staging: iio: resolver: ad2s1210: read excitation frequency from control register

This modifies the ad2s1210_show_fexcit() function to read the excitation
frequency from the control register. This way we don't have to keep
track of the value and don't risk returning a stale value.
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20230929-ad2s1210-mainline-v3-16-fa4364281745@baylibre.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/resolver/ad2s1210.c

@@ -76,7 +76,6 @@ struct ad2s1210_state {
 	struct regmap *regmap;
 	/** The external oscillator frequency in Hz. */
 	unsigned long clkin_hz;
-	unsigned int fexcit;
 	bool hysteresis;
 	u8 resolution;
 	/** For reading raw sample value via SPI. */
@@ -207,8 +206,6 @@ static int ad2s1210_reinit_excitation_frequency(struct ad2s1210_state *st,
 	if (ret < 0)
 		return ret;
 
-	st->fexcit = fexcit;
-
 	/*
 	 * Software reset reinitializes the excitation frequency output.
 	 * It does not reset any of the configuration registers.
@@ -233,8 +230,22 @@ static ssize_t ad2s1210_show_fexcit(struct device *dev,
 				    char *buf)
 {
 	struct ad2s1210_state *st = iio_priv(dev_to_iio_dev(dev));
+	unsigned int value;
+	u16 fexcit;
+	int ret;
 
-	return sprintf(buf, "%u\n", st->fexcit);
+	mutex_lock(&st->lock);
+	ret = regmap_read(st->regmap, AD2S1210_REG_EXCIT_FREQ, &value);
+	if (ret < 0)
+		goto error_ret;
+
+	fexcit = value * st->clkin_hz / (1 << 15);
+
+	ret = sprintf(buf, "%u\n", fexcit);
+
+error_ret:
+	mutex_unlock(&st->lock);
+	return ret;
 }
 
 static ssize_t ad2s1210_store_fexcit(struct device *dev,