iio: adc: aspeed: Add func to set sampling rate.

Add the function to set the sampling rate and keep the sampling period
for a driver used to wait the fresh value.
In addition, since the ADC clock is required when initializing the ADC
device, move clk_prepare_enable ahead of the initialization phase.
Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Link: https://lore.kernel.org/r/20210922081520.30580-9-billy_tsai@aspeedtech.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/aspeed_adc.c

@@ -73,6 +73,12 @@
 
 #define ASPEED_ADC_INIT_POLLING_TIME	500
 #define ASPEED_ADC_INIT_TIMEOUT		500000
+/*
+ * When the sampling rate is too high, the ADC may not have enough charging
+ * time, resulting in a low voltage value. Thus, the default uses a slow
+ * sampling rate for most use cases.
+ */
+#define ASPEED_ADC_DEF_SAMPLING_RATE	65000
 
 struct aspeed_adc_model_data {
 	const char *model_name;
@@ -96,6 +102,7 @@ struct aspeed_adc_data {
 	struct clk_hw		*clk_scaler;
 	struct reset_control	*rst;
 	int			vref_mv;
+	u32			sample_period_ns;
 };
 
 #define ASPEED_CHAN(_idx, _data_reg_addr) {			\
@@ -127,6 +134,24 @@ static const struct iio_chan_spec aspeed_adc_iio_channels[] = {
 	ASPEED_CHAN(15, 0x2E),
 };
 
+static int aspeed_adc_set_sampling_rate(struct iio_dev *indio_dev, u32 rate)
+{
+	struct aspeed_adc_data *data = iio_priv(indio_dev);
+
+	if (rate < data->model_data->min_sampling_rate ||
+	    rate > data->model_data->max_sampling_rate)
+		return -EINVAL;
+	/* Each sampling needs 12 clocks to convert.*/
+	clk_set_rate(data->clk_scaler->clk, rate * ASPEED_CLOCKS_PER_SAMPLE);
+	rate = clk_get_rate(data->clk_scaler->clk);
+	data->sample_period_ns = DIV_ROUND_UP_ULL(
+		(u64)NSEC_PER_SEC * ASPEED_CLOCKS_PER_SAMPLE, rate);
+	dev_dbg(data->dev, "Adc clock = %d sample period = %d ns", rate,
+		data->sample_period_ns);
+
+	return 0;
+}
+
 static int aspeed_adc_read_raw(struct iio_dev *indio_dev,
 			       struct iio_chan_spec const *chan,
 			       int *val, int *val2, long mask)
@@ -157,17 +182,9 @@ static int aspeed_adc_write_raw(struct iio_dev *indio_dev,
 				struct iio_chan_spec const *chan,
 				int val, int val2, long mask)
 {
-	struct aspeed_adc_data *data = iio_priv(indio_dev);
-
 	switch (mask) {
 	case IIO_CHAN_INFO_SAMP_FREQ:
-		if (val < data->model_data->min_sampling_rate ||
-			val > data->model_data->max_sampling_rate)
-			return -EINVAL;
-
-		clk_set_rate(data->clk_scaler->clk,
-				val * ASPEED_CLOCKS_PER_SAMPLE);
-		return 0;
+		return aspeed_adc_set_sampling_rate(indio_dev, val);
 
 	case IIO_CHAN_INFO_SCALE:
 	case IIO_CHAN_INFO_RAW:
@@ -391,6 +408,19 @@ static int aspeed_adc_probe(struct platform_device *pdev)
 	if (ret)
 		return ret;
 
+	ret = clk_prepare_enable(data->clk_scaler->clk);
+	if (ret)
+		return ret;
+	ret = devm_add_action_or_reset(data->dev,
+				       aspeed_adc_clk_disable_unprepare,
+				       data->clk_scaler->clk);
+	if (ret)
+		return ret;
+	ret = aspeed_adc_set_sampling_rate(indio_dev,
+					   ASPEED_ADC_DEF_SAMPLING_RATE);
+	if (ret)
+		return ret;
+
 	adc_engine_control_reg_val =
 		readl(data->base + ASPEED_REG_ENGINE_CONTROL);
 	adc_engine_control_reg_val |=
@@ -417,16 +447,6 @@ static int aspeed_adc_probe(struct platform_device *pdev)
 			return ret;
 	}
 
-	ret = clk_prepare_enable(data->clk_scaler->clk);
-	if (ret)
-		return ret;
-
-	ret = devm_add_action_or_reset(data->dev,
-				       aspeed_adc_clk_disable_unprepare,
-				       data->clk_scaler->clk);
-	if (ret)
-		return ret;
-
 	/* Start all channels in normal mode. */
 	adc_engine_control_reg_val =
 		readl(data->base + ASPEED_REG_ENGINE_CONTROL);