pwm: lpc18xx: Fix period handling

The calculation:

val = (u64)NSEC_PER_SEC * LPC18XX_PWM_TIMER_MAX;
do_div(val, lpc18xx_pwm->clk_rate);
lpc18xx_pwm->max_period_ns = val;

is bogus because with NSEC_PER_SEC = 1000000000,
LPC18XX_PWM_TIMER_MAX = 0xffffffff and clk_rate < NSEC_PER_SEC this
overflows the (on lpc18xx (i.e. ARM32) 32 bit wide) unsigned int
.max_period_ns. This results (dependant of the actual clk rate) in an
arbitrary limitation of the maximal period. E.g. for clkrate =
333333333 (Hz) we get max_period_ns = 9 instead of 12884901897.

So make .max_period_ns an u64 and pass period and duty as u64 to not
discard relevant digits. And also make use of mul_u64_u64_div_u64()
which prevents all overflows assuming clk_rate < NSEC_PER_SEC.

Fixes: 841e6f90bb78 ("pwm: NXP LPC18xx PWM/SCT driver")
Signed-off-by: Uwe Kleine-König <[e-mail]>
Signed-off-by: Thierry Reding <[e-mail]>

drivers/pwm/pwm-lpc18xx-sct.c

···
 	unsigned long clk_rate;
 	unsigned int period_ns;
 	unsigned int min_period_ns;
-	unsigned int max_period_ns;
+	u64 max_period_ns;
 	unsigned int period_event;
 	unsigned long event_map;
 	struct mutex res_lock;
···
 	mutex_unlock(&lpc18xx_pwm->res_lock);
 }
 
-static void lpc18xx_pwm_config_period(struct pwm_chip *chip, int period_ns)
+static void lpc18xx_pwm_config_period(struct pwm_chip *chip, u64 period_ns)
 {
 	struct lpc18xx_pwm_chip *lpc18xx_pwm = to_lpc18xx_pwm_chip(chip);
-	u64 val;
+	u32 val;
 
-	val = (u64)period_ns * lpc18xx_pwm->clk_rate;
-	do_div(val, NSEC_PER_SEC);
+	/*
+	 * With clk_rate < NSEC_PER_SEC this cannot overflow.
+	 * With period_ns < max_period_ns this also fits into an u32.
+	 * As period_ns >= min_period_ns = DIV_ROUND_UP(NSEC_PER_SEC, lpc18xx_pwm->clk_rate);
+	 * we have val >= 1.
+	 */
+	val = mul_u64_u64_div_u64(period_ns, lpc18xx_pwm->clk_rate, NSEC_PER_SEC);
 
 	lpc18xx_pwm_writel(lpc18xx_pwm,
 			   LPC18XX_PWM_MATCH(lpc18xx_pwm->period_event),
-			   (u32)val - 1);
+			   val - 1);
 
 	lpc18xx_pwm_writel(lpc18xx_pwm,
 			   LPC18XX_PWM_MATCHREL(lpc18xx_pwm->period_event),
-			   (u32)val - 1);
+			   val - 1);
 }
 
 static void lpc18xx_pwm_config_duty(struct pwm_chip *chip,
-				    struct pwm_device *pwm, int duty_ns)
+				    struct pwm_device *pwm, u64 duty_ns)
 {
 	struct lpc18xx_pwm_chip *lpc18xx_pwm = to_lpc18xx_pwm_chip(chip);
 	struct lpc18xx_pwm_data *lpc18xx_data = &lpc18xx_pwm->channeldata[pwm->hwpwm];
-	u64 val;
+	u32 val;
 
-	val = (u64)duty_ns * lpc18xx_pwm->clk_rate;
-	do_div(val, NSEC_PER_SEC);
+	/*
+	 * With clk_rate < NSEC_PER_SEC this cannot overflow.
+	 * With duty_ns <= period_ns < max_period_ns this also fits into an u32.
+	 */
+	val = mul_u64_u64_div_u64(duty_ns, lpc18xx_pwm->clk_rate, NSEC_PER_SEC);
 
 	lpc18xx_pwm_writel(lpc18xx_pwm,
 			   LPC18XX_PWM_MATCH(lpc18xx_data->duty_event),
-			   (u32)val);
+			   val);
 
 	lpc18xx_pwm_writel(lpc18xx_pwm,
 			   LPC18XX_PWM_MATCHREL(lpc18xx_data->duty_event),
-			   (u32)val);
+			   val);
 }
 
 static int lpc18xx_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
···
 		goto disable_pwmclk;
 	}
 
+	/*
+	 * If clkrate is too fast, the calculations in .apply() might overflow.
+	 */
+	if (lpc18xx_pwm->clk_rate > NSEC_PER_SEC) {
+		ret = dev_err_probe(&pdev->dev, -EINVAL, "pwm clock to fast\n");
+		goto disable_pwmclk;
+	}
+
 	mutex_init(&lpc18xx_pwm->res_lock);
 	mutex_init(&lpc18xx_pwm->period_lock);
 
-	val = (u64)NSEC_PER_SEC * LPC18XX_PWM_TIMER_MAX;
-	do_div(val, lpc18xx_pwm->clk_rate);
-	lpc18xx_pwm->max_period_ns = val;
+	lpc18xx_pwm->max_period_ns =
+		mul_u64_u64_div_u64(NSEC_PER_SEC, LPC18XX_PWM_TIMER_MAX, lpc18xx_pwm->clk_rate);
 
 	lpc18xx_pwm->min_period_ns = DIV_ROUND_UP(NSEC_PER_SEC,
 						  lpc18xx_pwm->clk_rate);