iio: adc: stm32-adc: adapt clock duty cycle for proper operation

For proper operation, STM32 ADC should be used with a clock duty cycle
of 50%, in the range of 49% to 51%. Depending on the clock tree, divider
can be used in case clock duty cycle is out of this range.
In case clk_get_scaled_duty_cycle() returns an error, kindly apply a
divider by default (don't make the probe fail).
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Link: https://lore.kernel.org/r/1604681846-31234-1-git-send-email-fabrice.gasnier@st.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/stm32-adc-core.c

@@ -202,7 +202,7 @@ static int stm32h7_adc_clk_sel(struct platform_device *pdev,
 {
 	u32 ckmode, presc, val;
 	unsigned long rate;
-	int i, div;
+	int i, div, duty;
 
 	/* stm32h7 bus clock is common for all ADC instances (mandatory) */
 	if (!priv->bclk) {
@@ -226,6 +226,11 @@ static int stm32h7_adc_clk_sel(struct platform_device *pdev,
 			return -EINVAL;
 		}
 
+		/* If duty is an error, kindly use at least /2 divider */
+		duty = clk_get_scaled_duty_cycle(priv->aclk, 100);
+		if (duty < 0)
+			dev_warn(&pdev->dev, "adc clock duty: %d\n", duty);
+
 		for (i = 0; i < ARRAY_SIZE(stm32h7_adc_ckmodes_spec); i++) {
 			ckmode = stm32h7_adc_ckmodes_spec[i].ckmode;
 			presc = stm32h7_adc_ckmodes_spec[i].presc;
@@ -234,6 +239,13 @@ static int stm32h7_adc_clk_sel(struct platform_device *pdev,
 			if (ckmode)
 				continue;
 
+			/*
+			 * For proper operation, clock duty cycle range is 49%
+			 * to 51%. Apply at least /2 prescaler otherwise.
+			 */
+			if (div == 1 && (duty < 49 || duty > 51))
+				continue;
+
 			if ((rate / div) <= priv->max_clk_rate)
 				goto out;
 		}
@@ -246,6 +258,10 @@ static int stm32h7_adc_clk_sel(struct platform_device *pdev,
 		return -EINVAL;
 	}
 
+	duty = clk_get_scaled_duty_cycle(priv->bclk, 100);
+	if (duty < 0)
+		dev_warn(&pdev->dev, "bus clock duty: %d\n", duty);
+
 	for (i = 0; i < ARRAY_SIZE(stm32h7_adc_ckmodes_spec); i++) {
 		ckmode = stm32h7_adc_ckmodes_spec[i].ckmode;
 		presc = stm32h7_adc_ckmodes_spec[i].presc;
@@ -254,6 +270,9 @@ static int stm32h7_adc_clk_sel(struct platform_device *pdev,
 		if (!ckmode)
 			continue;
 
+		if (div == 1 && (duty < 49 || duty > 51))
+			continue;
+
 		if ((rate / div) <= priv->max_clk_rate)
 			goto out;
 	}