pwm: sunxi: choose best prescaler to improve PWM resolution

Choose best prescaler to improve PWM resolution. Without this change
driver chooses first prescaler that gives us period value within
range, but it could be not the best one.
Signed-off-by: NVasily Khoruzhick <anarsoul@gmail.com>
Tested-by: NVagrant Cascadian <vagrant@debian.org>
Acked-by: NMaxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: NJagan Teki <jagan@openedev.com>

drivers/pwm/sunxi_pwm.c

@@ -67,49 +67,55 @@ static int sunxi_pwm_set_config(struct udevice *dev, uint channel,
 {
 	struct sunxi_pwm_priv *priv = dev_get_priv(dev);
 	struct sunxi_pwm *regs = priv->regs;
-	int prescaler;
-	u32 v, period = 0, duty;
-	u64 scaled_freq = 0;
+	int best_prescaler = 0;
+	u32 v, best_period = 0, duty;
+	u64 best_scaled_freq = 0;
 	const u32 nsecs_per_sec = 1000000000U;
 
 	debug("%s: period_ns=%u, duty_ns=%u\n", __func__, period_ns, duty_ns);
 
-	for (prescaler = 0; prescaler <= SUNXI_PWM_CTRL_PRESCALE0_MASK;
+	for (int prescaler = 0; prescaler <= SUNXI_PWM_CTRL_PRESCALE0_MASK;
 	     prescaler++) {
+		u32 period = 0;
+		u64 scaled_freq = 0;
 		if (!prescaler_table[prescaler])
 			continue;
 		scaled_freq = lldiv(OSC_24MHZ, prescaler_table[prescaler]);
 		period = lldiv(scaled_freq * period_ns, nsecs_per_sec);
-		if (period - 1 <= SUNXI_PWM_CH0_PERIOD_MAX)
-			break;
+		if ((period - 1 <= SUNXI_PWM_CH0_PERIOD_MAX) &&
+		    best_period < period) {
+			best_period = period;
+			best_scaled_freq = scaled_freq;
+			best_prescaler = prescaler;
+		}
 	}
 
-	if (period - 1 > SUNXI_PWM_CH0_PERIOD_MAX) {
+	if (best_period - 1 > SUNXI_PWM_CH0_PERIOD_MAX) {
 		debug("%s: failed to find prescaler value\n", __func__);
 		return -EINVAL;
 	}
 
-	duty = lldiv(scaled_freq * duty_ns, nsecs_per_sec);
+	duty = lldiv(best_scaled_freq * duty_ns, nsecs_per_sec);
 
-	if (priv->prescaler != prescaler) {
+	if (priv->prescaler != best_prescaler) {
 		/* Mask clock to update prescaler */
 		v = readl(&regs->ctrl);
 		v &= ~SUNXI_PWM_CTRL_CLK_GATE;
 		writel(v, &regs->ctrl);
 		v &= ~SUNXI_PWM_CTRL_PRESCALE0_MASK;
-		v |= (prescaler & SUNXI_PWM_CTRL_PRESCALE0_MASK);
+		v |= (best_prescaler & SUNXI_PWM_CTRL_PRESCALE0_MASK);
 		writel(v, &regs->ctrl);
 		v |= SUNXI_PWM_CTRL_CLK_GATE;
 		writel(v, &regs->ctrl);
-		priv->prescaler = prescaler;
+		priv->prescaler = best_prescaler;
 	}
 
-	writel(SUNXI_PWM_CH0_PERIOD_PRD(period) |
+	writel(SUNXI_PWM_CH0_PERIOD_PRD(best_period) |
 	       SUNXI_PWM_CH0_PERIOD_DUTY(duty), &regs->ch0_period);
 
 	debug("%s: prescaler: %d, period: %d, duty: %d\n",
 	      __func__, priv->prescaler,
-	      period, duty);
+	      best_period, duty);
 
 	return 0;
 }