pwm: th1520: Remove requirement for mul_u64_u64_div_u64_roundup

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>
Link: https://patch.msgid.link/20260605-pwm-th1520-fix-v2-1-5921e3a595f7@mailbox.org
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>

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 @@ impl pwm::PwmOps for Th1520PwmDriverData {
             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).