hwmon: (pwm-fan) Fix RPM calculation

To convert the number of pulses counted into an RPM estimation, we need
to divide by the width of our measurement interval instead of
multiplying by it. If the width of the measurement interval is zero we
don't update the RPM value to avoid dividing by zero.

We also don't need to do 64-bit division, with 32-bits we can handle a
fan running at over 4 million RPM.
Signed-off-by: Paul Barker <pbarker@konsulko.com>
Link: https://lore.kernel.org/r/20201111164643.7087-1-pbarker@konsulko.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/pwm-fan.c

@@ -54,16 +54,18 @@ static irqreturn_t pulse_handler(int irq, void *dev_id)
 static void sample_timer(struct timer_list *t)
 {
 	struct pwm_fan_ctx *ctx = from_timer(ctx, t, rpm_timer);
+	unsigned int delta = ktime_ms_delta(ktime_get(), ctx->sample_start);
 	int pulses;
-	u64 tmp;
 
+	if (delta) {
 		pulses = atomic_read(&ctx->pulses);
 		atomic_sub(pulses, &ctx->pulses);
-	tmp = (u64)pulses * ktime_ms_delta(ktime_get(), ctx->sample_start) * 60;
-	do_div(tmp, ctx->pulses_per_revolution * 1000);
-	ctx->rpm = tmp;
+		ctx->rpm = (unsigned int)(pulses * 1000 * 60) /
+			(ctx->pulses_per_revolution * delta);
 
 		ctx->sample_start = ktime_get();
+	}
+
 	mod_timer(&ctx->rpm_timer, jiffies + HZ);
 }