
> division can use Rust's u64::div_ceil() directly instead of the
> open-coded numerator/denominator form.
> 
> This also drops the TODO referring to a future
> mul_u64_u64_div_u64_roundup kernel helper, which is no longer required.
> 
> Reviewed-by: Michal Wilczynski <m.wilczynski@samsung.com>
> Signed-off-by: Maurice Hieronymus <mhi@mailbox.org>

Applied to

https://git.kernel.org/pub/scm/linux/kernel/git/ukleinek/linux.git pwm/for-next

Thanks
Uwe