pwm: rcar: Improve register calculation

There were several issues in the function rcar_pwm_set_counter():

- The u64 values period_ns and duty_ns were cast to int on function
call which might loose bits on 32 bit architectures.
Fix: Make parameters to rcar_pwm_set_counter() u64
- The algorithm divided by the result of a division which looses
precision.
Fix: Make use of mul_u64_u64_div_u64()
- The calculated values were just masked to fit the respective register
fields which again might loose bits.
Fix: Explicitly check for overlow

Implement the respective fixes.

A side effect of fixing the 2nd issue is that there is no division by 0
if clk_get_rate() returns 0.

Fixes: ed6c1476bf7f ("pwm: Add support for R-Car PWM Timer")
Signed-off-by: Uwe Kleine-König <[e-mail]>
Link: https://lore.kernel.org/r/ab3dac7[e-mail]
[ukleinek: Added an explicit #include <linux/bitfield.h> to please the
0day build bot]
Link: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Reviewed-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Uwe Kleine-König <[e-mail]>

drivers/pwm/pwm-rcar.c

···
  * - The hardware cannot generate a 0% duty cycle.
  */
 
+#include <linux/bitfield.h>
 #include <linux/clk.h>
 #include <linux/err.h>
 #include <linux/io.h>
···
 	rcar_pwm_write(rp, value, RCAR_PWMCR);
 }
 
-static int rcar_pwm_set_counter(struct rcar_pwm_chip *rp, int div, int duty_ns,
-				int period_ns)
+static int rcar_pwm_set_counter(struct rcar_pwm_chip *rp, int div, u64 duty_ns,
+				u64 period_ns)
 {
-	unsigned long long one_cycle, tmp;	/* 0.01 nanoseconds */
+	unsigned long long tmp;
 	unsigned long clk_rate = clk_get_rate(rp->clk);
 	u32 cyc, ph;
 
-	one_cycle = NSEC_PER_SEC * 100ULL << div;
-	do_div(one_cycle, clk_rate);
+	/* div <= 24 == RCAR_PWM_MAX_DIVISION, so the shift doesn't overflow. */
+	tmp = mul_u64_u64_div_u64(period_ns, clk_rate, (u64)NSEC_PER_SEC << div);
+	if (tmp > FIELD_MAX(RCAR_PWMCNT_CYC0_MASK))
+		tmp = FIELD_MAX(RCAR_PWMCNT_CYC0_MASK);
 
-	tmp = period_ns * 100ULL;
-	do_div(tmp, one_cycle);
-	cyc = (tmp << RCAR_PWMCNT_CYC0_SHIFT) & RCAR_PWMCNT_CYC0_MASK;
+	cyc = FIELD_PREP(RCAR_PWMCNT_CYC0_MASK, tmp);
 
-	tmp = duty_ns * 100ULL;
-	do_div(tmp, one_cycle);
-	ph = tmp & RCAR_PWMCNT_PH0_MASK;
+	tmp = mul_u64_u64_div_u64(duty_ns, clk_rate, (u64)NSEC_PER_SEC << div);
+	if (tmp > FIELD_MAX(RCAR_PWMCNT_PH0_MASK))
+		tmp = FIELD_MAX(RCAR_PWMCNT_PH0_MASK);
+	ph = FIELD_PREP(RCAR_PWMCNT_PH0_MASK, tmp);
 
 	/* Avoid prohibited setting */
 	if (cyc == 0 || ph == 0)