iio: adc: stm32: allow to tune analog clock

Add new optional dt property to tune analog clock prescaler.
Driver looks for optional "st,max-clk-rate-hz", then computes
best approximation below that rate, using ADC internal prescaler.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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

@@ -79,6 +79,7 @@ struct stm32_adc_priv_cfg {
  * @domain:		irq domain reference
  * @aclk:		clock reference for the analog circuitry
  * @bclk:		bus clock common for all ADCs, depends on part used
+ * @max_clk_rate:	desired maximum clock rate
  * @booster:		booster supply reference
  * @vdd:		vdd supply reference
  * @vdda:		vdda analog supply reference
@@ -95,6 +96,7 @@ struct stm32_adc_priv {
 	struct irq_domain		*domain;
 	struct clk			*aclk;
 	struct clk			*bclk;
+	u32				max_clk_rate;
 	struct regulator		*booster;
 	struct regulator		*vdd;
 	struct regulator		*vdda;
@@ -141,7 +143,7 @@ static int stm32f4_adc_clk_sel(struct platform_device *pdev,
 	}
 
 	for (i = 0; i < ARRAY_SIZE(stm32f4_pclk_div); i++) {
-		if ((rate / stm32f4_pclk_div[i]) <= priv->cfg->max_clk_rate_hz)
+		if ((rate / stm32f4_pclk_div[i]) <= priv->max_clk_rate)
 			break;
 	}
 	if (i >= ARRAY_SIZE(stm32f4_pclk_div)) {
@@ -230,7 +232,7 @@ static int stm32h7_adc_clk_sel(struct platform_device *pdev,
 			if (ckmode)
 				continue;
 
-			if ((rate / div) <= priv->cfg->max_clk_rate_hz)
+			if ((rate / div) <= priv->max_clk_rate)
 				goto out;
 		}
 	}
@@ -250,7 +252,7 @@ static int stm32h7_adc_clk_sel(struct platform_device *pdev,
 		if (!ckmode)
 			continue;
 
-		if ((rate / div) <= priv->cfg->max_clk_rate_hz)
+		if ((rate / div) <= priv->max_clk_rate)
 			goto out;
 	}
 
@@ -655,6 +657,7 @@ static int stm32_adc_probe(struct platform_device *pdev)
 	struct device *dev = &pdev->dev;
 	struct device_node *np = pdev->dev.of_node;
 	struct resource *res;
+	u32 max_rate;
 	int ret;
 
 	if (!pdev->dev.of_node)
@@ -731,6 +734,13 @@ static int stm32_adc_probe(struct platform_device *pdev)
 	priv->common.vref_mv = ret / 1000;
 	dev_dbg(&pdev->dev, "vref+=%dmV\n", priv->common.vref_mv);
 
+	ret = of_property_read_u32(pdev->dev.of_node, "st,max-clk-rate-hz",
+				   &max_rate);
+	if (!ret)
+		priv->max_clk_rate = min(max_rate, priv->cfg->max_clk_rate_hz);
+	else
+		priv->max_clk_rate = priv->cfg->max_clk_rate_hz;
+
 	ret = priv->cfg->clk_sel(pdev, priv);
 	if (ret < 0)
 		goto err_hw_stop;