iio: adc: ad7124: do not require mclk

Make the "mclk" clock optional in the ad7124 driver. The MCLK is an
internal counter on the ADC, so it is not something that should be
coming from the devicetree. However, existing users may be using this
to essentially select the power mode of the ADC from the devicetree.
In order to not break those users, we have to keep the existing "mclk"
handling, but now it is optional.

Now, when the "mclk" clock is omitted from the devicetree, the driver
will default to the full power mode. Support for an external clock
and dynamic power mode switching can be added later if needed.

Signed-off-by: David Lechner <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/ad7124.c

···
 	struct ad_sigma_delta sd;
 	struct ad7124_channel *channels;
 	struct regulator *vref[4];
-	struct clk *mclk;
 	unsigned int adc_control;
 	unsigned int num_channels;
 	struct mutex cfgs_lock; /* lock for configs access */
···
 {
 	unsigned int fclk, odr_sel_bits;
 
-	fclk = clk_get_rate(st->mclk);
+	fclk = ad7124_master_clk_freq_hz[FIELD_GET(AD7124_ADC_CONTROL_POWER_MODE,
+						   st->adc_control)];
+
 	/*
 	 * FS[10:0] = fCLK / (fADC x 32) where:
 	 * fADC is the output data rate
···
 static int ad7124_setup(struct ad7124_state *st)
 {
 	struct device *dev = &st->sd.spi->dev;
-	unsigned int fclk, power_mode;
+	unsigned int power_mode;
+	struct clk *mclk;
 	int i, ret;
 
-	fclk = clk_get_rate(st->mclk);
-	if (!fclk)
-		return dev_err_probe(dev, -EINVAL, "Failed to get mclk rate\n");
+	/*
+	 * Always use full power mode for max performance. If needed, the driver
+	 * could be adapted to use a dynamic power mode based on the requested
+	 * output data rate.
+	 */
+	power_mode = AD7124_ADC_CONTROL_POWER_MODE_FULL;
 
-	/* The power mode changes the master clock frequency */
-	power_mode = ad7124_find_closest_match(ad7124_master_clk_freq_hz,
-					ARRAY_SIZE(ad7124_master_clk_freq_hz),
-					fclk);
-	if (fclk != ad7124_master_clk_freq_hz[power_mode]) {
-		ret = clk_set_rate(st->mclk, fclk);
-		if (ret)
-			return dev_err_probe(dev, ret, "Failed to set mclk rate\n");
+	/*
+	 * This "mclk" business is needed for backwards compatibility with old
+	 * devicetrees that specified a fake clock named "mclk" to select the
+	 * power mode.
+	 */
+	mclk = devm_clk_get_optional_enabled(dev, "mclk");
+	if (IS_ERR(mclk))
+		return dev_err_probe(dev, PTR_ERR(mclk), "Failed to get mclk\n");
+
+	if (mclk) {
+		unsigned long mclk_hz;
+
+		mclk_hz = clk_get_rate(mclk);
+		if (!mclk_hz)
+			return dev_err_probe(dev, -EINVAL,
+					     "Failed to get mclk rate\n");
+
+		/*
+		 * This logic is a bit backwards, which is why it is only here
+		 * for backwards compatibility. The driver should be able to set
+		 * the power mode as it sees fit and the f_clk/mclk rate should
+		 * be dynamic accordingly. But here, we are selecting a fixed
+		 * power mode based on the given "mclk" rate.
+		 */
+		power_mode = ad7124_find_closest_match(ad7124_master_clk_freq_hz,
+			ARRAY_SIZE(ad7124_master_clk_freq_hz), mclk_hz);
+
+		if (mclk_hz != ad7124_master_clk_freq_hz[power_mode]) {
+			ret = clk_set_rate(mclk, mclk_hz);
+			if (ret)
+				return dev_err_probe(dev, ret,
+						     "Failed to set mclk rate\n");
+		}
 	}
 
 	/* Set the power mode */
···
 		if (ret)
 			return ret;
 	}
-
-	st->mclk = devm_clk_get_enabled(&spi->dev, "mclk");
-	if (IS_ERR(st->mclk))
-		return dev_err_probe(dev, PTR_ERR(st->mclk), "Failed to get mclk\n");
 
 	ret = ad7124_soft_reset(st);
 	if (ret < 0)